Women should start taking vitamin and mineral supplements "as soon as they know they are pregnant," Dr. Prakesh S. Shah of the Mount Sinai Hospital in Toronto, the lead researcher on the study, told Reuters Health. He noted that about 70 percent to 80 percent of women in the U.S. and Canada use prenatal supplements.

Currently, the World Health Organization recommends supplementation with folic acid and iron during pregnancy, Shah and his team note in CMAJ. But subsequent research has suggested that other nutrients may also be important for preventing low birth weight and premature birth.

To see if multimicronutrient supplements might benefit babies more than iron and folic acid supplements alone, Shah and his colleagues reviewed 13 published trials conducted in Asia, Africa, Europe and North America. The number of micronutrients women received in the various studies ranged from 8 to 28.

Compared with women who took a placebo, women who took multimicronutrients were 19 percent less likely to have a low birth weight baby. When compared with women taking iron and folic acid only, the researchers found the multimicronutrient users had a 17 percent lower risk of having a low birth weight baby.

Babies born to women taking multimicronutrient supplements weighed 54 grams (about two ounces) more, on average, than babies born to women taking iron and folic acid alone.

However, multimicronutrient supplements didn't appear to affect the risk of preterm birth or infants who were "small-for-gestational-age."

In an interview, Shah said that certain nutrients, in certain amounts, stood out as being the most important: vitamin A (2,640 IU), vitamin D (200 IU), vitamin E (10 milligrams), vitamin B1 (1.4 milligrams), folic acid (400 micrograms), vitamin C (70 milligrams), zinc (15 milligrams) and iron (30 milligrams).

Low birth weight infants -- those born weighing less than 2.5 kilograms (about 5.5 pounds) --face a greater risk of health problems that extends into adulthood, Shah said. For example, they are more vulnerable to infections as infants, more likely to have developmental problems in childhood, and at greater risk of diabetes and high blood pressure as adults.

Giving pregnant women these vitamins and minerals most likely prevents babies from gaining too little weight by improving their nutrition, strengthening a woman's immune system, and reducing fetal growth restriction, Shah said.

Shah said the WHO should revisit its guidelines to recommend multimicronutrients, not just iron and folic acid, for all pregnant women. "It is important that we implement this change."

In an editorial accompanying the study, Drs. Zulfiqar A. Bhutta and Batool Azra Haider of The Aga Khan University in Karachi, Pakistan, say "maternal undernutrition is one of the most neglected aspects of nutrition in public health globally," and note that a recent study found up to 19 percent of reproductive age women worldwide may be "seriously undernourished."

If studies bear out the findings of Shah and his team, they agree that "multimicronutrient supplementation should replace iron-folic acid supplementation in susceptible populations."