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For decades, doctors have wondered why teenage mothers tend to have premature babies and babies who weigh less at birth - that is, generally less-healthy children. Is the cause purely social, or does biology play a part as well?

A study by three Utahns, published Thursday by the New England Journal of Medicine, Boston, apparently has the answer: The mother's age itself is a factor, regardless of social conditions."We can only speculate" about the precise reason for that, said the study's principal author, Alison M. Fraser, research associate at the University of Utah's Department of Human Genetics. "It could be because the mother's still growing, there's a competition for nutrients with the fetus."

Other possibilities are that the young woman's uterus is not mature, or that problems with the cervical blood supply may predispose the young teen mothers to infection, she said. Mild infections could prompt the mothers' bodies to produce hormones that make premature birth more likely.

Joining Fraser in the study, "Association of young maternal age with adverse reproductive outcomes," were John E. Brockert with the Utah Department of Health and Ryk Ward of the University of Utah.

Approximately 10 percent of 15- to 19-year-old girls get pregnant, according to the report. Birth to mothers in that age group account for about 13 percent of all live births in the country.

This is of concern to health professionals because teen mothers have an increased risk of low birth-weight babies, premature babies and babies who die within the first year of life, the study says.

"It's fairly well-known that teenagers who get pregnant are more likely to be poor and unmarried and have inadequate education, and they're less likely to get adequate prenatal care," Fraser said Thursday.

What has been controversial is whether the tendency for young mothers to have smaller or premature babies is because of social factors alone, or "whether there's some increased risk due to their biological immaturity," she said.

Utah was a perfect place to test whether the differences were because of poor social conditions or the mothers' immature bodies. That's because this state's mothers are largely married, with healthful lifestyles and appropriate education for their age, she said.

Among the states, Utah is 47th in percentage of children living below the poverty line, and 10th in proportion of high-school graduates.

The rates of drinking, smoking and drug use among mothers-to-be are much lower here than nationwide, Fraser said.

"We were able to control and essentially eliminate the compounding effects" of outside causes so the study could focus on the differences resulting from age of the mother, she added.

The team used computers to extract information about birth weight, race and mother's age from Utah birth certificates covering a 20-year period. Nearly 753,000 babies born in the state from 1970 to 1990.

The study further selected groups to compare by focusing on the 130,508 who were white, first-born children not part of multiple births, who were born to mothers 13 to 24 years old.

Altogether, 90,315 were born to mothers ages 20 to 24. Mothers under 17 totaled 15,106, and those 18 and 19 amounted to 28,667.

The scientists discovered what their paper terms a "significantly higher risk" for babies of young mothers.

Teen mothers 13 to 17 "were almost twice as likely to have premature babies as a comparable group of mothers ages 20 to 24," she said. "And they also had a 70 percent increase in their risk for low birth weight.

"This indicates that young age alone may be an independent risk factor for adverse outcomes of pregnancy."

Researchers have been developing the data base for the study since 1987 or 1988, she said. "The actual study itself didn't take that long, but it's a long process."

Use of computers speeded the process, she said.

Young teen mothers in the best circumstances - that is, with good prenatal care, husbands, educations appropriate to their ages - have a bit over a 5 percent risk of low birth-weight babies. For a comparable group of mothers ages 20 to 24, the risk is about 3 percent. For premature infants, mothers 20 to 24 have a risk of around 4 percent, while the younger teens face a 7.5 percent risk.

"That's in the best possible circumstance," Fraser told the Deseret News. Factors such as poor prenatal care cause the risks to increase.

Asked at what age a woman is biologically mature enough to bear a healthy child, Fraser said, "I think more research needs to be done in this area."

Some women may be ready at age 18, some even at 16, while others might not be until 20. "I'd say to be safe, 20 to 24 seems to have the lowest rate of these adverse outcomes," she said.