Teen-age mothers who are poor, undereducated and do not get prenatal care are known to give birth to babies who are at much higher risk of health problems. But new research points to another possible cause for the high rates of prematurity and low birth weight: the mother's age.
According to a study in today's New England Journal of Medicine, an adolescent's physical immaturity may be a factor affecting the infant's health. While several earlier and much smaller studies have suggested as much, the study of 134,088 Utah women is the first large epidemiological analysis to confirm the association between biological age and birth outcome.
The finding serves as a reminder of the importance of early prenatal care and the need for pregnant teen-agers to adopt healthful lifestyles, said the study's co-author, Alison M. Fraser, of the University of Utah's department of human genetics.
"This message needs to get across to young teens: Not only are they not emotionally ready for a baby, but they are not physically ready," she said.
The study prompted an accompanying editorial in the medical journal warning lawmakers that health and welfare reform proposals restricting benefits to pregnant or parenting teen-agers may backfire. "We suspect that many (adolescents) will still become pregnant, and that they and their infants will be at increased risk for health problems if benefits are reduced or eliminated," the editorial said.
Of about 1 million pregnancies among teen-age girls each year in the United States, about half result in birth, one-third in abortions and the rest in miscarriages.
Health experts have long recognized that pregnant teen-agers are less likely to receive prenatal care and are more likely to be poor and undereducated.
The Utah study analyzed white women ages 13 to 24 who delivered their first babies between 1970 and 1990. The researchers identified the 13- to 17-year-olds who appeared to be best prepared to give birth to healthy babies: They were married, had received adequate prenatal care and had an appropriate level of education for their ages. These adolescents were compared to similar women ages 20 to 24.
Despite the apparent absence of socioeconomic risk factors, the teen-agers under 17 still had almost double the risk of giving birth three or more weeks prematurely or having a low birth-weight baby (less than 5 1/2 pounds) compared to older women. The risk of having a baby born small for its gestational age was almost 1 1/2 times greater for younger teen-agers.
And the incidence of health problems appeared to correlate to the age of the teen-age mothers: The risk was highest among those 13 to 15 and lowest among those 18 and 19.
Overall, 7% of the women under 17 gave birth to low birth-weight babies compared to 4% of the women 20 to 24, while 10% of the younger women and only 5% of the older women gave birth prematurely.
"Our data suggest that adequate prenatal care does not completely eliminate the risks inherent in teen-age pregnancy, presumably because biologic immaturity increases the risk of a poor outcome," the authors said.
Fraser noted that researchers could only infer that the teen-agers with optimal circumstances were not poor. "That's our assumption. . . . Generally speaking, Utah does not have huge pockets where people are dramatically poor."
The researchers could not address whether other risk factors, such as drug or alcohol use, could have caused more adverse outcomes among the babies of younger mothers. But the authors noted that other surveys in Utah have shown very low rates of drug or alcohol use in pregnant women, so "it is unlikely that illicit drug use by pregnant teen-agers influenced our results."
However, other researchers cautioned that the study leaves many questions unanswered. For example, the analysis included no data on the incidence of toxemia, a condition that can lead to premature delivery and low birth weight. Adolescents are at higher risk of developing that disorder, said Dr. Anita Nelson, an expert on reproductive health at Harbor-UCLA Medical Center.
Nelson questioned the assumption that a significant number of the Utah teen-agers were not poor or did not abuse drugs or alcohol.
"There is poverty in Utah," she said. "There is substance abuse, and sometimes young women are not forthcoming about that."
Nevertheless, the study's conclusions are troubling, Nelson said. "Because we've always thought that many things that lead to poor outcomes with adolescents--poor nutrition or smoking--could be dealt with with good prenatal care," she said.
The role that biological immaturity could play in birth outcomes is unclear, Fraser acknowledged. The authors suggested that a pregnant adolescent may still be growing, causing her body to compete for nutrients with the fetus. Or, an immature reproductive system might cause an increased risk of infection, which could affect birth outcome.
In an editorial, Dr. Robert L. Goldenberg of the University of Alabama, Birmingham, suggested that being underweight at conception or during pregnancy might cause the higher risks.
The study findings are also problematic because they contradict earlier research comparing mostly minority teen-agers to older mothers living in inner-city communities. In general, those studies have not found differences in outcomes between the age groups when factors such as marital status and prenatal care are similar.