Researchers at the Italian University of Insubria looked into previous studies and concluded that the risk of preterm labor and low birth weight is negligible when compared to regular, 9 to 5 jobs. The researchers, who were led by Matteo Bonzini, noted that shift workers did have a slightly higher chance of delivering a small baby but not by a margin that allowed a definite conclusion to be drawn as to whether shift work was the true cause of this.
The theory is that working night shifts could interfere with hormone production and confuse the body. Previous studies suggest that nurses working the night shift are more likely to suffer from irregular periods (See: Does rotating shift work cause irregular periods?). "On balance, the evidence currently available about the investigated birth outcomes does not make a compelling case for mandatory restrictions on shift-working in pregnancy," the researchers who reviewed 23 studies involving from 700 to over 35,000 women per study.
The research team found serious flaws in some of studies, which failed to take into account that some women smoke through their pregnancies and did not look at socio-economic status. In addition, some studies used the reports of study participants rather than their medical records. These studies were found to be of substandard quality, and were not used in the review. The study team did advise women to avoid working night shifts during their pregnancy if they had that possibility, given the fact that more research is needed to make an accurate conclusion.
But if you are a night shift worker and you are pregnant or trying to get pregnant, perhaps you don't need to worry about your working hours too much and you should instead focus on your health and nutrition.