When I asked my OB what I should do about prenatal vitamins—those revered nutritional supplements—he shocked me by reaching under his desk and pulling out the garbage can.

"Put them in here," he said.

My doctor's advice made my jaw drop.

On one hand, that was a huge relief, as the hyperemesis gravidarum (extreme morning sickness) that characterized all my pregnancies prevented me from swallowing almost any pill.

But I thought you were a bad mother if you didn't take prenatal vitamins. In fact, before I became so ill, I purchased a jug of prenatal vitamins that included every possible herb that might benefit my brand new embryo. Because you're better safe than sorry, right? Shouldn't everyone take a multivitamin?

According to my doctor, that's not right. And he's not alone. In February, a study published in Archives of Internal Medicine showed that daily multivitamin use doesnt ward off cancer or heart disease, even though a nutrient-rich diet does.

Charles Lockwood, MD, the chief of obstetrics and gynecology at Yale–New Haven Hospital, confirmed my doctor's advice: "If you eat a balanced diet and are not iron deficient, it is not clear that prenatal vitamins result in any health benefits during pregnancy."

My doctor then referred me to Williams Obstetrics, the essential source of information for ob-gyns. In the 17th edition, I discovered this passage:

"The practice of supplying vitamin supplements prenatally is a deeply ingrained habit of many obstetricians, even though scientific evidence to show that the usual vitamin supplements are of benefit to either the mother or her fetus is quite meager."

Why doesn't anyone know this? I read on to discover the Committee on Maternal Nutrition of the National Research Council pointed out that pregnancy supplements are of doubtful value, "except for iron and possibly folic acid."

So we're not completely off the hook—we still benefit from popping a few pills. But which pills, and how much? That seems to be quite a topic of debate. What does seem certain is that we're better off taking our vitamins more discriminatingly, if we take them at all.

For example, if we take large doses of vitamin C, we could give our unborn babies scurvy, according to William Obstetrics. And supplementing iodine beyond our usual salt intake can "induce a sizeable goiter in the fetus." Yikes!

I learned that the pills I should take while pregnant include the iron and folic acid mentioned in Williams Obstetrics, as well as the new darling of nutritional experts, vitamin D.

Next Page: The trifecta of vitamins [ pagebreak ]Iron
By midpregnancy, women need approximately 7 milligrams more of iron per day, according to Williams Obstetrics, bringing their recommended intake to 27 milligrams. Pregnancy increases your volume of blood, and very few women have sufficient iron stores (or iron-rich diets) to meet this new demand.

For this reason, the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists endorse the recommendation by the National Academy of Sciences that women take a ferrous iron supplement daily.

Typical prenatal multivitamins often contain calcium and magnesium, which inhibit the absorption of the iron you actually need (but only after the first few months of pregnancy, according to Williams Obstetrics' 22nd edition). My own pregnancy anemia didn't kick in until I was about five months along, and I finally had great success taking an iron supplement alone.

And, contrary to popular belief, a calcium supplement isn't recommended for routine use during pregnancy. Simply getting the 1,000 milligrams of calcium recommended for all females should be sufficient. The fetus only uses "2.5% of the maternal calcium, most of which is in the bone, and which is readily mobilized for fetal growth." It's not only unnecessary, but it prevents the absorption of iron, which you actually do need.

Folic acid
There is little debate that a certain amount of folate is recommended during early pregnancy. "I would recommend extra folate in the periconceptional period [before conception through early pregnancy] as this has been shown to reduce the occurrence of spina bifida and congenital heart defects," Dr. Lockwood says.

Apparently, 1 milligram of folic acid a day is ample to help raise the odds of a healthy baby, but if a woman hasn't taken this supplement preconception or in early pregnancy, there is no need to panic. Folic acid is readily available in many foods you're probably already eating—spinach, fortified cereals, and white beans. The March of Dimes has a helpful website with a list of those foods and additional recommendations.

Vitamin D
The research regarding pregnancy and vitamin D is relatively new, and fascinating. The journal Fertility and Sterility recently published a study linking replete stores of vitamin D to successful outcomes in in vitro fertilization.

In my next entry, I'll lay out what I learned from Ludna Pal, MD, one of the authors of the study, about the connections this vitamin may have to conception and health.