If that little bundle of joy has had such a profound impact on your life that you just can't wait to have another, take a breath. A new study suggests that you might want to wait at least six months before getting pregnant again, and that more than 11 months could be even better. The study found that when the interval between pregnancies was less than six months, the chances of the baby dying, being born prematurely, having birth defects or having a low birth weight were significantly higher than among those who waited more than 11 months between pregnancies. "Recommending optimal pregnancy spacing as a health intervention represents an easy, accessible and low-cost means to improve pregnancy outcomes," said the study's lead author, Dr. Sorina Grisaru-Granovsky, director of the maternal-fetal medicine division in the obstetrics and gynecology department at the Shaare Zedek Medical Center in Jerusalem. The study, published in the December issue of Contraception, recommends more than 11 months as the optimal interval between pregnancies. Past research had suggested that a short time between pregnancies was associated with premature birth, growth restriction in the womb, stillbirth and other causes of early infant deaths and even maternal death. Theories abound as to why a short interval would cause such problems, including depletion of critical nutrient stores in the mother during the initial pregnancy, a hormonal imbalance during the postpartum period and the physical stress associated with breast-feeding, according to the study. To better assess the effects that time between pregnancies might have, Grisaru-Granovsky and her research colleagues reviewed information from an Israeli national database. They included information from 440,838 live births that occurred in Israel from 2000 to 2005 among women who had previously given birth. Of that group, 36,020 women became pregnant again in less than six months, 77,899 were pregnant again six to 11 months later, 124,152 waited 12 to 23 months for a subsequent pregnancy and 158,636 waited 24 to 59 months. Only 44,131 women waited five years or more between their pregnancies. The chances of having a preterm delivery increased by 23 percent in pregnancies where women had waited less than six months, the study found. The odds of having a baby who was small or very small for gestational age increased by about 15 percent. Of more concern, the researchers said, was that the chances of early infant death increased 64 percent in the group that had waited less than six months between pregnancies and 22 percent among those who had waited six to 11 months. The risk of birth defects was 14 percent higher in the group that had waited less than six months. Putting substantially more time between pregnancies, however, wasn't necessarily better, the study found. Women who waited more than five years before having another baby had about a 40 percent higher risk of delivering that baby early. "Pregnancy interval has been reported to influence preterm delivery, weight for gestation age and neonatal death, and this paper probably adds more information because it's such a large study, and they looked at births over time," said Dr. Steven Allen, chairman of obstetrics and gynecology at Scott & White Healthcare in Temple, Texas. "There are problems with a short interpregnancy interval, and six months appears to be the most important time frame," he said. "The good news is that most patients probably have their babies between those time frames anyway." However, he pointed out that because the study wasn't done prospectively, experts still don't really know what the perfect spacing between pregnancies might be. "But waiting more than six months is a relatively reasonable thing to do," Allen said. The important thing to realize, he said, is that having just had a child or breast-feeding offer no natural protection against pregnancy. "If you're making an active decision to avoid two deliveries close together," Allen said, "the logical follow-up is that contraception is important."