Do iron supplements help in pregnancy?

What the trials found

A 2024 Cochrane review included 57 trials with 48,971 pregnant women.^[1] Compared with placebo or no iron, daily iron:

• may reduce anemia at term: 4.0% against 7.4% of women (14 trials, 13,543 women, low certainty)^[1]
• may reduce iron deficiency at term: 44.0% against 66.0% (low certainty)^[1]
• probably reduces iron deficiency anemia at term: 5.0% against 18.4% (moderate certainty)^[1]
• probably lowers the share of low-birthweight babies slightly: 5.2% against 6.1% (12 trials, 18,290 babies, moderate certainty)^[1]

Iron probably makes little or no difference to preterm birth (7.6% against 8.2%) or to deaths of mothers.^[1] It may make little or no difference to deaths of newborns (1.4% against 1.5%), and its effect on average birthweight is very uncertain.^[1] Iron taken together with folic acid probably reduced anemia at term from 25.5% to 12.1%.^[1]

Women who start with normal iron

A 2023 review looked only at pregnant women who had no anemia and no iron deficiency at the start.^[3] It found 8 trials with 2,822 women. In 4 trials with 1,670 women, daily iron probably reduced iron deficiency anemia at term (risk ratio 0.51).^[3] In 2 trials with 361 babies, it probably reduced low birthweight (risk ratio 0.30).^[3]

Why the need rises

In pregnancy, the volume of blood plasma and the number of red blood cells grow to supply the fetus and the placenta.^[4] In US survey data from 1999 to 2006, 18% of pregnant women had iron deficiency: 6.9% in the first trimester, 14.3% in the second and 29.7% in the third.^[4] Iron deficiency in pregnancy raises the risk of premature birth, low birthweight and death of mother and baby.^[4]

What official bodies say

• WHO recommends 30 to 60 mg of elemental iron with 400 µg of folic acid a day for pregnant women.^[2]
• The US recommended intake in pregnancy is 27 mg a day, and the upper limit is 45 mg.^[4]
• The American College of Obstetricians and Gynecologists recommends low-dose iron from the first trimester. The US Preventive Services Task Force found the evidence not enough to judge routine iron for pregnant women without symptoms.^[4]
• EFSA sets the same reference intake in pregnancy as for other women before menopause, 16 mg a day. It assumes that iron stores and higher absorption cover the extra need.^[5]

Side effects

In 7 trials with 1,028 pregnant women, ferrous sulfate raised the odds of stomach and gut side effects about 3 times (odds ratio 3.33).^[6] In pregnant women, ferrous bisglycinate caused fewer of these side effects than other iron supplements (incidence rate ratio 0.36) and raised hemoglobin more.^[7] In the 2024 Cochrane review, the evidence on side effects was very uncertain (21.6% with iron against 18.0% without).^[1]

What is still unknown

The Cochrane authors call for more research on the baby's iron status, growth and development.^[1] Of the trials, 19 were done in places with some malaria risk, and none reported data on clinical malaria in the mothers.^[1]

The Dietary Guidelines for Americans advise pregnant women to take an iron supplement when an obstetrician or other health care provider recommends it.^[4]