In our study we calculated pooled risk estimates for three clinically important pregnancy outcomes (PTD, SGA, and LBW) among women exposed to shift work, after systematically reviewing all available epidemiological studies. We found small elevations of risk (pooled RRs between 1.1 and 1.3).
Our risk estimate for PTD was based on a reasonable number of studies, but may have been exaggerated by publication bias, with more complete reporting of positive than non-positive findings from smaller studies. Moreover, when poorer quality studies were excluded, the pooled risk estimate did not differ significantly from unity. Regarding SGA and LBW, the number of studies identified was smaller, but results were more homogeneous across studies, and exclusion of poor quality studies did not materially change the pooled risk estimates. However, the pooled risk estimates were not sufficiently high to allow confident conclusions about a hazard, and spurious associations from bias or residual confounding cannot be ruled out. For pregnancy-induced hypertension and pre-eclampsia too few studies were available to allow any firm conclusion. Our findings are largely consistent with those of previous reviews 3, 9
and indicate that any effects of shift work on PTD, SGA and LBW are likely to be small.
Estimates of risk for PTD have tended to reduce with inclusion of more recent studies in meta-analyses: Mozurkewich3
in 2000 pooled risk estimates from six studies and found a 24% elevation of risk among exposed women; in our previous review9
(2007, based on 13 studies), we calculated a 20% increase in risk; and with addition of three further studies published over the past 5 years, the excess RR has fallen to 15%. If there is a hazard, then one explanation of this time trend could be that in most countries, precautionary legislation was introduced, allowing pregnant workers to be assigned to other tasks (without shift work) or to take earlier antenatal leave, so that fewer women continued shift work during the later stages of pregnancy. An occupational exposure such as night shifts may have been voluntarily suspended if women suspected that it could be detrimental to their pregnancy. In this situation, risks in women who continue to work shifts late in pregnancy might be underestimated because of a healthy pregnant worker effect,32
healthier women with uncomplicated pregnancies being less likely to modify their work schedules. In support of this, in two studies 20, 21
the risk of PTD associated with shift work was higher in women whose work conditions did not change in the course of pregnancy. Finally, in a study comparing European countries, significant associations of PTD with shift work were mainly observed in countries where long prenatal leaves were infrequent and legal support for preventive measures was weaker.30
An important limitation of the available epidemiological evidence is that many authors have not properly distinguished between different types of shift work. There is a possibility that rotating shifts (with or without work at night) may have varying level of job demand or could lead to a different degree of misalignment of maternal endogenous circadian rhythms compared with fixed night work. We did not calculate pooled estimates of risk for specific patterns of work-schedule because of the small number of studies available with sufficient information. However, Croteau et al20
presented risk estimates separately for night workers and for rotating shift workers (both compared with fixed day-time workers), and found that for both SGA and PTD risks were elevated only among rotating shift workers.
The mechanisms whereby shift work might result in adverse pregnancy outcomes are not entirely understood. Both direct (through disturbances of circadian rhythm) and indirect (through psychosocial stresses and sleep disruption) mechanisms have been proposed to explain a causal relationship between shift work and obstetric complications that are inherently multifactorial in nature. There is a sound basis for the notion that stressors may impact on PTD and LBW through several intersecting pathways, which include neuroendocrine, behavioral, immune, and vascular mechanisms.33, 34
Moreover, shift work modifies peak values and rhythm amplitudes of serum melatonin, whose function seems essential for successful pregnancy. 35
Recently, melatonin was found to act synergistically with oxytocin to increase membrane-bound phospholipase C activity and associated signaLling mechanisms, thereby enhancing myometrial contractility and gap junction-associated intercellular communication.36
Further investigation is needed to ascertain whether and to what extent disruption of sleep and circadian rhythms and daylight-night working cycles poses a significant threat to pregnant women and their fetuses.
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. Further studies are needed to address the question whether adverse birth outcomes are related to different types of rotating work schedules (separating between night- and day-time shift), or to fixed night work. In the meantime, we suggest that, it would be prudent, insofar as job circumstances allow to permit pregnant women who wish to do so, to reduce their exposure to shift and night working.