It's a safe bet that Dr. David Barker has never craved pickles and ice cream. Still, if there's one man in the world who understands the peculiar dietary needs of pregnant women, it's him. Barker rose to fame in the '90s as the main backer of "fetal programming," the idea that conditions in the womb, influenced by the mother's diet, can determine a baby's likelihood of chronic adult disease. Today the Barker Hypothesis is the subject of hundreds of studies--and one very comprehensive guidebook, "The Best Start in Life," just out in England and available at amazon.co.uk. On the surface, much of the advice in Barker's book seems obvious (there can't be a woman in the world who thinks eating nothing but rice cakes is good for a fetus--well, at least not outside of Hollywood). But some issues are more complex than they first seem. Dieting during pregnancy, or even a few months before conception, creates an in utero environment short on the nutrients a baby needs for organ growth. The fetus may react by slowing its metabolism to conserve what energy-rich fat it can--which could lead to obesity, heart disease and diabetes later in life. Or it may put most of the scarce energy it gets into brain-building, not leaving enough for other organs like the liver. On the flip side, fetuses that are overnourished, particularly in late pregnancy, may also be at risk for adult obesity and breast cancer. And high sugar levels may stall placental growth. The main message: stick to a balanced diet, and lay off the ice cream, pickles or no. Your baby may thank you later.