Iron deficiency is the most common nutritional deficiency worldwide, and is estimated to affect a quarter of the pregnant population.1 Low systemic iron levels have been associated with several maternal pregnancy complications, including infection and preeclampsia.2 However, these results from traditional observational studies may be affected by confounding. A systematic review of trials found that daily iron supplementation in pregnancy reduced iron deficiency anemia, but there was no clear effect on other maternal complications, which in large part may be due to low statistical power.3 To robustly evaluate whether systemic iron status affects risk of maternal pregnancy complications, we conducted a Mendelian randomization (MR) study. By leveraging genetic instruments for systemic iron status, the approach greatly reduced the risk of confounding and allowed for a large study sample.