Iron is crucial because women build up blood volume during pregnancy, and deficiencies can lead to anemia, which “increases the risk of having a low birthweight baby, and increases the risk of preterm labor and delivery,” explained Dr. Clark, a maternal-fetal medicine specialist at the University of Texas Medical Branch in Galveston.

Expecting women need double the iron a woman who isn’t pregnant needs, according to A.C.O.G. So during pregnancy, vegetarians and vegans should take special care to eat plenty of iron-rich foods, like dried beans and peas and fortified cereals. Because the iron in plant-based foods is not as easily taken up by the body as the iron from meat, you should “cook them, soak them” or eat them with foods high in vitamin C to increase absorption, said Susan Levin, the director of nutrition education for the Physicians Committee for Responsible Medicine, an advocacy group that promotes a plant-based diet. Prune juice is also high in iron. Ms. Levin said she’d encourage any pregnant woman “to take the prenatal vitamin and focus on iron-rich foods such as spinach, lentils, beets and raisins.”

B vitamins, including vitamin B12 and folic acid, are also critical in pregnancy. Vitamin B12 is not found in plants, but it’s in tofu, soy milk, some cereals and nutritional yeast, which some vegans eat as a cheese substitute. Folic acid prevents neural tube defects to the spine and brain, which occur in the first month of pregnancy, so the United States Preventive Services Task Force recommends that any woman of childbearing age take a daily supplement of folic acid of 400 micrograms daily, at least.

Being deficient both in vitamin B12 and folic acid “has been shown to further increase the risk of having a baby with neural tube defects,” Dr. Clark said. Physicians often advise women to take prenatal vitamins, which usually contain iron, vitamin B12, folic acid and other nutrients.

Not eating meat during gestation may even have upsides. A vegetarian diet in the first trimester was linked to a lower risk of excessive gestational weight gain, a 2010 study found. Furthermore, Ms. Levin said, “Maternal diets high in plant foods may reduce risk of complications, including gestational diabetes.” This includes both vegetarians and meat-eaters who eat a lot of vegetables during pregnancy, she said.