Properly spacing pregnancies may improve the chance of having healthier babies. Researchers analyzed 67 studies from around the world and found that intervals between pregnancies briefer than 18 months or longer than 59 months--counting from delivery to next conception--increased the risk of low birth weight and preterm birth. Waiting a long time between pregnancies wasn't as problematic as having the next baby too soon: Intervals shorter than six months boosted risk of preterm birth by 40 percent and low birth weight by 61 percent. "If you conceive too quickly after giving birth, your nutritional stores may be too depleted to nourish another fetus," says researcher Agustin Conde-Agudelo, M.D., M.P.H. "Wait too long, and your reproductive capacity could decline."