Association of supine going-to-sleep position in late pregnancy with reduced birthweight: data from an individual participant data meta-analysis.

Importance: Supine maternal position in the third trimester is associated with reduced uterine blood-flow and increased risk of late stillbirth. As reduced uterine blood flow is also associated with fetal growth restriction we explored the relationship between going-to-sleep position and birthweight. Objective: We hypothesised that women who reported they usually went-to-sleep supine after 28 weeks of pregnancy, would have infants with lower birthweight and birthweight centiles. Design: Pre-specified sub-group analysis using data from controls in an individual participant data meta-analysis of four case-control studies investigating sleep and stillbirth. Setting: New Zealand, Australia and United Kingdom. Participants: 1760 women with ongoing pregnancies ≥28 weeks’ gestation at interview.Main Outcomes and Measures: The primary outcome was adjusted mean difference (aMD) inbirthweight. Secondary outcomes were birthweight centiles [INTERGROWTH (IG) 21st  and customised], and adjusted odds ratios (aOR) for birthweight <50th and <10th  centile [small-forgestational-age (SGA)] for supine versus non-supine going-to-sleep position in the last one to four weeks, adjusted for variables known to be associated with birth size. Results:5Of the 1760 women, 57 (3.2%) reported they usually went-to-sleep supine during the previous one to four weeks. Adjusted mean birthweight among women who reported supine going-to-sleep position was 144g (aMD -253 to -36g) less than women who reported non-supine going-to-sleep position (3410g versus 3554g, p=0.009), representing an approximate 10 percentile reduction in adjusted mean IG (48.5 versus 58.6, aMD -10.1 [-17.1, -3.1]) and customised (40.7 versus 49.7, aMD -9.0 [-16.6, -1.4]) centiles. There was a non-significant increase in birthweight <50th IG centile (aOR 1.90, 95% CI0.83, 4.34) and two-fold increase in birthweight <50th customised (aOR 2.12, 95% CI 1.20, 3.76). Going-to-sleep supine was associated with a threefold increase in SGA by IG (aOR 3.23, 95% CI90 1.37, 7.59) and non-significant increase in SGA customised (aOR 1.63, 95% CI 0.77, 3.44). Conclusions and Relevance: Going-to-sleep supine in late pregnancy is independently associated with reduced birthweight and birthweight centile. This novel association is biologically plausible and likely to be modifiable. Public health campaigns that encourage women in the third trimester to settle to sleep on their side have potential to optimise birthweight.