Bending forwards and walking a lot at work in the early stages of pregnancy may increase the risk of miscarriage, a study suggests.

Miscarriage affects about 15% of women. Risk factors include parental age, smoking, night shift work and exposure to air pollution and various chemical compounds.

There is also some evidence to suggest forward bending, walking and standing in the workplace may raise the risk, but previous findings have been inconclusive. To explore this further, researchers drew on data from the pregnancies of almost half a million women in Denmark.

Each additional hour in an eight-hour working day of bending forwards, particularly at a 30-degree angle, was associated with a 36% higher risk of miscarriage, the researchers found. Every extra hour of walking was associated with an 18% higher risk.

The findings of the study, in which 803,829 pregnancies among 475,312 women between 2004 and 2018 were analysed, were published in the journal Occupational and Environmental Medicine.

Researchers from Bispebjerg hospital in Copenhagen and the University of Copenhagen used an improved pregnancy-specific job exposure model that combined activity tracker readings and expert evaluations of the time spent standing, walking and forward bending at a 30-degree angle or more.

About one in 10 (81,307) of the pregnancies ended in miscarriage, which is lower than the usual prevalence. Analysis of the data showed bending forwards, walking and standing were all associated with a higher risk of miscarriage.

Each additional hour of forward bending, particularly at a 30-degree angle, was associated with a 36% higher risk, while one more hour of walking or standing was associated with 18% and 3% higher risks respectively.

This was an observational study and therefore no firm conclusions can be drawn about cause and effect. There were also other limitations, including the fact that there was no individual data about smoking during pregnancy.

Further studies including potentially influential factors are needed to confirm the findings, but they highlight the importance of including the early pregnancy stage in guidelines for pregnant workers, say the researchers.

“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,” they said.

Prof Asma Khalil, professor of obstetrics and maternal medicine and consultant obstetrician at City St George’s, University of London, who was not involved with the study, said the findings should not be interpreted as meaning that ordinary movement in early pregnancy was unsafe.

“This is a large, well conducted nationwide register-based cohort study using Danish data from more than 800,000 pregnancies. A key strength is the use of a pregnancy-specific job exposure matrix to estimate occupational standing, walking and forward bending, which reduces recall bias compared with self-reported exposure.

“The main finding is that forward bending at work was associated with an increased risk of miscarriage, with a more consistent dose-response pattern than was seen for standing or walking. However, this is an observational study, so it cannot prove causation.”

Key limitations included the lack of individual level data on smoking, inability to adjust for lifting, and potential confounding from other occupational exposures such as shift work or chemical exposures, Khalil said.

“While the study raises an interesting hypothesis regarding occupational postures and miscarriage risk, the findings need replication and confirmation in other populations before they can be translated into precise workplace guidance.”