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Ethnicity-specific birthweight distributions improve identification of term newborns at risk for short-term morbidity

      Objective

      We aimed to determine whether ethnicity-specific birthweight distributions more accurately identify newborns at risk for short-term neonatal morbidity associated with small for gestational age (SGA) birth than population-based distributions not stratified on ethnicity.

      Study Design

      We examined 100,463 singleton term infants born to parents in Washington State between Jan. 1, 2006, and Dec. 31, 2008. Using multivariable logistic regression models, we compared the ability of an ethnicity-specific growth distribution and a population-based growth distribution to predict which infants were at increased risk for Apgar score <7 at 5 minutes, admission to the neonatal intensive care unit, ventilation, extended length of stay in hospital, hypothermia, hypoglycemia, and infection.

      Results

      Newborns considered SGA by ethnicity-specific weight distributions had the highest rates of each of the adverse outcomes assessed—more than double those of infants only considered SGA by the population-based standards. When controlling for mother's age, parity, body mass index, education, gestational age, mode of delivery, and marital status, newborns considered SGA by ethnicity-specific birthweight distributions were between 2 and 7 times more likely to suffer from the adverse outcomes listed above than infants who were not SGA. In contrast, newborns considered SGA by population-based birthweight distributions alone were at no higher risk of any adverse outcome except hypothermia (adjusted odds ratio, 2.76; 95% confidence interval, 1.68–4.55) and neonatal intensive care unit admission (adjusted odds ratio, 1.40; 95% confidence interval, 1.18–1.67).

      Conclusion

      Ethnicity-specific birthweight distributions were significantly better at identifying the infants at higher risk of short-term neonatal morbidity, suggesting that their use could save resources and unnecessary parental anxiety.

      Key words

      For Editors’ Commentary, see Contents
      See related editorial, page 397
      Conventionally, small for gestational age (SGA) birthweight has been defined as below the lowest 10th percentile of the population adjusted for gestational age.
      • Kierans W.J.
      • Kramer M.S.
      • Wilkins R.
      • et al.
      Charting bith outcome in British Columbia: determinants of optimal health and ultimate risk—An expansion and update.
      • Beeby P.
      • Bhutap T.
      • Taylor L.
      New South Wales population-based birthweight percentile charts.
      • Ferdynus C.
      • Quantin C.
      • Abrahamowicz M.
      • et al.
      Can birth weight standards based on healthy populations improve the identification of small-for-gestational-age newborns at risk of adverse neonatal outcomes?.
      SGA can be a consequence of intrauterine growth restriction, defined as reduced growth during fetal life relative to the genetic growth potential of the fetus. However, infants classified as SGA may be constitutionally small but healthy as opposed to growth restricted. A number of factors influence birthweight, including infant sex, gestational age at delivery and maternal variables such as parity, ethnicity, prepregnancy weight and height.
      • Ego A.
      • Subtil D.
      • Grange G.
      • et al.
      Customized versus population-based birth weight standards for identifying growth restricted infants: a French multicenter study.
      • Gardosi J.
      Clinical strategies for improving the detection of fetal growth restriction.
      • Gardosi J.
      Customized fetal growth standards: rationale and clinical application.
      • Gardosi J.
      • Clausson B.
      • Francis A.
      The value of customised centiles in assessing perinatal mortality risk associated with parity and maternal size.
      Two recent Canadian studies have focused on the role of ethnicity in fetal growth. The first suggests that birthweight distributions need to be modified for newborns of immigrant mothers
      • Ray J.G.
      • Sgro M.
      • Mamdani M.
      • et al.
      Birth weight curves tailored to maternal world region.
      and the second indicates that use of ethnicity-specific birthweight distributions improves differentiation between infants who are pathologically small, and thus at risk of adverse neonatal outcomes, from those who are constitutionally small but healthy.
      • Hanley G.E.
      • Janssen P.A.
      Ethnicity-specific growth distributions for prediction of newborn morbidity.
      In 2007, Janssen et al
      • Janssen P.A.
      • Thiessen P.
      • Klein M.C.
      • Whitfield M.F.
      • MacNab Y.C.
      • Cullis-Kuhl S.C.
      Standards for the measurement of birth weight, length and head circumference at term in neonates of European, Chinese and South Asian ancestry.
      reported significant differences in weight, length, and head circumference of infants at birth according to whether their parents were of European, Chinese or South Asian descent. They developed new birthweight distributions for singleton term infants born to healthy mothers of European, Chinese, and South Asian descent. In a subsequent article,
      • Hanley G.E.
      • Janssen P.A.
      Ethnicity-specific growth distributions for prediction of newborn morbidity.
      we demonstrated that these ethnicity-specific birthweight distributions predicted neonatal morbidity more often than the population-based birthweight distributions in British Columbia.
      • Kierans W.J.
      • Kramer M.S.
      • Wilkins R.
      • et al.
      Charting bith outcome in British Columbia: determinants of optimal health and ultimate risk—An expansion and update.
      This study was based on 2647 infants born in British Columbia. In the current study, we examine infants born in Washington State between 2006 and 2008 to determine whether the ethnicity-specific birthweight distributions developed by Janssen et al
      • Janssen P.A.
      • Thiessen P.
      • Klein M.C.
      • Whitfield M.F.
      • MacNab Y.C.
      • Cullis-Kuhl S.C.
      Standards for the measurement of birth weight, length and head circumference at term in neonates of European, Chinese and South Asian ancestry.
      improve the identification of fetuses at risk for neonatal morbidity compared with a population-based growth distribution that is not stratified on ethnicity developed by Kierans.
      • Kierans W.J.
      • Kramer M.S.
      • Wilkins R.
      • et al.
      Charting bith outcome in British Columbia: determinants of optimal health and ultimate risk—An expansion and update.

      Materials and Methods

      This study uses a large retrospective cohort consisting of all births in Washington State between 2006 and 2008 (the most recent data available at the time of our request). The Washington Birth Certificate data contains information collected in hospital and birth centers about the mother, the father, the pregnancy, and the infant. These data were linked to the Comprehensive Hospital Abstract Reporting System (CHARS), an inpatient discharge database that includes hospitalization information from all Washington State hospitals.
      To test the Janssen ethnicity-specific growth distribution, we restricted our sample to singleton infants between 37 and 41 weeks' gestational age born to parents who both identified their ethnicity as white, Chinese, or South Asian. We were restricted to these ethnicity groups as the Janssen ethnicity-specific standards were developed in a hospital in British Columbia, Canada, where 50% of delivering women are white, approximately 40% are Chinese, and approximately 10% are South Asian. We excluded mothers with known exposure to alcohol or tobacco during pregnancy, diabetes (preexisting or gestational), or hypertensive disorders to minimize confounding by these factors.
      We classified newborns into 4 categories; SGA according to the ethnicity-specific distributions developed by Janssen et al
      • Janssen P.A.
      • Thiessen P.
      • Klein M.C.
      • Whitfield M.F.
      • MacNab Y.C.
      • Cullis-Kuhl S.C.
      Standards for the measurement of birth weight, length and head circumference at term in neonates of European, Chinese and South Asian ancestry.
      (SGA-J ethnicity specific); SGA according to the Kierans population-based charts (SGA-K population-based); meeting neither criterion (not SGA); and, SGA according to the SGA-K designation but not the Janssen distribution (SGA-K-only population-based). SGA-K-only differs from SGA-K in that the infants who were also identified as SGA by Janssen's ethnicity-specific standards (n = 2277) were removed from this group. There were no SGA-J infants that were not also considered SGA-K. The Kierans population-based charts are stratified by infant sex and provide birthweight and head circumferences for percentiles. The Janssen ethnicity-specific distribution is stratified by sex and ethnicity and refers to weight, length, and head circumference. We present results examining infants who are considered SGA according to their birthweight (less than the 10th percentile). We also performed a sensitivity analysis using the 5th percentile of the Kierans population-based distribution to classify infants as SGA-K.
      Our primary outcomes were Apgar score <7 at 5 minutes, admission to the neonatal intensive care unit (NICU), ventilation using endotracheal intubation postdelivery (yes, no), ventilation for more than 6 hours, and extended length of stay in hospital ( ≥3 days for vaginal delivery and ≥4 days for cesarean delivery). Using International Classification of Diseases, Ninth Revision (ICD-9) diagnostic codes, we identified infants who suffered from hypothermia (ICD-9 778.3), hypoglycemia (ICD-9 775.6), and infection (ICD-9 771). We were unable to report infant death, hypoxic ischemic encephalopathy, and intraventicular hemorrhage as our data agreement with the Washington Department of Health states that we cannot release data with less than 5 individuals per cell size for privacy reasons. We calculated crude and adjusted odds ratios (aORs) and their 95% confidence intervals (CIs) to estimate risk of outcomes. We adjusted for maternal age, parity, education level, body mass index (BMI), gestational age, mode of delivery (cesarean section vs vaginal), labor induction, and marital status. All analyses were performed using Stata version 11 software (StataCorp, College Station, TX). Women with missing data were excluded from the adjusted logistic regression analyses. We obtained our data from the Washington State Department of Health. Institutional ethics approval was not required because these data are deidentified and publicly available on request.

      Results

      We started with a sample of 198,439 women who delivered in Washington State between 2006 and 2008. We excluded 9105 (4.6%) mothers because they gave birth to multiples; 43,395 (22%) mothers gave birth preterm or postterm; 12,268 (6.2%) had diabetes; 10,611 (5.3%) had hypertension and 22,597 (11.4%) smoked during pregnancy. Of our final sample of 100,463 included infants, 93,689 were not considered SGA by the population-based or the ethnicity-specific distributions (not SGA), 6774 were defined as SGA by the Kierans population-based distributions (SGA-K) among which 2277 were also defined as SGA by the Janssen ethnicity-specific distribution (SGA-J), thus there were 4497 newborns identified as SGA by Kierans distribution alone (SGA-K-only). Janssen's ethnicity-specific distribution did not identify any SGA infants in addition to those classified as SGA by Kierans distribution. We also compared infants identified as SGA by the 5th percentile of the Kierans distribution to the Janssen's ethnicity specific distribution, and our results did not meaningfully change, thus we present the 10th percentile here as this is more clinically relevant.
      Although maternal groups were statistically significantly different with respect to parity, age, BMI, and mode of delivery, the absolute differences between groups were small. Mothers of SGA groups were similar in terms of marital status, but mothers of not SGA newborns were less likely to be unmarried (P < .001). Groups were not significantly different with respect to maternal education and induction of labor. On average, SGA-J infants weighed less than SGA-K-only infants (2537.1 g vs 2825.1 g, P < .001) and both groups were on average lighter than the not SGA group (P < .001) (Table 1). Head circumference was also significantly different between groups (P < .001). SGA-K-only infants were more likely to be Chinese or South Asian (12.2%) compared with SGA-J infants (4.8%) (P < .001). More SGA-K-only infants were female compared with SGA-J infants (51.6% vs 38.4% respectively, P < .001).
      Table 1Infant and maternal characteristics by SGA group
      CharacteristicNot SGA n = 93,689SGA-K-only (population-based) n = 4497SGA-J (ethnicity-specific) n = 2277
      All SGA-J (ethnicity-specific) newborns were also considered SGA by population-based standards.
      P value
      MATERNAL
      Parity
       037,763 (40.3)2513 (55.9)1312 (57.6)
       132,240 (34.4)1190 (26.5)523 (23.0)
       ≥222,523 (24.0)724 (16.1)411 (19.4)< .001
       Missing1163 (1.2)70 (1.6)31 (1.4)
      Mother's age, y28.9 (5.6)28.3 (5.7)27.9 (6.0)< .001
      BMI
       <208759 (9.4)666 (14.8)304 (13.4)
       20-2540,384 (43.1)2102 (46.7)1066 (46.8)
       25-3021,315 (23.8)874 (19.4)452 (9.9)
       >3015,831 (16.9)525 (11.7)301 (13.2)< .001
       Missing6400 (6.8)330 (7.3)154 (6.8)
      Mother's education
       High School or less17,682 (18.9)875 (19.5)507 (22.3)
       Some postsecondary31,356 (33.5)1397 (31.5)718 (31.5)
       Postsecondary9915 (10.6)552 (12.3)211 (9.3)
       Postgraduate2583 (2.8)137 (3.0)50 (2.2)
       Missing304 (0.3)15 (0.3)5 (0.2).061
      Marriage status
       Not married14,276 (15.2)860 (19.1)479 (21.0)< .001
       Missing30 (0.03)0 (0.0)0 (0.0)
      Mode of delivery
       Cesarean section24,158 (25.8)1097 (24.4)634 (27.8)
       Vaginal delivery69,531 (74.2)3400 (75.6)1643 (72.2).008
      Labor induction20,339 (21.7)992 (22.1)536 (23.5).100
      INFANT
      Size
       Mean weight, g (SD)3577.1 (410.4)2825.1 (178.5)2537.1 (349.9)< .001
       Mean head, cm (SD)34.5 (1.5)33.1 (1.4)32.6 (1.7)< .001
      Ethnicity
       White90,292 (96.4)3949 (87.8)2168 (95.2)
       Chinese1296 (1.4)147 (3.3)24 (1.1)
       South Asian2101 (2.2)401 (8.9)85 (3.7)< .001
      Sex
       Male47,726 (50.9)2177 (48.4)1402 (61.6)
       Female45,963 (49.1)2320 (51.6)875 (38.4)< .001
      Mean gestational age, wks (SD)39.2 (1.1)39.4 (1.1)39.5 (1.3)< .001
      BMI, body mass index; SGA, small for gestational age; SD, standard deviation.
      Hanley. Ethnicity-specific birthweight distributions improve identification of SGA infants. Am J Obstet Gynecol 2013.
      a All SGA-J (ethnicity-specific) newborns were also considered SGA by population-based standards.
      SGA-J infants had the highest rates of each of the adverse outcomes assessed—more than double those of infants considered SGA-K-only except for ventilation required after delivery (Table 2). Rates of NICU admission and hypoglycemia, occurred more than 3 times as often in SGA-J infants compared with SGA-K-only infants. Rates of adverse outcomes in the SGA-K-only infants were similar to the rates of the not SGA newborns.
      Table 2Adverse neonatal outcomes by SGA group
      VariableNot SGA n = 93,689SGA-K-only (population-based) n = 4497SGA-J (ethnicity-specific) n = 2277
      All SGA-J (ethnicity-specific) newborns were also considered SGA by population-based standards
      5 min Apgar <71344 (1.4)75 (1.7)84 (3.7)
      Infection498 (0.5)36 (0.8)40 (1.8)
      Hypoglycemia1004 (1.1)55 (1.2)112 (4.9)
      Hypothermia126 (0.1)18 (0.4)25 (1.1)
      NICU admission2124 (2.3)151 (3.4)255 (11.2)
      Extended length of stay
      Extended length of stay refers to stays of ≥3 days for vaginal delivery and ≥4 days for cesarean delivery.
      12,249 (13.1)627 (13.9)597 (26.2)
      Ventilation required postdelivery2678 (2.9)143 (3.2)127 (5.6)
      Ventilation needed for >6 h291 (0.3)27 (0.6)45 (2.0)
      NICU, neonatal intensive care unit; SGA, small for gestational age.
      Hanley. Ethnicity-specific birthweight distributions improve identification of SGA infants. Am J Obstet Gynecol 2013.
      a All SGA-J (ethnicity-specific) newborns were also considered SGA by population-based standards
      b Extended length of stay refers to stays of ≥3 days for vaginal delivery and ≥4 days for cesarean delivery.
      In a multivariable logistic regression analysis, odds of all adverse outcomes, adjusted for maternal age, parity, prepregnancy BMI, education, mode of delivery, gestational age, labor induction, and marital status were significantly elevated for infants who were considered SGA-J (Table 3). The aORs of a 5 minute Apgar score <7, need for ventilation, and extended length of stay were approximately 2 times higher for SGA-J infants than infants who were not SGA; (aOR, 2.18; 95% CI, 1.69–2.79; aOR, 1.88; 95% CI, 1.55–2.27, and aOR, 2.28; 95% CI, 2.06–2.52, respectively). The odds of infection were 3 times higher among SGA-J infants (aOR, 3.04; 95% CI, 2.17–4.25) and nearly 5 times higher for hypoglycemia (aOR, 4.71; 95% CI, 3.81–5.81). The odds of NICU admission, ventilation for more than 6 hours and hypothermia were more than 5 times higher in SGA-J infants than in infants who were not SGA (aOR, 5.05; 95% CI, 4.37–5.83; aOR, 5.83; 95% CI, 4.14–8.20, and aOR, 6.52; 95% CI, 4.10–10.36, respectively).
      Table 3Unadjusted and adjusted ORs in each SGA group for each adverse infant outcome with 95% CI
      VariableNot SGA n = 93,689SGA-K-only (population-based) n = 4497SGA-K (population-based) n = 6774
      SGA-K infants includes the 4497 who were identified as SGA only by the Kierans population-based distribution and the 2277 who were identified as SGA by both the Kierans population-based distribution and the Janssen ethnicity-specific distribution
      SGA-J (ethnicity-specific) n = 2277
      All SGA-J (ethnicity-specific) newborns were also considered SGA by population-based standards
      Unadjusted ORs
       5 min Apgar <71.00 (ref)1.12 (0.89–1.42)1.65 (1.39–1.95)
      Statistically significantly at 95% CI
      2.68 (2.12–2.33)
      Statistically significantly at 95% CI
       Infection1.00 (ref)1.41 (1.00–1.70)2.12 (1.67–2.71)
      Statistically significantly at 95% CI
      3.36 (2.42–4.65)
      Statistically significantly at 95% CI
       Hypoglycemia1.00 (ref)1.05 (0.80–1.38)2.33 (1.98–2.75)
      Statistically significantly at 95% CI
      4.70 (3.85–5.74)
      Statistically significantly at 95% CI
       Hypothermia1.00 (ref)2.55 (1.56–4.16)
      Statistically significantly at 95% CI
      4.74 (3.35–6.71)
      Statistically significantly at 95% CI
      8.45 (5.48–13.03)
      Statistically significantly at 95% CI
       NICU admission1.00 (ref)1.37 (1.15–1.62)
      Statistically significantly at 95% CI
      2.75 (2.46–3.07)
      Statistically significantly at 95% CI
      5.44 (4.74–6.24)
      Statistically significantly at 95% CI
       Extended length of stay
      Extended length of stay refers to stays of ≥3 days for vaginal delivery and ≥4 days for cesarean delivery.
      1.00 (ref)1.05 (0.96–1.14)1.47 (1.37–1.56)
      Statistically significantly at 95% CI
      2.36 (2.15–2.59)
      Statistically significantly at 95% CI
       Ventilation required postdelivery1.00 (ref)1.09 (0.92–1.30)1.41 (1.24–1.60)
      Statistically significantly at 95% CI
      1.99 (1.66–2.39)
      Statistically significantly at 95% CI
       Ventilation needed for more than 6 h1.00 (ref)1.30 (0.95–1.80)3.45 (2.66–4.47)
      Statistically significantly at 95% CI
      6.40 (4.66–8.79)
      Statistically significantly at 95% CI
      Adjusted ORs
      n = 86,384n = 4116n = 6224n = 2108
       5 min Apgar <71.00 (ref)1.09 (0.85–1.40)1.49 (1.24–1.79)
      Statistically significantly at 95% CI
      2.18 (1.69–2.79)
      Statistically significantly at 95% CI
       Infection1.00 (ref)1.34 (0.93–1.93)2.03 (1.58–2.62)
      Statistically significantly at 95% CI
      3.04 (2.17–4.25)
      Statistically significantly at 95% CI
       Hypoglycemia1.00 (ref)1.12 (0.84–1.48)2.44 (2.05–2.90)
      Statistically significantly at 95% CI
      4.71 (3.81–5.81)
      Statistically significantly at 95% CI
       Hypothermia1.00 (ref)2.76 (1.68–4.55)
      Statistically significantly at 95% CI
      4.67 (3.24–6.75)
      Statistically significantly at 95% CI
      6.52 (4.10–10.36)
      Statistically significantly at 95% CI
       NICU admission1.00 (ref)1.40 (1.18–1.67)
      Statistically significantly at 95% CI
      2.77 (2.47–3.10)
      Statistically significantly at 95% CI
      5.05 (4.37–5.83)
      Statistically significantly at 95% CI
       Extended length of stay
      Extended length of stay refers to stays of ≥3 days for vaginal delivery and ≥4 days for cesarean delivery.
      1.00 (ref)1.03 (0.94–1.14)1.44 (1.35–1.55)
      Statistically significantly at 95% CI
      2.28 (2.06–2.52)
      Statistically significantly at 95% CI
       Ventilation required postdelivery1.00 (ref)1.04 (0.87–1.25)1.35 (1.18–1.55)
      Statistically significantly at 95% CI
      1.88 (1.55–2.27)
      Statistically significantly at 95% CI
       Ventilation needed for >6 h1.00 (ref)1.35 (0.97–1.83)3.51 (2.65–4.65)
      Statistically significantly at 95% CI
      5.83 (4.14–8.20)
      Statistically significantly at 95% CI
      Adjusted ORs are adjusted for mother's age, parity, body mass index, education, gestational age, mode of delivery, labor induction, and marital status.
      CI, confidence interval; NICU, neonatal intensive care unit; OR, odds ratio; ref, reference; SGA, small for gestational age.
      Hanley. Ethnicity-specific birthweight distributions improve identification of SGA infants. Am J Obstet Gynecol 2013.
      a SGA-K infants includes the 4497 who were identified as SGA only by the Kierans population-based distribution and the 2277 who were identified as SGA by both the Kierans population-based distribution and the Janssen ethnicity-specific distribution
      b All SGA-J (ethnicity-specific) newborns were also considered SGA by population-based standards
      c Statistically significantly at 95% CI
      d Extended length of stay refers to stays of ≥3 days for vaginal delivery and ≥4 days for cesarean delivery.
      When examining all infants who were considered SGA by the population-based Kierans distribution, including those who are SGA-J, the odds ratios suggest that these infants are at higher risk for all adverse outcomes than infants who are not considered SGA (Table 3). However, when the SGA-J infants were excluded from this analysis, the infants who are only SGA by the Kierans standard were not at higher risk of any adverse outcomes except hypothermia and NICU admission (aOR, 2.76; 95% CI, 1.68–4.55 and aOR, 1.40; 95% CI, 1.18–1.67, respectively). Because the SGA-J infants were smaller, on average, than the SGA-K and SGA-K-only infants, we performed a sensitivity analysis using the 5th percentile of the Kierans population-based distribution to identify infants as SGA to determine whether our results were simply the result of identifying smaller infants. However, even when comparing SGA-J babies with the 5th percentile of the Kierans distribution the direction and magnitude of our odds ratios did not meaningfully change.

      Comment

      In our study, term infants born to healthy mothers identified as SGA on the basis of ethnicity-specific birthweight distributions were significantly more likely to have a 5 minute Apgar score <7, infection, hypoglycemia, hypothermia, NICU admission, extended length of stay in hospital, and ventilatory support than infants who were considered SGA by population-based nonethnicity-specific standards. These findings are consistent with previous work suggesting that ethnicity-specific birthweight distributions are better at predicting adverse infant outcomes, largely by eliminating the healthy but constitutionally small infants from the SGA category resulting in a smaller group of SGA infants who are more likely to suffer from short-term morbidity.
      • Ray J.G.
      • Sgro M.
      • Mamdani M.
      • et al.
      Birth weight curves tailored to maternal world region.
      • Hanley G.E.
      • Janssen P.A.
      Ethnicity-specific growth distributions for prediction of newborn morbidity.
      • Kierans W.
      • Joseph K.
      • Luo Z.C.
      • Platt R.
      • Wilkins R.
      • Kramer M.
      Does one size fit all? The case for ethnic-specific standards of fetal growth.
      Our results are also consistent with literature suggesting that customized population weight percentiles incorporating information on maternal weight, height, parity size, and ethnicity improve the associations between SGA and complications including low Apgar score, perinatal morbidity, admission to the NICU, stillbirths, and neonatal deaths.
      • Ego A.
      • Subtil D.
      • Grange G.
      • et al.
      Customized versus population-based birth weight standards for identifying growth restricted infants: a French multicenter study.
      • Gardosi J.
      • Clausson B.
      • Francis A.
      The value of customised centiles in assessing perinatal mortality risk associated with parity and maternal size.
      • Gardosi J.
      • Francis A.
      Adverse pregnancy outcome and association with small for gestational age birthweight by customized and population-based percentiles.
      • Clausson B.
      • Gardosi J.
      • Francis A.
      • Cnattingius S.
      Perinatal outcome in SGA births defined by customised versus population-based birthweight standards.
      • McCowan L.M.E.
      • Harding J.E.
      • Stewart A.W.
      Customised birthweight centiles predict SGA pregnancies with perinatal morbidity.
      However, not all researchers agree that the customized weight percentiles incorporating maternal characteristics improve the detection of SGA. Hutcheon et al
      • Hutcheon J.A.
      • Zhang X.
      • Platt R.W.
      • Cnattingius S.
      • Kramer M.S.
      The case against customized birthweight standards.
      suggest that the benefit of customized percentiles primarily results from the use of intrauterine standards to derive birthweights for gestational ages other than 280 days rather than using the weight of newborn infants at those gestational ages. Given that we examine term infants and use birthweights derived from newborn infants (not intrauterine standards), this cannot explain our findings that ethnicity-specific birthweight distributions were better at identifying newborns at risk for short-term morbidity. It remains to be tested whether customized percentiles incorporating all of the maternal information outlined above are superior when predicting adverse outcomes than ethnicity-specific weight distributions.
      Given our finding that all SGA-J infants were also considered SGA-K, to understand whether the SGA-K designation conveys some level of risk that is not captured by the SGA-J designation, we examined the infants identified as SGA by Kierans population-based weight distribution alone (SGA-K-only). Our results suggest that it did not. SGA-K-only infants were at significantly elevated risk for hypothermia and NICU admission only. This finding indicates that the higher odds of adverse outcomes in the entire SGA-K population compared with not SGA newborns were primarily driven by the SGA-J newborns. Our findings further suggest that the Kierans population-distribution misclassifies a large percentage of healthy but constitutionally small newborns as abnormal, in contrast to the Janssen ethnicity specific distribution that significantly more often identified infants at higher risk of neonatal morbidity.
      Our study is not without limitations. Data for age, parity, BMI, education, and marital status were missing for some patients, although less than 7% of mothers were missing data overall. Sensitivity analyses excluding the variables with missing data did not change our findings, suggesting that these missing data were unlikely to have biased our results. We were missing information to differentiate cesarean sections between those that were scheduled and those that were not, which may be important as many newborns thought to be at risk for adverse outcomes may be scheduled for cesarean delivery. However, the majority of elective cesarean sections are done for the indication of previous cesarean section. As we used ethnicity-specific weight distributions developed in British Columbia Canada, our ethnicity-specific distributions were stratified only on white, Chinese, and South Asian subgroups. Although these groups represent 60% of the newborns born in Washington State in the years 2006 to 2008, future work should develop birthweight distributions for newborns of additional indigenous and ethnic groups and should examine whether they are useful for newborns of mixed ethnicity. We were also unable to determine which babies were specifically tested for hypoglycemia and hypothermia as we relied on ICD codes for these diagnoses. The American Academy of Pediatrics guidelines recommend screening infants who are “at risk” for hypoglycemia, including infants who are SGA.
      • Adamkin D.H.
      Committee on fetus and newborn. Postnatal glucose homeostasis in late-preterm and term infants.
      This suggests that there was likely preferential testing for hypoglycemia among the smaller infants, suggesting that our results for this outcome could be biased. Finally, the Janssen ethnicity-specific distribution refers only to infants at term. Ethnicity-specific standards also need to be developed for preterm gestations.
      Our results add to the evidence suggesting that, when stratified by ethnicity, weight distributions more accurately identify newborns who are growth restricted, and at elevated risk for adverse outcomes.
      • Ferdynus C.
      • Quantin C.
      • Abrahamowicz M.
      • et al.
      Can birth weight standards based on healthy populations improve the identification of small-for-gestational-age newborns at risk of adverse neonatal outcomes?.
      • Figueras F.
      • Gardosi J.
      Should we customize fetal growth standards?.
      • Lyon V.
      • Howatson A.
      • Khan K.S.
      • Owen P.
      Unadjusted and customised weight centiles in the identification of growth restriction among stillborn infants.
      • Odibo A.O.
      • Francis A.
      • Cahill A.G.
      • Macones G.A.
      • Crane J.P.
      • Gardosi J.
      Association between pregnancy complications and small-for-gestational-age birth weight defined by customized fetal growth standard versus a population-based standard.
      Our study also suggests that population-based weight distributions not stratified on ethnicity misclassify a considerable number of healthy term newborns as SGA. Infants who are SGA are more closely monitored following birth, including continuous temperature monitoring and meticulous attention to infection control. They are also often provided with extra care, including nutrient and iron supplementation.
      • Carlo W.A.
      Chapter 91: the high risk infant.
      More accurate identification of SGA newborns who are at risk of short-term morbidity through use of the ethnicity-specific birthweight distributions may minimize monitoring and intervention in hospital for healthy newborns, importantly also minimizing anxiety on the part of parents.

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