Introduction Whilst it has long been recognised that posture has a profound impact on maternal haemodynamics, few studies have extrapolated these practices to maternal sleep. Recently, findings from several countries have shown that self-report of maternal going-to-sleep in the supine position increases the odds 2-8-fold for 3rd trimester stillbirth. However, none of the prior research has been able to objectively assess sleep position and it is unclear how many pregnant women spend time supine. The goal of the present study was to objectively assess sleep position and respiratory disturbance in pregnant women and to determine how these variables change across pregnancy.

Methods Pregnant women were recruited from a large academic medical centre prior to 19 weeks’ gestation and invited to undergo home sleep testing using the WatchPAT device. Home sleep testing was repeated in late pregnancy. The proportion of supine sleep time was calculated and OSA was defined as an apnoea-hypopnoea index (AHI) ≥5 events/hour.

Results Overall, 174 women were studied at a mean gestational age of 15.3±3.0 weeks (early pregnancy) and home sleep testing was repeated at a mean gestational age of 35.5±1.9 weeks (late pregnancy). Sleep duration decreased as pregnancy progressed (6.8hr vs 6.0 hr, p<0.001) while both the AHI and the oxygen desaturation index increased (2.2 vs. 5.4, p<0.001 and 0.5 vs 1.8, p<0.001 respectively). The proportion of women spending some time in the supine sleep position was 36% in early pregnancy, rising to 50% in late pregnancy (p=0.03). of note, 16% of women spent at least half the night supine in early pregnancy and 20% of women did so in late pregnancy.

Discussion Half of women spend some time in the supine sleep position in late pregnancy. Given the emerging associations between supine sleep and stillbirth, maternal sleep practices offer a modifiable risk factor.