Decent interval: well-spaced babies may have advantage.

Babies conceived 18 months to 5 years after a previous birth are healthier than those conceived at shorter or longer intervals, a massive data analysis shows.

Studies to determine the optimal spacing between a birth and the mother's next pregnancy have yielded mixed results. In an attempt to settle the question, researchers in Colombia analyzed 67 studies that included more than 11 million births worldwide.

Compared with babies conceived 18 to 23 months after a sibling's birth, those conceived within 6 months were 40 percent more likely to be born prematurely, 61 percent more likely to be underweight (less than 2.5 kilograms, or 5.5 pounds), and 26 percent more likely to be small for their gestational age at birth. Babies conceived between 6 and 17 months after their mothers' previous birth also had a slightly increased risk of the three health measures, whereas babies conceived 24 to 59 months after a previous birth had no increased risk.

Premature birth, low birthweight, and small size for gestational age are all risk factors for poor health in infancy and can lead to complications later in life.

Health officials should counsel women who've just given birth to delay their next conception by 18 to 59 months, concludes study leader Augustin Conde-Agudelo of the Santa Fe Foundation in Bogota.

"This is the first, and certainly the most ambitious, attempt to do this sort of rigorous meta-analysis" of the birth interval, says physician C. Johannes van Dam of the Population Council in New York City.

Curiously, babies conceived 5 years or more after a birth were 20, 29, and 43 percent more likely to be premature, small for gestational age, or underweight, respectively, than were the babies conceived after 18 to 23 months, the researchers report in the April 19 Journal of the American Medical Association.

It's unclear why a long interval would impart such risks.

The problems of a baby born after a short interval are better understood. The second baby might suffer because the mother is nutritionally depleted and physically stressed by simultaneously recovering from the first birth, handling an infant, and carrying a new fetus, van Dam says.

Epidemiologist Mark A. Klebanoff of the National Institute of Child Health and Human Development in Bethesda, Md., questions whether getting pregnant soon after a birth is the primary factor that places a pregnancy at high risk. "Is there something about women who get pregnant right away that imparts the risk?" he asks.

He cites earlier research showing that women with very long or short birth intervals are more likely to be unmarried and have less education than other women. Although Conde-Agudelo's team tried to account for these and other differences, many of the studies in their analysis relied on birth certificates, which, Klebanoff says, "are notoriously incomplete and inaccurate."

Worldwide, roughly half of all births are unplanned. Doctors should counsel new mothers against having another child right away, van Dam says. The best results will come in countries with birth control programs and prevalent breast-feeding, which naturally delays a woman's return to fertility.