CHICAGO (AP)  -- Women can maximize their chances of having healthy babies by spacing their pregnancies at least 18 months but no more than five years apart, researchers say.

The researchers reached that conclusion after an analysis of 67 international studies involving more than 11 million pregnancies.

The analysis found that spacing babies too close together or too far apart raises the risk of complications such as premature births and low birth weight.

The findings suggest that millions of infant deaths could be avoided worldwide with better family planning, said co-author Dr. Agustin Conde-Agudelo of Santa Fe de Bogota Foundation in Colombia.
 
"The financial cost of birth spacing programs is lower than the financial cost of infant deaths, of course," Conde-Agudelo said in an e-mail.

Pregnancy and nursing use up nutrients in a woman's body, he said, and for a woman to get pregnant again before she has a chance to recover nutritionally may mean higher risks for the baby.

As for why long intervals between births cause problems too, he speculated that time could diminish a woman's reproductive capacity and that factors that decrease fertility also could lead to poor fetal development.

The analysis found that for each month under 18 months between pregnancies, the risk of premature birth increased 1.9 percent. For each month longer than 59 months between pregnancies, the chances of premature birth climbed 0.6 percent.

The analysis appears in Wednesday's Journal of the American Medical Association.

Conde-Agudelo said the risks of short and long birth intervals held true for both developing and developed nations, showing up in U.S. studies and in research from Europe, Africa and Latin America.

The World Health Organization should consider recommending longer intervals between births, said Rachel Royce, an epidemiologist at Research Triangle Institute International in North Carolina who wrote an accompanying editorial in the journal.

Dr. Sarah Kilpatrick of the University of Illinois-Chicago, who was not involved in the research, said doctors should educate women about why it is better to space their pregnancies and make sure they have access to contraception.

"The U.S. is lucky because we have access to many more contraceptive methods than many other countries," she said.

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