Smoking during pregnancy and its effect on placental weight: A Mendelian randomization study.

It is well known that maternal smoking in pregnancy causes a lower birth weight on average, but the relationship between maternal smoking and placental weight is less clear, with observational studies showing conflicting results.Our Mendelian randomization study suggests that for pre-pregnancy smokers, continuing smoking during pregnancy causes higher placental weight at term than quitting smoking.Our study also suggests that a greater number of cigarettes smoked per day during pregnancy causes a larger placental weight at term.A possible explanation for our findings is that the placenta grows larger in mothers who smoke during pregnancy to compensate for the lower oxygen availability, but further work is needed to confirm and further investigate this hypothesis.