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Association between parity and breastfeeding with maternal high blood pressure

Published:February 11, 2013DOI:https://doi.org/10.1016/j.ajog.2013.02.014

      Objective

      The objective of this study was to determine how parity and breastfeeding were associated with maternal high blood pressure, and how age modifies this association.

      Study Design

      Baseline data for 74,785 women were sourced from the 45 and Up Study, Australia. These women were 45 years of age or older, had an intact uterus, and had not been diagnosed with high blood pressure before pregnancy. Odds ratios (ORs) and 99% confidence intervals (CIs) for the association between giving birth, breastfeeding, lifetime breastfeeding duration, and average breastfeeding per child with high blood pressure were estimated using logistic regression.

      Results

      The combination of parity and breastfeeding was associated with lower odds of having high blood pressure (adjusted OR, 0.89; 99% CI, 0.82–0.97; P < .001), compared with nulliparous women, whereas there was no significant difference between mothers who did not breastfeed and nulliparous women (adjusted OR, 1.06; 99% CI, 0.95–1.18; P = .20). Women who breastfed for longer than 6 months in their lifetime, or greater than 3 months per child, on average, had significantly lower odds of having high blood pressure when compared with parous women who never breastfed. The odds were lower with longer breastfeeding durations and were no longer significant in the majority of women over the age of 64 years.

      Conclusion

      Women should be encouraged to breastfeed for as long as possible and a woman's breastfeeding history should be taken into account when assessing her likelihood of high blood pressure in later life.

      Key words

      Pregnancy and breastfeeding are associated with large changes to the female hormonal profile. A correlation between these events and a woman's risk of cardiovascular disease has been reported since the 1950s
      • Winkelstein Jr, W.
      • Stenchever M.A.
      • Lilienfeld A.M.
      Occurrence of pregnancy, abortion, and artificial menopause among women with coronary artery disease: a preliminary study.
      with more recent large cohort studies showing breastfeeding is beneficial to maternal cardiovascular health.
      • Lee S.Y.
      • Kim M.T.
      • Jee S.H.
      • Yang H.P.
      Does long-term lactation protect premenopausal women against hypertension risk? A Korean women's cohort study.
      • Ram K.T.
      • Bobby P.
      • Hailpern S.M.
      • et al.
      Duration of lactation is associated with lower prevalence of the metabolic syndrome in midlife—SWAN, the study of women's health across the nation.
      • Schwarz E.B.
      • Ray R.M.
      • Stuebe A.M.
      • et al.
      Duration of lactation and risk factors for maternal cardiovascular disease.
      • Stuebe A.M.
      • Schwarz E.B.
      • Grewen K.
      • et al.
      Duration of lactation and incidence of maternal hypertension: a longitudinal cohort study.
      For Editors' Commentary, see Contents
      Cardiovascular diseases are the leading cause of death among women in developed countries. Hypertension is one of the most prevalent cardiovascular diseases and is a strong predictor of other cardiovascular diseases including atherosclerosis, myocardial infarction, and stroke. Age is a powerful predictor of hypertension, with increasing age associated with increasing rates of hypertension in industrialized countries.
      • Franklin S.S.
      • Gustin Wt
      • Wong N.D.
      • et al.
      Hemodynamic patterns of age-related changes in blood pressure. The Framingham Heart Study.
      The association between pregnancy and hypertension in later life remains unclear. Early work within the field found an association between increasing number of pregnancies and lower odds of hypertension,
      • Ness R.B.
      • Kramer R.A.
      • Flegal K.M.
      Gravidity, blood pressure, and hypertension among white women in the Second National Health and Nutrition Examination Survey.
      whereas other studies have shown no association between parity and blood pressure.
      • Lawlor D.A.
      • Emberson J.R.
      • Ebrahim S.
      • et al.
      Is the association between parity and coronary heart disease due to biological effects of pregnancy or adverse lifestyle risk factors associated with child-rearing? Findings from the British Women's Heart and Health Study and the British Regional Heart Study.
      • Ness R.B.
      • Harris T.
      • Cobb J.
      • et al.
      Number of pregnancies and the subsequent risk of cardiovascular disease.
      These studies had relatively small sample sizes in comparison to the large cohorts that have since been established.
      Breastfeeding has been reported to confer many benefits upon the newborn, including reduced levels of childhood obesity,
      • Kramer M.S.
      Do breast-feeding and delayed introduction of solid foods protect against subsequent obesity?.
      hypertension,
      • Martin R.M.
      • Gunnell D.
      • Smith G.D.
      Breastfeeding in infancy and blood pressure in later life: systematic review and meta-analysis.
      • Singhal A.
      • Cole T.J.
      • Lucas A.
      Early nutrition in preterm infants and later blood pressure: two cohorts after randomised trials.
      • Taittonen L.
      • Nuutinen M.
      • Turtinen J.
      • Uhari M.
      Prenatal and postnatal factors in predicting later blood pressure among children: cardiovascular risk in young Finns.
      and hyperlipidemia.
      • Owen C.G.
      • Whincup P.H.
      • Odoki K.
      • Gilg J.A.
      • Cook D.G.
      Infant feeding and blood cholesterol: a study in adolescents and a systematic review.
      • Kark J.D.
      • Troya G.
      • Friedlander Y.
      • Slater P.E.
      • Stein Y.
      Validity of maternal reporting of breast feeding history and the association with blood lipids in 17 year olds in Jerusalem.
      More recent studies have found an association between breastfeeding and a lower risk of maternal hypertension. This was first reported in 2005 by the Korean Women's Cohort (KWC) Study
      • Lee S.Y.
      • Kim M.T.
      • Jee S.H.
      • Yang H.P.
      Does long-term lactation protect premenopausal women against hypertension risk? A Korean women's cohort study.
      and has since been reported by the Study of Women's Health Across the Nation (SWAN),
      • Ram K.T.
      • Bobby P.
      • Hailpern S.M.
      • et al.
      Duration of lactation is associated with lower prevalence of the metabolic syndrome in midlife—SWAN, the study of women's health across the nation.
      the Women's Health Initiative (WHI) Study,
      • Schwarz E.B.
      • Ray R.M.
      • Stuebe A.M.
      • et al.
      Duration of lactation and risk factors for maternal cardiovascular disease.
      and the US Nurses' Health Study II.
      • Stuebe A.M.
      • Schwarz E.B.
      • Grewen K.
      • et al.
      Duration of lactation and incidence of maternal hypertension: a longitudinal cohort study.
      No studies have examined whether the combined effect of parity and breastfeeding is associated with high blood pressure, and how age modifies the association between breastfeeding and high blood pressure.
      The present study aimed to determine: (a) the association between the event of giving birth with high blood pressure in later life, using observational data from the 45 and Up Study, Australia, (b) whether the combination of giving birth and breastfeeding is associated with high blood pressure in later life, and (c) if the duration of breastfeeding was associated with high blood pressure in later life, and how this association is modified as women age.

      Materials and Methods

      This study obtained data from women participating in the 45 and Up Study, a large scale cohort study of 267,153 men and women aged 45 and over in New South Wales, Australia. Participants were randomly selected from the Australian Medicare Database, which provides near complete coverage of the population, and they were enrolled into the study by completing a baseline questionnaire (available at www.45andUp.org.au) and providing a signed consent form. People aged 80 years and over, and residents of rural and remote areas were oversampled. Study recruitment commenced in 2006 and was completed in 2009. The methods for the 45 and Up Study have been described elsewhere.
      • Banks E.
      • Redman S.
      • Jorm L.
      • et al.
      Cohort profile: the 45 and Up Study.
      The 45 and Up Study received ethics approval from the University of NSW Human Ethics Committee, and the current study was approved by the University of Western Sydney Human Research Ethics Committee. Exposure-outcome relationships estimated from the 45 and Up Study data have been shown to be consistent with another large study of the same population, regardless of the underlying response rate or mode of questionnaire administration.
      • Mealing N.M.
      • Banks E.
      • Jorm L.R.
      • Steel D.G.
      • Clements M.S.
      • Rogers K.D.
      Investigation of relative risk estimates from studies of the same population with contrasting response rates and designs.
      All of the data used in this study were acquired from the 45 and Up Study baseline questionnaire. Women were included in this study if: they were age 45 years or more; had never given birth or had given birth after 18 years of age and before 45 years of age; had not had a hysterectomy or both ovaries removed; and had responded “No” to the question “Has a doctor ever told you that you have: high blood pressure–when pregnant?” (Figure 1).
      Figure thumbnail gr1
      FIGURE 1Participants included in the study
      Flow chart of participant inclusion.
      Lupton. Parity, breastfeeding, and high blood pressure. Am J Obstet Gynecol 2013.
      Women were defined as having high blood pressure if they answered “Yes” to the question “In the last month have you been treated for: high blood pressure.” Women were excluded if: they answered “Yes” to the question “Has a doctor ever told you that you have: high blood pressure – when not pregnant?” and the “Age when condition was first found” was younger than, the age reported on the question “How old were you when you gave birth to your FIRST child?”; answered “Yes” to “Has a doctor ever told you that you have: high blood pressure – when not pregnant?”, but were not being treated for high blood pressure; they failed to provide an age of onset for high blood pressure; they provided invalid data for family history; or they provided invalid data for the number of children they had given birth to in their specified age range (Figure 1). Classification of demographic and lifestyle characteristics have been described elsewhere.
      • Chiu C.L.
      • Lujic S.
      • Thornton C.
      • et al.
      Menopausal hormone therapy is associated with having high blood pressure in postmenopausal women: observational cohort study.
      Women were classified as never having given birth if they answered “0’ to the question “How many children have you given birth to?”, with the further instruction to “please include stillbirths but do not include miscarriages, please write ‘0’ if you have not had any children.” Total breastfeeding duration was obtained from the response to the question “For how many months, in total, have you breastfed?”. Average breastfeeding duration was obtained by dividing the total breastfeeding duration by the reported number of children for each woman.
      Odds ratios (ORs) and 99% confidence intervals (CIs) for the association between giving birth, breastfeeding, lifetime breastfeeding duration, and average breastfeeding per child with high blood pressure were estimated using logistic regression. For the analysis of whether giving birth is associated with having high blood pressure, women who had never given birth were the reference group. Analysis of lifetime breastfeeding and breastfeeding duration included only women who had given birth, with women who had never breastfed as the reference group. Both crude and adjusted OR were calculated and descriptions refer to adjusted OR unless otherwise specified. OR were adjusted for demographic and lifestyle factors using the categories in Table 1, with an additional category for missing values. There was a significant interaction between whether a woman breastfed and current age, with having high blood pressure. As a result, women were stratified according to current age and divided into 3 groups (45 to <54 years, 54 to <64 years, 64 years or older) when testing the association between lifetime breastfeeding and breastfeeding duration with high blood pressure status. All statistical tests were 2-sided, using a significance level of P < .01 to partially account for multiple testing issues.
      • Rothman K.J.
      No adjustments are needed for multiple comparisons.
      • Geissinger J.D.
      Letter: medical treatment of subdural hematomas challenged.
      All statistical analyses were carried out using SPSS software (version 20; SPSS, Inc, Chicago, IL).
      TABLE 1Sociodemographic factors associated with past breastfeeding behavior
      CharacteristicsGroupsCases
      Percentages do not consistently total to 100% because of missing values;
      (% column)
      % BF
      Percentages do not consistently total to 100% because of missing values;
      OR
      Analysis adjusted for current age, country of origin, income level, family history of HBP, BMI, smoking status, alcohol consumption, physical activity, oral contraceptive use, HRT use, and number of children;
      (99% CI)
      Current age, y45 to <5422,361 (35)931.00
      54 to <6421,454 (33)870.58 (0.53–0.64)
      P < .01.
      64+20,384 (32)870.60 (0.54–0.66)
      P < .01.
      Country of originAustralia48,291 (75)901.00
      Other15,368 (24)860.78 (0.72–0.84)
      P < .01.
      Income<$30K16,416 (26)861.00
      $30–$70K15,984 (25)901.29 (1.17–1.42)
      P < .01.
      $70K+15,472 (24)942.09 (1.86–2.35)
      P < .01.
      Not disclosed16,327 (25)860.94 (0.86–1.02)
      Family history of HBPNo31,098 (48)891.00
      Yes33,101 (52)891.01 (0.94–1.08)
      BMI<2529,344 (46)901.00
      25-3018,868 (29)890.88 (0.81–0.95)
      P < .01.
      30+10,839 (17)860.69 (0.63–0.75)
      P < .01.
      Smoking statusNever41,733 (65)901.00
      Past17,909 (28)890.86 (0.80–0.93)
      P < .01.
      Current4272 (7)830.57 (0.50–0.64)
      P < .01.
      Alcohol (drinks/wk)<124,191 (38)861.00
      1-517,760 (28)911.55 (1.42–1.69)
      P < .01.
      6-1012,990 (20)911.52 (1.37–1.67)
      P < .01.
      11+7963 (12)901.36 (1.22–1.53)
      P < .01.
      Physical activityInsufficient19,792 (31)871.00
      Sufficient44,407 (69)901.19 (1.11–1.28)
      P < .01.
      Oral contraceptive useNever11,277 (18)871.00
      Ever51,964 (81)901.02 (0.93–1.12)
      HRT useNever44,261 (69)901.00
      Ever18,674 (29)870.93 (0.87–1.01)
      Number of children16908 (11)791.00
      226,371 (41)881.90 (1.73–2.09)
      P < .01.
      319,078 (30)922.94 (2.65–3.27)
      P < .01.
      4+11,842 (18)923.66 (3.24–4.13)
      P < .01.
      BF, breastfed; BMI, body mass index; CI, confidence interval; HBP, high blood pressure; HRT, hormone replacement therapy; OR, odds ratio.
      bThe percentage of women who responded yes to having breastfed;
      Lupton. Parity, breastfeeding, and high blood pressure. Am J Obstet Gynecol 2013.
      a Percentages do not consistently total to 100% because of missing values;
      c Analysis adjusted for current age, country of origin, income level, family history of HBP, BMI, smoking status, alcohol consumption, physical activity, oral contraceptive use, HRT use, and number of children;
      d P < .01.

      Results

      A total of 74,785 women aged 45 and over were included in the analysis of which 64,199 gave birth (85.8%), and 12,456 (16.7%) reported current treatment for high blood pressure. Of the women who had given birth, 57,097 (88.9%) reported they had breastfed, with the youngest women in the cohort (45 to <54 year olds) having the highest prevalence of breastfeeding (92.7%) (Figure 1).
      Demographic and lifestyle characteristics of women who had given birth are shown in Table 1. Women who were born in Australia, had an income greater than $30,000, consumed greater than 1 alcoholic drink per week, never smoked, had sufficient physical activity levels and had a current body mass index (BMI) of less than 25, had higher odds of having breastfed. Increasing number of children was also associated with increased odds of having breastfed.
      There was an association between parity and high blood pressure, with women who had given birth having significantly lower odds of high blood pressure when compared with women who had never given birth (adjusted OR, 0.91; 99% CI, 0.84–0.99; P = .004). Further analysis, stratifying mothers according to whether they had breastfed, found mothers who breastfed had significantly lower odds of having high blood pressure, compared with women who had never given birth (adjusted OR, 0.89; 99% CI, 0.82–0.97; P < .001), whereas the odds of having high blood pressure for mothers who did not breastfeed were not significantly different from women who had never given birth (adjusted OR, 1.06; 99% CI, 0.95–1.18; P = .195) (Table 2).
      TABLE 2Parity, breastfeeding history, and the odds of having high blood pressure
      VariableCases, nCrude OR (99% CI)Adjusted OR (99% CI)
      Analysis adjusted for current age, country of origin, income level, body mass index, smoking status, alcohol consumption, physical activity, family history of high blood pressure, history of oral contraceptive use, and history of hormone replacement therapy use;
      Given birth
       No10,5861.00 (reference)
       Yes64,1991.00 (0.93–1.07)0.91 (0.84–0.99)
      P < .01.
      Given birth stratified by breastfeeding
       No10,5861.00 (reference)
       Yes, never breastfed71021.31 (1.18–1.45)
      P < .01.
      1.06 (0.95–1.18)
       Yes, breastfed57,0970.96 (0.89–1.03)0.89 (0.82–0.97)
      P < .01.
      CI, confidence interval; OR, odds rstio.
      Lupton. Parity, breastfeeding, and high blood pressure. Am J Obstet Gynecol 2013.
      a Analysis adjusted for current age, country of origin, income level, body mass index, smoking status, alcohol consumption, physical activity, family history of high blood pressure, history of oral contraceptive use, and history of hormone replacement therapy use;
      b P < .01.
      Analysis including only women who had given birth, stratified by current age, found no association between the number of children a woman gave birth to and having high blood pressure. There was a significant association between lifetime breastfeeding duration and having high blood pressure. Lifetime breastfeeding duration of 6 months or greater was associated with lower odds of having high blood pressure in women aged 45 to <64 years, when compared with parous women who did not breastfeed. The odds of having high blood pressure decreased with longer durations of breastfeeding. The lowest odds were observed in women aged 45 to <54 years whose lifetime breastfeeding duration was 24 months or greater (adjusted OR, 0.58; 99% CI, 0.44–0.77; P < .001). No significant association between lifetime breastfeeding duration and high blood pressure was observed in women aged 64 years and over (Figure 2).
      Figure thumbnail gr2
      FIGURE 2Association of lifetime breastfeeding with high blood pressure
      A, 45 to <54 years of age, B, 54 to <64 years of age, C, 64 or more years of age.
      Lupton. Parity, breastfeeding, and high blood pressure. Am J Obstet Gynecol 2013.
      The average breastfeeding duration per child was also associated with having high blood pressure, with 3 months or more breastfeeding per child associated with lower odds of having high blood pressure, in women aged 45 to <64 years, when compared with parous women who never breastfed. The odds of having high blood pressure within these women decreased with longer durations of breastfeeding per child. Within the 64 years and over age group, there was a significant association between average breastfeeding duration per child and having high blood pressure, only for women who had breastfed for 18 months or more per child (adjusted OR, 0.38; 99% CI, 0.17–0.84; P = .002), compared with parous women who did not breastfeed (Figure 3).
      Figure thumbnail gr3
      FIGURE 3Breastfeeding per child associated with high blood pressure
      A, 45 to <54 years of age, B, 54 to <64 years of age, C, 64 or more years of age.
      Lupton. Parity, breastfeeding, and high blood pressure. Am J Obstet Gynecol 2013.
      A final analysis including only participants with a family history of high blood pressure, stratified by age, found that breastfeeding is significantly associated with reduced odds of having high blood pressure in women aged 45 to 64 years (Table 3).
      TABLE 3Breastfeeding history and high blood pressure among women with reported family history of blood pressure, stratified by age
      Age group, yCases, nCrude OR (99% CI)Adjusted OR (99% CI)
      Odds ratios refer to the odds of high blood pressure among breastfeeding compared with nonbreastfeeding women, adjusted for country of origin, income level, body mass index, smoking status, alcohol consumption, physical activity, history of oral contraceptive use, history of hormone replacement therapy use, and number of children;
      45 to <5412,7300.59 (0.46–0.76)
      P < .01.
      0.68 (0.52–0.89)
      P < .01.
      54 to <6411,7210.69 (0.59–0.82)
      P < .01.
      0.80 (0.67–0.95)
      P < .01.
      64+86501.00 (0.84–1.18)1.00 (0.84–1.18)
      CI, confidence interval; OR, odds ratio.
      Lupton. Parity, breastfeeding, and high blood pressure. Am J Obstet Gynecol 2013.
      a Odds ratios refer to the odds of high blood pressure among breastfeeding compared with nonbreastfeeding women, adjusted for country of origin, income level, body mass index, smoking status, alcohol consumption, physical activity, history of oral contraceptive use, history of hormone replacement therapy use, and number of children;
      b P < .01.

      Comment

      This study showed the combination of giving birth and breastfeeding was associated with lower odds of having high blood pressure in later life, compared with women who had never given birth. There was no significant difference in odds of having high blood pressure in women who had given birth and did not breastfeed, compared with women who had never given birth. We also showed that within parous women, the number of children a woman gave birth to was not associated with her odds of having high blood pressure. In addition, 6 months or more lifetime breastfeeding duration, and 3 months or more average breastfeeding per child, were associated with significantly lower odds of having high blood pressure in later life. The association between breastfeeding duration and high blood pressure diminished with increasing age, demonstrating age as a significant predictor for having high blood pressure.
      Previous studies examining the association between parity and high blood pressure have been inconclusive, with some studies showing no association
      • Lawlor D.A.
      • Emberson J.R.
      • Ebrahim S.
      • et al.
      Is the association between parity and coronary heart disease due to biological effects of pregnancy or adverse lifestyle risk factors associated with child-rearing? Findings from the British Women's Heart and Health Study and the British Regional Heart Study.
      • Ness R.B.
      • Harris T.
      • Cobb J.
      • et al.
      Number of pregnancies and the subsequent risk of cardiovascular disease.
      and others showing decreased odds of having high blood pressure with increasing numbers of pregnancies.
      • Ness R.B.
      • Kramer R.A.
      • Flegal K.M.
      Gravidity, blood pressure, and hypertension among white women in the Second National Health and Nutrition Examination Survey.
      Our large cohort study, which included 74,785 women, showed that the number of children a woman gave birth to was not associated with having high blood pressure. Previous studies did not account for breastfeeding duration among the mothers when examining the association between parity and high blood pressure and this may explain the inconsistent results between studies, because of varying rates of breastfeeding in different populations. Our study had a breastfeeding rate of 88.9% with women born outside Australia having lower odds of having breastfed (adjusted OR, 0.78; 99% CI, 0.72–0.84; P < .001) compared with women born in Australia. Only parous women who breastfed had significantly lower odds of having high blood pressure, when compared with nulliparous women. The reduced odds of having high blood pressure in these women are therefore more likely to be a result of breastfeeding, rather than parity alone.
      Breastfeeding may modify the risk of having high blood pressure through alterations in vessel structure or in the maternal hormonal and lipid profiles. These alterations may induce changes in the maternal cardiovascular system because of the repeated oxytocin exposure produced by each feeding.
      • Uvnas-Moberg K.
      Oxytocin may mediate the benefits of positive social interaction and emotions.
      Exposure to oxytocin has an inhibitory effect on adrenocorticotropic hormones and cortisol secretion, reducing sympathetic nervous system stimulation and reducing blood pressure.
      • Amico J.A.
      • Johnston J.M.
      • Vagnucci A.H.
      Suckling-induced attenuation of plasma cortisol concentrations in postpartum lactating women.
      Within rats there is evidence that repeated oxytocin exposure affects central α2-adrenergic activity, decreasing sympathetic nervous system drive on both the heart and arteries, lowering blood pressure.
      • Petersson M.
      • Alster P.
      • Lundeberg T.
      • Uvnas-Moberg K.
      Oxytocin causes a long-term decrease of blood pressure in female and male rats.
      The positive social interactions and emotions induced in the mother by breastfeeding, including the touching sensation and warmth created by the offspring, may also have long-term benefits to the mother's cardiovascular health because of the psychologic antistress effects produced by breastfeeding.
      • Uvnas-Moberg K.
      Oxytocin may mediate the benefits of positive social interaction and emotions.
      The relationship between breastfeeding duration and blood pressure has been reported in 4 large cohort studies to date.
      • Lee S.Y.
      • Kim M.T.
      • Jee S.H.
      • Yang H.P.
      Does long-term lactation protect premenopausal women against hypertension risk? A Korean women's cohort study.
      • Ram K.T.
      • Bobby P.
      • Hailpern S.M.
      • et al.
      Duration of lactation is associated with lower prevalence of the metabolic syndrome in midlife—SWAN, the study of women's health across the nation.
      • Schwarz E.B.
      • Ray R.M.
      • Stuebe A.M.
      • et al.
      Duration of lactation and risk factors for maternal cardiovascular disease.
      • Stuebe A.M.
      • Schwarz E.B.
      • Grewen K.
      • et al.
      Duration of lactation and incidence of maternal hypertension: a longitudinal cohort study.
      All studies have found an association between breastfeeding and maternal high blood pressure, with increased durations of breastfeeding associated with reduced likelihood of having high blood pressure. The WHI study was the only study to include women over 60 years of age,
      • Schwarz E.B.
      • Ray R.M.
      • Stuebe A.M.
      • et al.
      Duration of lactation and risk factors for maternal cardiovascular disease.
      and reported that the association between increased duration of breastfeeding and reduced cardiovascular risk diminished as women aged. The WHI study did not specifically examine the interaction between age and high blood pressure. Our study supports the above mentioned studies, and extends the research by showing that the association between breastfeeding duration (both lifetime and per child) and having high blood pressure is only significant in women up to 64 years of age. The 1 exception being women aged 64 years and over had significantly lower odds of having high blood pressure if they breastfed each child for an average of 18 months or more (OR, 0.38; 99% CI, 0.17–0.84; P = .002), compared with parous women of the same age who never breastfed. The per child breastfeeding duration is reported in the present paper to make the results more accessible for physicians and midwifes when discussing the benefits of breastfeeding to a mother after a single pregnancy. A recommendation of at least 3 months of breastfeeding per child is advocated by the authors and is more meaningful to a mother than a recommendation about how long she should breastfeed in her lifetime.
      A family history of high blood pressure is known to increase an individual's risk of having high blood pressure, possibly because of the inheritance of a number of genetic polymorphisms that make the individual susceptible to high blood pressure.
      • Lupton S.J.
      • Chiu C.L.
      • Lind J.M.
      A hypertension gene: are we there yet?.
      We showed that breastfeeding significantly reduces the odds of having high blood pressure in women with a family history of high blood pressure (Table 3), up to 64 years of age. Women who have a family history of high blood pressure could be encouraged to breastfeed their children to help delay the onset of high blood pressure.
      Ageing is the single largest risk factor for high blood pressure because of the stiffening of elastane in the internal lamina of arteries and calcification of the arterial intima.
      • Mitchell G.F.
      • Parise H.
      • Benjamin E.J.
      • et al.
      Changes in arterial stiffness and wave reflection with advancing age in healthy men and women: the Framingham Heart Study.
      Both of these factors cause an increase in peripheral resistance, leading to an increase in the pressure wave reflection and a rise in systolic blood pressure as individuals age.
      • Mitchell G.F.
      • Parise H.
      • Benjamin E.J.
      • et al.
      Changes in arterial stiffness and wave reflection with advancing age in healthy men and women: the Framingham Heart Study.
      • O'Rourke M.F.
      • Nichols W.W.
      Aortic diameter, aortic stiffness, and wave reflection increase with age and isolated systolic hypertension.
      Our results show that the benefits afforded to the cardiovascular system by breastfeeding are no longer present in the majority of older women.
      The current WHO recommendations for breastfeeding encourage breastfeeding for a minimum of 6 months per child.

      Feig C. Exclusive breastfeeding for six months best for babies everywhere. World Health Organisation Online 2011.

      This study provides further support for these recommendations, as both the lifetime and per child durations of breastfeeding were associated with significant reduction in the odds of future maternal high blood pressure. Our study showed that 3 months or more of breastfeeding per child, and 6 months or more lifetime breastfeeding, significantly decreased a woman's odds of having high blood pressure, and the odds continued to lower with longer breastfeeding durations. Women should therefore be encouraged to breastfeed for as long as possible to reduce the likelihood of high blood pressure before 64 years of age. A woman's breastfeeding history is also important when assessing her likelihood of having high blood pressure in later life.
      The key strength of this study is the large cohort size, which enabled us to examine the duration of breastfeeding in different age groups. This study used self-reported data, which is prone to recall bias, and may have led to under- or overreporting of breastfeeding duration. It has been reported in the literature that women who breastfeed are more likely to have a healthy BMI
      • Lepe M.
      • Bacardi Gascon M.
      • Castaneda-Gonzalez L.M.
      • Perez Morales M.E.
      • Jimenez Cruz A.
      Effect of maternal obesity on lactation: systematic review.
      and are less likely to smoke.
      • Gerd A.T.
      • Bergman S.
      • Dahlgren J.
      • Roswall J.
      • Alm B.
      Factors associated with discontinuation of breastfeeding before 1 month of age.
      Our study showed that women who currently had a lower BMI, had never smoked, had sufficient levels of exercise, and had a higher income were more likely to have breastfed. Women who breastfeed may therefore represent a healthier cohort. From our data it is not possible to determine whether women who breastfed were a healthier cohort during their childbearing years, or whether breastfeeding results in healthier behaviors beyond a woman's childbearing years. Either way, there is no difference in odds of having high blood pressure in women over 64 years of age when comparing women who breastfed with women who never breastfed.
      In conclusion, women who breastfed their children had significantly reduced odds of having high blood pressure, compared with women who had never given birth and varying rates of breastfeeding in different populations may account for inconsistent reporting of the association between parity and high blood pressure. Women who breastfed for longer than 6 months in their lifetime, or greater than 3 months per child, had significantly lower odds of having high blood pressure when compared with parous women who had never breastfed. This held true in women with a family history of high blood pressure. The odds were lower with longer breastfeeding durations and the association diminished in women over 64 years of age. Women should be encouraged to breastfeed for as long as possible, and a woman's breastfeeding history should be taken into account when assessing her likelihood of high blood pressure in later life.

      Acknowledgments

      The 45 and Up Study is managed by the Sax Institute in collaboration with major partner Cancer Council New South Wales; and partners the National Heart Foundation of Australia (NSW Division); NSW Ministry of Health; beyondblue: the national depression initiative; Ageing, Disability and Home Care, NSW Family and Community Services; and Australian Red Cross Blood Service. We thank the many thousands of people participating in the 45 and Up Study. J.M.L. is supported by a National Health and Medical Research Council - Australian Biomedical Fellowship. S.J.L. is the recipient of a University of Western Sydney Postgraduate Research Award and an Ingham Health Research Institute scholarship.

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