Mums-to-be who work shifts have smaller babies and longer pregnancies, a study warned.
And shift work affects the health of their unborn child from the earliest stages of gestation.
Rotating night and day shifts impairs the mothers glucose tolerance which meant mothers had poorer control of their blood glucose levels.
It also disrupted the maternal circadian rhythms, or the 24-hour body clock, and metabolism.
The findings suggested early pregnancy might be a time of particular susceptibility to disrupted circadian rhythms and shed light on the impact of unsocial working hours on the health of their unborn babies.
It is estimated between a sixth and a third of shift workers work hours outside the 8am to 6pm slot.
And two thirds of shift workers were women of reproductive age between 15 and 44 years old, Australian labour figures showed.
Lead author Dr Kathy Gatford at the University of Adelaide explained: “Shift workers are at increased risk of developing type 2 diabetes and obesity, however the impact
during pregnancy on maternal metabolism is unknown
“Shift work is associated with increased risks of chronic disease including diabetes and obesity in epidemiological studies and short term simulated shift work in a controlled laboratory environment impairs metabolic control.
“It has been proposed that circadian disruption is a mechanism underlying development of chronic disease in shift workers.
“Shift work interferes with normal patterns of sleep–activity, feeding–fasting and light–dark exposure, all key input signals to the circadian timing system.
“Consistent with effects of shift work in humans, circadian disruption in rodents increases adiposity and perturbs glucose metabolism.
“Associations between shift work and metabolic dysfunction during pregnancy have not been reported in women, although sleep duration of less then seven hours/night more than doubles the risk of developing gestational diabetes, and sleep disruption is a well-recognised consequence of shift work.
“Epidemiological studies demonstrate an association between shift work and increased risk of miscarriage, preterm birth and intrauterine growth restriction.”
The senior lecturer in paediatrics and reproductive health said while shift work has been associated with impaired pregnancy outcomes, until now the mechanisms have not been understood.
So the study involving the South Australian Health and Medical Research Institute looked how a disrupted sleep patterns affect pregnancy outcomes in sheep -an animal model for human pregnancy.
A flock of pregnant sheep were exposed to simulating shift patterns, with either light on during the day and food provided each morning, or the timing of light and feeding changed to mimic a rotation between day and night shift during each week.
Their maternal circadian rhythms and glucose control in early and late pregnancy were measured and all lambs at birth were weighed.
Those sheep on the shift work rota had impaired glucose tolerance during early pregnancy, just as it does in non-pregnant human adults.
Reduced fetal growth was seen in pregnancies with a single foetus and longer pregnancies in mothers carrying twins.
Importantly, this was seen even if mothers stopped simulated shift work after the first third of pregnancy.
This suggested exposure to rotating night and day shifts, even if only in early pregnancy, may adversely affect maternal metabolic and pregnancy outcomes.
The researchers now hope to identify shift work patterns that do not adversely affect the mother’s metabolism or pregnancy outcomes.
Dr Gatford concluded: “The effects of shift work on pregnancy are not well understood.
“We found that exposure to rotating night and day shifts, even if only early in pregnancy, altered both maternal metabolic and pregnancy outcomes.
“This study has implications for the large number of women working shifts, and highlights the need to further interrogate the impact of shift work on maternal health and pregnancy outcomes in human populations.
“Future studies should also address the long term impact upon progeny health.
“We are now assessing whether maternal shift work affects the health of their children by looking at circadian rhythms, cardiometabolic health and body composition in the progeny in this study.’
The study was published in The Journal of Physiology.
By Ben Gelblum and Tony Whitfield