Iron supplementation was typically initiated between 13 and 20 weeks of pregnancy and continued until term or postpartum. Daily doses ranged from 30 mg to 100 mg of elemental iron. Ferrous sulfate was the most commonly used form, followed by ferrous fumarate, ferrous gluconate, and heme iron.

On average, supplementation increased hemoglobin by 6.95 grams/liter and ferritin by 12.2 ng/mL. The participants who took iron supplements also were 50% less likely to develop anemia, with one case of anemia prevented for every 10 participants who received supplementation. However, these results showed high heterogeneity.

Four studies reported gastrointestinal side effects, including nausea, vomiting, constipation, diarrhea, and abdominal pain.

Iron supplements in pregnancy: A precaution or an overreach? Original paper