June 27, 2012

fetal growth and physical work while pregnant

This analysis accompanied a roundup which can be viewed here.

Title, Date of Publication & Journal

Physically demanding work, fetal growth and the risk of adverse birth outcomes. The Generation R Study, 27 June 2012, Occup Environ Med

Claim supported by evidence?

The paper should not be used to conclude that standing for long periods during pregnancy may curb fetal growth.

Summary

• Previous studies suggest certain work-related risk factors adversely influence pregnancy outcomes and this study aimed to gain insight into how physically demanding work influences birth outcomes by looking at fetal growth characteristics during pregnancy. 
• Large prospective cohort study examining the associations between physically demanding work with fetal growth and the risks of adverse outcomes
• Results do not fully support claim that physically demanding work during pregnancy was associated with lower fetal growth rates
• Results are ambiguous and no more than hypothesis-generating
• The study findings were not associated with adverse birth outcomes

Study Conclusions

The paper concludes that “physically demanding work during pregnancy was associated with lower fetal growth rates”. The study also found that “these finding were not reflected in adverse birth outcomes”.

Head circumference: Working between 25 and 39 hours did show statistically significant difference in growth rates of head circumference, but working greater than 40 hours was not significantly different from working 1-24 hours. Long periods of standing occasionally and often showed statistically different growth rates in head circumference compared to no long periods of standing; however, there appeared to be no difference between long periods of standing occasionally and standing often. This is contrary to what we might reasonably expect if there were a causal link.

Fetal weight: Working 25-39h and >40h per week showed statistically different growth rates in fetal weight. The clinical relevance of the difference is unclear. Long periods of standing did not show statistically significant differences in fetal weight.

Abdominal circumference: No associations between long periods of standing and working hours were observed for abdominal circumference.

Authors also looked at physical workload as measured by heavy lifting and no associations were found for fetal weight, head circumference or abdominal circumference.
Overall, authors do not state what the significance of lower fetal growth rates during pregnancy are, given that they were not associated with adverse birth outcomes.

Strengths/Limitations

Large prospective study, suitably designed

Most potential confounders (e.g. gestational age, maternal age, mother’s height/weight, educational level, ethnicity, smoking/alcohol etc) were accounted for. Study did acknowledge some that were not considered because they were not available (stress, fatigue, physical activity outside employment).

Conducting many statistical tests (72 in Table 3 alone) will always produce some false positive results in a study like this. It is always dangerous to produce a high number of data points and then focus on the ones that appear to show a correlation.

No consistent pattern can be seen across the data points, and this further weakens the hypothesis of an overall link.

Glossary

None

Any specific expertise relevant to studied paper (beyond statistical)?

None

‘Before the headlines’ is a service provided to the SMC by volunteer statisticians: members of the Royal Statistical Society (RSS), Statisticians in the Pharmaceutical Industry (PSI) and experienced statisticians in academia and research.  A list of contributors, including affiliations, is available here.