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I recently saw the Oscar-nominated Juno, an otherwise enjoyable movie that unfortunately perpetrated the misunderstanding. When Juno discloses her pregnancy to Allison Janney, who plays her mother, Janney responds supportively by making a doctors' appointment and tells Juno to begin prenatal vitamins. I wanted to yell in the crowded theater, "It's too late!"
The human spinal cord and heart form in the first few weeks after conception—thus, defects happen before most women even realize they are pregnant. So taking prenatal vitamins once a woman is already pregnant, like Juno, does nothing to prevent birth defects. You have to take them for several weeks before becoming pregnant. That's a problem since most American pregnancies, like Juno's, are unplanned.
Many grains today are fortified with folic acid, but for complicated reasons, not enough is added to prevent most birth defects even in women who eat grains regularly. That's why the March of Dimes, Centers for Disease Control, and other groups advise almost all menstruating women to take a daily vitamin that containing folic acid.
A helpful note: Folic acid is found in almost any commercial multivitamin, not just "prenatal" vitamins that a gossipy busybody might stumble across in your cabinet.
The second lesson: To prevent the most common major birth defects, health authorities recommend that menstruating women should take daily folic acid even if they are not planning a pregnancy.
Finally, many parents know to hold off on cow's milk until a baby's first birthday, since cow's milk contains very little iron, which babies need to make blood cells. Breast milk contains adequate iron, but most infants today get weaned to formula by the time they're 6 months old. Confronted by a dizzying array of formula choices, up to one-third of parents choose "low iron" formulas, often from unfounded fears that iron causes constipation.