Association of fetal activity patterns in utero with postnatal sleep behavior

The human circadian clock, a key mechanism governing the sleep-wake cycle, starts oscillating before birth. Distinct fetal behavioral states have been described using short-duration ultrasound recordings, but the alignment of fetal activity with day and night cycles (i.e., diurnal rhythms) remains poorly understood due to the lack of long-term monitoring. This study characterizes fetal diurnal activity patterns using continuous third-trimester monitoring over at least five days across 32 mother-child pairs, yielding over 20,000 combined sensor-hours of maternal (acceleration and temperature) and fetal (acceleration) wearable data. We tracked diurnal pattern development from in utero to 6 months of age, assessing infant sleep at three postnatal time points (first weeks, 3 months, and 6 months) with sleep diaries and questionnaires. Multi-level modeling revealed early diurnal rhythms in fetuses that align with maternal activity and circadian rhythms. Random Forest analyses identified fetal day/night sleep duration ratio – a simplified proxy for fetal circadian entrainment – as the strongest predictor of postnatal infant sleep, with fetuses preferring nighttime sleep maintaining this preference postnatally. Maternal sleep regularity (i.e., consistent sleep patterns characterized by low day-to-day variability in sleep timing) during pregnancy also predicted infant sleep, being associated with a stronger preference for nighttime sleep in infants. These findings highlight the potential influence of intrauterine and maternal factors on early circadian entrainment and highlight maternal sleep as an actionable target to improve infant sleep and family well-being. A limitation of this study is the relatively small cohort size and, consequently, the findings should be interpreted with caution until they are replicated in larger, independent cohorts.