P024 High resolution measurement of sleep position during pregnancy and the impact on infant birthweight

Abstract Introduction Pregnant women are advised to sleep laterally from 28-weeks of gestation, due to increased risk of stillbirth in those who report “going-to-sleep” in the supine position. Historically, objectively-measured sleep position during pregnancy was classified as supine or lateral, potentially missing important subtleties regarding the mechanisms behind sleeping position and fetal wellbeing. This study aimed to characterise sleep position during pregnancy with precise measurement techniques and assess the impact of discretisation of supine sleep on infant birthweight. Methods A tri-axial accelerometer was worn around the abdomen for 7 nights, between 32-36 weeks of pregnancy. Body position was measured continuously in degrees of roll. Supine was defined crudely as 0°= fully supine to 45° left or right tilt versus more precisely as 0-15°, 15-30° and 30-45°. Birthweights were converted into customised centiles. Results A total of 368 nights were recorded across 56 women. The median (IQR) proportion of supine positioning (0-45°) across the week was 15.6% (4.9,26.5), equivalent to 74.5 (24.0,128.6) minutes per night. Of time spent supine, 32% was within 0-15°, compared to 28% and 41% in more tilted positions of 15-30° and 30-45° respectively. Birthweight centile was not related to supine (0-45°) time overnight (β=0.01, p=.85) or when constrained to 0-15° (β=0.17, p=.07). Discussion Half the women studied spent over an hour in the supine position overnight, although a 30-45° lateral tilt was favoured. Supine sleep was not associated with birthweight, suggesting that, at this supine exposure, the fetus is minimally impacted from aortocaval compression potentially impacting uteroplacental supply.