An individual participant data meta-analysis of maternal going-to-sleep position, interactions with fetal vulnerability, and the risk of late stillbirth.

Background: Maternal supine going-to-sleep position has been associated with increased risk of late stillbirth (>28 weeks’), but it is unknown if the risk differs between right and left side, and if some pregnancies are more vulnerable. Methods: Systematic searches were undertaken for an individual-level participant data (IPD) meta-analysis of case-control studies, prospective cohort studies and randomised trials undertaken up until 26 Jan, 2018, that reported data on maternal going-to-sleep position and stillbirth. Participant inclusion criteria included gestation >28 weeks’, non-anomalous, singleton pregnancies. The primary outcome was stillbirth. A one-stage approach stratified by study and site was used for the meta-analysis. The interaction between supine going-to-sleep position and fetal vulnerability was assessed by bi-variable regression. The multivariable model was adjusted for a priori confounders. Registration number: PROSPERO, CRD42017047703. Findings: Six case-control studies were identified, with data obtained from five (cases, n=851; controls, n=2257). No data was provided by a sixth study (cases, n=100; controls, n=200). Supine going-tosleep position was associated with increased odds of late stillbirth (adjusted odds ratio [aOR] 2·63, 95% CI 1·72–4·04, p<0·0001) compared with left side. Right side had similar odds to left (aOR 1·04, 95% CI 0·83–1·31, p=0·75). There were no significant interactions between supine going-to-sleep position and assessed indicators of fetal vulnerability, including small-for-gestational-age infants (p=0·32), maternal obesity (p=0·08), and smoking (p=0·86). The population attributable risk for supine going-to-sleep position was 5·8% (3·2–9·2). Interpretation: This IPD meta-analysis confirms that supine going-to-sleep position is independently associated with late stillbirth. Going-to-sleep on left or right side appears equally safe. No significant interactions with our assessed indicators of fetal vulnerability were identified, therefore, supine going-to-sleep position can be considered a contributing factor for late stillbirth in all pregnancies. This finding could reduce late stillbirth by 5·8% if every pregnant woman >28 weeks’ gestation settled to sleep on her side.