Japanese Study Links Night-Shift Work During Pregnancy to Possible Developmental Difficulties
Babies receive signals about day and night before birth, including melatonin passed from the mother through the placenta. A study of 31,528 mothers and children found an association between night-shift work during pregnancy and developmental screening results at age four, but did not establish that night work caused difficulties.
Babies receive signals about day and night before birth, including melatonin passed from the mother through the placenta, a finding that may help explain how an infant’s body clock develops.
A Japanese study of 31,528 mothers and children found an association between night-shift work during pregnancy and possible developmental difficulties flagged at age four. The study did not show that night work caused those difficulties.
Melatonin, a hormone released mainly at night, can pass from mother to fetus through the placenta. The developing body clock is not yet fully working before birth, and maternal signals may help it distinguish day from night.
Researchers compared mothers who did not work night shifts, those who stopped between two pregnancy surveys and those who reported night shifts at both stages. Working even one night shift a month counted as night work.
Children whose mothers reported night shifts at both stages had a 29% to 34% higher relative risk of being flagged for possible communication, fine motor or social difficulties than children whose mothers did not work nights.
That relative increase does not mean a third of the children had difficulties. Across all groups, about 13% of children were flagged for a possible difficulty in at least one area.
Parents reported their children’s abilities through screening questions, not medical diagnoses. A flagged result means a child might need further assessment.
The researchers did not know how many nights the mothers worked, how demanding their jobs were or why some stopped working nights. The study also did not test whether more frequent shifts carried greater risks.
The association appeared stronger among mothers who reported sleeping at least ten hours. However, only 80 women both continued night shifts and reported sleeping that long, making the finding uncertain. It does not show that longer sleep is harmful.
Researchers said the study could not determine whether stopping night shifts would reduce any risk. Night work can disrupt the body’s daily timing, and light and meal times can also affect it.
Animal studies offer clues about how timing signals may reach a fetus. In a 2025 rat study, substances from the mother’s food rose and fell during the day in a part of the unborn rat’s brain that would later help control its body clock.
In a 2026 mouse study, researchers gave pregnant mice a stress-related hormone at different times of day. The timing changed daily rhythms in their offspring.
