Many women in the United State delay marriage to jump-start their careers, but women in numerous other countries tend to follow ancient traditions and marry young, sometimes too young. A recent report by Boston University School of Public Health researcher Anita Raj and colleagues claims that half of Indian women aged 20 to 24 had been brides when they were under 18, the legal age for marriage in India; and more than 22 percent were married before age 16. Although young marriage might be accepted in India and frowned upon by other cultures, the real problem, the study shows, is that the bodies of women that age are simply not ready for reproduction, and so they suffer. Reaching menarche, that is experiencing a menstrual period, is often considered the leap to womanhood for girls. But that leap is much slower than a monthly period might indicate, and not really a sign that a human female is ready to reproduce. Instead, all young women experience a time of "adolescent sterility" during which they have periods, but no ovulation, or when conception is a risky business because the reproductive organs are not yet fully formed in spite of menstrual cycling. Interestingly, the same sort of time where female bodies seem adult, but are not, occurs in our non-humans primate cousins, the monkeys and apes. Young female macaques might display big red swellings on their backsides that in adult females signal ovulation and the possibility of conception, but in young female monkeys it's just practice. These "little girls" might also be in the throes of hormonal urges, but behavioral research shows they have no idea what to do with their urges. A young baboon with her first swelling will walk up to a male, present her bright red behind as if she is ready for sex, and then run away screaming. Underage monkeys do become pregnant, but when they do, but they are sometimes inadequate mothers, and their bodies take a long time to reproduce again. Although mating and pregnancy are the heart of reproductive success (and evolution), nobody said it was the perfect system in any species. Instead, it's a process, and pushing that process only asks for trouble. The girls in Raj's study who married young experienced more unwanted pregnancies, they terminated their pregnancies more often, and more than 13 percent opted for sterilization. No wonder. Marriage under age 18 in India means no contraception and at least three babies born two years apart, a reproductive schedule that would exhaust any woman, under or over 18. Many of those pregnancies also don't turn out well. The March of Dimes reports that babies of teens are more at risk for premature birth and low birth weight, major health issues, and these babies die more often than babies of older mothers. No matter the country, women are simply not ready to reproduce until they are adults, and that means after age 18 or even older when the body has had a few years of menstrual cycles under its belt and has become fully woman, rather than girl. Culturally, we can push reproduction all we want, but biology let's us know the limits.

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Meredith F. Small is an anthropologist at Cornell University. She is also the author of "Our Babies, Ourselves; How Biology and Culture Shape the Way We Parent" (link) and "The Culture of Our Discontent; Beyond the Medical Model of Mental Illness" (link). Her Human Nature column appears each Friday on LiveScience. [Human Nature Column Archive]