Objectively Measured Maternal Supine Sleep and Fetal Growth: A Prospective Cohort Study

To explore the relationship between different definitions of objectively measured supine sleep position during late pregnancy and customised birthweight centile, as a marker of fetal wellbeing.

Prospective cohort study.

Australian public hospital antenatal clinic.

Eighty-four pregnant women from 32-36 weeks of gestation.

Body position was measured in degrees of roll using a tri-axial accelerometer worn around the abdomen for seven sequential nights. Supine was defined as 'broad supine' (0° = fully supine to 45° left/right tilt), and 'supine low-tilt' (0°-15°). Two additional Risk Indices capturing risk proportional to duration spent in each degree of supine-to-lateral tilt were developed. Birthweights were converted into customised centiles using GROW software.

The primary outcome was customised birthweight centile. Secondary outcomes included birthweight, gestational age at delivery, and fetal growth trajectory.

Across 556 nights of data, the median nightly duration of broad supine positioning was 65.9 [25.0, 125.7] minutes compared to only 9.1 [0.7, 24.4] minutes of supine low-tilt. There was no relationship between customised birthweight centile and time spent in broad supine (R² = 0.0027, F(1,82) = 0.22, p = 0.64) or supine low-tilt (R² = 0.0029, F(1,82) = 0.24, p = 0.63); nor with any supine risk index and customised birthweight centile, birthweight, gestational age at delivery, or fetal growth trajectory.

In this cohort, there was no evidence of an association between objectively measured supine sleep and customised birthweight centile. Yet to be identified mechanisms unrelated to fetal growth may be responsible for supine going-to-sleep position being a risk factor for late stillbirth. Further prospective investigation is needed to untangle cause and effect; however, given the low rate of stillbirth, such a study may not be feasible.