Working night shifts during pregnancy is linked to a heightened risk of neurodevelopmental abnormalities detected by screening in the child by the age of 4, finds research published online in Occupational & Environmental Medicine.
But nightly prenatal sleep length during this period may modify this observed association, exploratory findings suggest.
Night shift work during pregnancy has been associated with unfavourable health outcomes for both mother and child, including neurodevelopmental issues, potentially related to body clock and sleep disturbances, note the researchers.
They therefore wanted to explore the associations between mums' night shift work and their children's neurodevelopment and to find out if there are potentially modifiable prenatal factors, such as changes in night shift patterns and sleep duration.
They drew on nationwide survey data for around 100,000 pregnant women from the Japan Environment and Children's Study (JECS) who were enrolled between January 2011 and March 2014.
The women were surveyed in the first and second/third trimesters and answered questions about the number of night shifts they worked in a month.
Their responses were divided into 3 groups: no night shifts reported in either survey (27,655 women); night shifts in the first, but not second survey ('discontinued night shifts', 1779 women); and night shifts in both surveys ('continued night shifts', 2084 women).
Prenatal sleep length was calculated from self-reported bedtime and wake-up times in the second/third trimester. Sleep length was categorised as short if less than 6 hours; moderate if 6–9 hours; and long if 10 or more hours.
Child neurodevelopmental abnormalities at the age of 4 were assessed using a developmental screening tool—the Japanese version of the Ages and Stages Questionnaire, Third Edition (J-ASQ-3).
This is made up of 5 domains: communication; gross motor skills; fine motor skills (ability to make small, precise, deliberate movements); problem solving; and personal–social functioning (interpersonal skills, emotional/behavioural development, and awareness of personal boundary/space).
In all, 31,528 mothers who worked in the first trimester and completed the child development questionnaire when their children were 4 years old were included in the analysis.
Some 4189 (just over 13%) of the 4 year olds were classified as high risk of an abnormality in at least one of the five J-ASQ-3 domains: 1210 (4%) for communication; 1599 (5%) for gross motor skills; 1906 (6%) for fine motor skills; 913 (3%) for problem solving; and 1495 (5%) for personal–social functioning.
Compared with the children of mothers who didn't work night shifts, the children of those who discontinued night shift work were more likely to have developmental screening abnormalities, but only in the fine motor skills domain (24% higher risk).
But the children of mothers who continued night shifts during their pregnancy had higher risks of developmental abnormalities in 3 domains compared with children whose mothers didn't work night shifts.
These were in the domains of communication skills (29% higher risk); fine motor skills (34% higher risk); and personal–social functioning (31% higher risk).
Only the fine motor skills domain showed a significant association, regardless of whether night shifts were continued or discontinued.
"One possible explanation is that maternal inflammation associated with persistent night work may preferentially affect neural pathways involved in fine motor function," suggest the researchers.
A separate exploratory analysis showed that the observed association between night shift pattern and the child's neurodevelopmental risk differed according to how long the mothers slept.
Prenatal sleep duration of 10 or more hours was associated with an 80% heightened risk of abnormalities in any of the 5 domains compared with prenatal sleep length of 6-9 hours.
Long prenatal sleep duration may also reflect underlying fatigue, subclinical depression, inflammation, or anaemia, rather than representing a harmful exposure in itself, suggest the researchers.
This is an observational study, and as such, can't establish cause and effect. And the researchers acknowledge that they weren't able to account for shift rotation systems, individual chronotype, workload, exposure to light, or the timing of night shift discontinuation, all of which might have influenced the findings.
But in light of the findings, it may be worth employers considering flexible work schedules for pregnant night shift workers, they say.
