Most doctors, many other healthcare professionals, and the March of Dimes recommend that all women of childbearing age supplement with 400 mcg per day of folic acid. Such supplementation could protect against the formation of neural tube defects (such as spina bifida) during the time between conception and when pregnancy is discovered.

The requirement for the B vitamin folic acid doubles during pregnancy, to 800 mcg per day from all sources. Deficiencies of folic acid during pregnancy have been linked to low birth weight and to an increased incidence of neural tube defects (e.g., spina bifida) in infants.

In one study, women who were at high risk of giving birth to babies with neural tube defects were able to lower their risk by 72% by taking folic acid supplements prior to and during pregnancy. Several preliminary studies have shown that a deficiency of folate in the blood may increase the risk of stunted growth of the fetus. This does not prove, however, that folic acid supplementation results in higher birth weights. Although some trials have found that folic acid and iron, when taken together, have improved birth weights, other trials have found supplementation with these nutrients to be ineffective.

The relationship between folate status and the risk of miscarriage is also somewhat unclear. In some studies, women who have had habitual miscarriages were found to have elevated levels of homocysteine (a marker of folate deficiency).  In a preliminary study, 22 women with recurrent miscarriages who had elevated levels of homocysteine were treated with 15 mg per day of folic acid and 750 mg per day of vitamin B6, prior to and throughout their next pregnancy. This treatment reduced homocysteine levels to normal and was associated with 20 successful pregnancies. It is not known whether supplementing with these vitamins would help prevent miscarriages in women with normal homocysteine levels. As the amounts of folic acid and vitamin B6 used in this study were extremely large and potentially toxic, this treatment should be used only with the supervision of a doctor.

In other studies, however, folate levels did not correlate with the incidence of habitual miscarriages.

Preliminary and double-blind evidence has shown that women who use a multivitamin-mineral formula containing folic acid beginning three months before becoming pregnant and continuing through the first three months of pregnancy have a significantly lower risk of having babies with neural tube defects (e.g., spina bifida) and other congenital defects.

In addition to achieving significant protection against birth defects, women who take folic acid supplements during pregnancy have been reported to have fewer infections, and to give birth to babies with higher birth weights and better Apgar scores. (An Apgar score is an evaluation of the well-being of a newborn, based on his or her color, crying, muscle tone, and other signs.) However, if a woman waits until after discovering her pregnancy to begin taking folic acid supplements, it will probably be too late to prevent a neural tube defect.

Biotin deficiency may occur in as many as 50% of pregnant women. As biotin deficiency in pregnant animals results in birth defects, it seems reasonable to use a prenatal multiple vitamin and mineral formula that contains biotin.

In a preliminary study, pregnant women who used a zinc-containing nutritional supplement in the three months before and after conception had a 36% decreased chance of having a baby with a neural tube defect, and women who had the highest dietary zinc intake (but took no vitamin supplement) had a 30% decreased risk.

Iron requirements increase during pregnancy, making iron deficiency in pregnancy quite common. Iron supplement use in the United States is estimated at 85% during pregnancy, with most women taking supplements three or more times per week for three months. Pregnant women with a documented iron deficiency need doctor-supervised treatment. In one study, 65% of women who were not given extra iron developed iron deficiency during pregnancy, compared with none who received an iron supplement. However, there is a clear increase in reported side effects with increasing supplement amounts of iron, especially iron sulfate.  Supplementation with large amounts of iron has also been shown to reduce blood levels of zinc. Although the significance of that finding is not clear, low blood levels of zinc have been associated with an increased risk of complications in both the mother and fetus.

Iron supplementation was associated in one study with an increased incidence of birth defects, possibly as a result of an iron-induced deficiency of zinc. Although additional research needs to be done, the evidence suggests that women who are supplementing with iron during pregnancy should also take a multivitamin-mineral formula that contains adequate amounts of zinc. To be on the safe side, pregnant women should discuss their supplement program with a doctor.

Supplementation with fish oil (providing either 2.7 g or 6.1 g per day of the omega-3 fatty acids EPA and DHA) significantly reduced recurrence of premature delivery, according to data culled from six clinical trials involving women with a high risk for such complications. Fish oil supplementation did not prevent premature delivery of twin pregnancies, nor did it have any preventive effect against intrauterine growth retardation or pregnancy-induced hypertension. Fish oils should be free of contaminants, such as mercury and organochlorine pesticides. Women who eat substantial amounts of certain types of seafood (e.g., swordfish, tuna) may be consuming contaminants that can increase the risk of brain and nervous system abnormalities in their offspring. Exposure to mercury and polychlorinated biphenyls (PCBs) was found to be increased in relation to maternal intake of seafood. Higher exposure to these toxic contaminants has been linked to an increased risk of deficits in the developing brains and nervous systems of the children.

S-adenosylmethionine (SAMe) supplementation has been shown to aid in the resolution of blocked bile flow (cholestasis), an occasional complication of pregnancy.

Calcium needs double during pregnancy. Low dietary intake of this mineral is associated with increased risk of preeclampsia, a potentially dangerous (but preventable) condition characterized by high blood pressure and swelling. Supplementation with calcium may reduce the risk of pre-term delivery, which is often associated with preeclampsia. Calcium may reduce the risk of pregnancy-induced hypertension, though these effects are more likely to occur in women who are calcium deficient. Supplementation with up to 2 grams of calcium per day by pregnant women with low dietary calcium intake has been shown to improve the bone strength of the fetuses.

Pregnant women should consume 1,500 mg of calcium per day from all sources-food plus supplements. Food sources of calcium include dairy products, dark green leafy vegetables, tofu, sardines (canned with edible bones), salmon (canned with edible bones), peas, and beans.