Bending forwards, and to a lesser extent, walking a lot at work in early pregnancy, may raise the risk of miscarriage, finds a large study of more than 470,000 Danish women, published online in the journal Occupational & Environmental Medicine.
Each additional hour of bending forwards, particularly at a 30 degree angle, was associated with a 36% higher risk, while each additional hour of walking was associated with an 18% higher risk, although the pattern was only consistent for bending forwards, the findings show.
Miscarriage is relatively common, affecting around 15% of women. Risk factors include parental age, smoking, night shift work and exposure to air pollution and various chemical compounds, note the researchers.
There is some evidence to suggest that workplace standing, walking, and forward bending may also heighten the risk, but the findings have been inconclusive. And given that almost 70% of women in Europe work, concerns have been raised about safe levels of occupational physical activity during pregnancy, they add.
To explore this further, the researchers drew on data from a large national register containing information on the pregnancies of working women living in Denmark between 1977 and 2018.
They focused on 475,312 women with 803,829 pregnancies between them from January 2004 to December 2018.
The researchers deployed an improved pregnancy-specific job exposure model that combines activity tracker readings and expert evaluations of the time spent standing, walking and forward bending at a 30 degree angle or more.
Just over 1 in 10 (81,307) of the pregnancies ended in miscarriage, which is lower than the usual prevalence, possibly because the data came from hospital registers and many early miscarriages are handled at home and not registered in hospital data, say the researchers.
Analysis of the results showed that standing, walking, and bending forwards were all associated with heightened risks of miscarriage.
Each additional hour of forward bending, particularly at a 30 degree angle, was associated with a 36% higher risk, while each additional hour of walking and standing were associated with 18% and 3% higher risks, respectively.
The risk was greater among those women who had been absent from work the preceding week than it was among those who hadn't been absent.
Further analyses indicated a consistent dose–response relationship only for forward bending.
This is an observational study and as such, no firm conclusions can be drawn about cause and effect. And the researchers acknowledge that they had no individual-level information on smoking during pregnancy nor were they able to account for lifting, which may partly explain the observed associations, or shift work and chemical exposures.
And because prolonged forward bending is relatively uncommon in most jobs, its overall impact is likely limited despite the higher risk estimates, they suggest.
Further studies, incorporating potentially influential factors, are needed to confirm the findings, they emphasise.
Nevertheless, they suggest: "Although the mechanisms are not completely understood, these occupational exposures may affect placental perfusion or hormonal regulation in ways that could increase the risk of miscarriage."
And they point out: "In Denmark, there are currently no formal guidelines addressing occupational standing or walking within the first 4 months of pregnancy, and no guidance regarding forward bending.
"Our findings highlight the importance of including the early pregnancy stage into guidelines for pregnant workers."
