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Activity restriction (AR) is commonly prescribed for preterm birth (PTB) prevention, based on an untested hypothesis of a direct correlation between activity and gestational age (GA) at delivery. Therefore, our objective was to determine if activity in pregnancy correlates with GA at delivery.
We conducted a prospective observational cohort study from October 2017 to July 2018 at a single institution to examine the association between steps/day and GA at delivery. We enrolled nulliparas between 10 0/7 and 23 6/7 wks without medical co-morbidities and measured activity using the Fitbit Flex 2. Subjects were not instructed to alter activity at enrollment. The primary exposure was steps/day, divided into low-(< 5000 steps/day) and high-activity groups (≥5000 steps/day). Our primary outcome was the rate of PTB< 37 wks in patients with low vs high activity. Secondary outcomes included peripartum complications, and median steps/day in the term vs preterm groups. Data on provider prescribed AR was also collected. Assuming women delivering at term take 5000±750 steps/day, the required sample size (a=0.05, 80% power) to detect a mean difference of 500 steps between term and preterm groups was 126 women (ClinicalTrials.gov NCT03304782).
Of 134 enrolled subjects, 25 (19%) and 109 (81%) were in the low and high activity groups, respectively. Overall, 11 (8.2%) delivered preterm. Women in the high activity group were older, partnered, employed and had higher education (Table). Our primary outcome, PTB, was similar between groups (aOR* 1.09, 95%CI 0.19, 6.19) as was the median GA at delivery (Table). There was no difference in median steps/day between preterm and term deliveries (7765, IQR 5188-10387 vs 6971, IQR 5412-8517, p=0.61) (Figure). Further, the median steps/day did not correlate with GA at delivery (r= -0.05, p=0.56). The 19 women prescribed AR were 2.5x more likely to deliver preterm than those who were not (OR 2.51; CI 95% 0.60-10.45).
In this prospective cohort study the rate of PTB and GA at delivery were not different among women with low vs high activity.