Best iron for pregnancy without constipation

How do the options compare?

Each option is compared on the elemental iron it delivers, the gastrointestinal complaint and constipation rates reported in randomized pregnancy trials, and how it fits alongside the iron already in a prenatal.

• Ferrous bisglycinate 25 mg produced fewer total gastrointestinal complaints than ferrous sulphate 50 mg in a randomized double-blind trial of pregnant women, 16.5 percent versus 20.1 percent at weeks 27 to 40 (p=0.042), and black stools were far less common on bisglycinate, 8.1 percent versus 30.9 percent. 1
• Constipation stayed near 20 percent at 20, 40 and 60 mg of ferrous fumarate in the same trial program (20.9, 20.8 and 20.1 percent) and jumped to 32.6 percent at 80 mg, with laxative use rising from 0 percent at 20 mg to 7.1 percent at 80 mg. 1
• Iron salts differ in how much elemental iron they deliver per milligram of compound: ferrous fumarate is 33 percent elemental iron by weight, ferrous sulfate is 20 percent and ferrous gluconate is 12 percent. Supplemental iron at 45 mg a day or more may cause nausea and constipation, taking iron with food helps minimize that, ascorbic acid increases nonheme iron absorption, and calcium reduces it, so experts suggest taking calcium and iron at different times of day. 2
• Underneath any iron choice sits the ordinary constipation plan: adults should get 22 to 34 grams of fiber a day depending on age and sex, and should drink plenty of water and other liquids when eating more fiber or taking a fiber supplement. Stimulant laxatives are reserved for severe constipation or when other approaches have not worked. 3
• The iron RDA in pregnancy is 27 mg, and 89 percent of prenatal products surveyed contained iron at a median of 27 mg per serving, so a separate iron supplement stacks on top of what the prenatal already delivers. 4

Which option makes sense?

Dose is the bigger lever and form is the smaller one. Low-dose iron, ferrous bisglycinate where it is available, taken with food and kept several hours away from calcium, is the combination with the least gastrointestinal cost in the pregnancy data that exists.

Disclosure: Pregnancy Smart offers a supplement that overlaps with one or more options above. This page describes the pregnancy evidence for each option on its own merits.