Studies suggest inherent risk of poor pregnancy outcomes for teenagers

Family Planning Perspectives, Nov 1995 by Hollander, Dore

Teenagers may be intrinsically more likely than older women to have adverse pregnancy outcomes, even if they receive adequate prenatal care, according to the results of research conducted in Utah.(1) In a population that reflected the characteristics of white, middle-class Americans, teenagers were significantly more likely than women in their early 20s to deliver an infant who was underweight, premature or small for gestational age: Additionally, a California study suggests that the risk of congenital malformations is elevated among infants born to teenagers.(2) The findings from both studies raise the question of whether young maternal age, irrespective of background or behavioral characteristics, increases the risk of poor pregnancy outcomes.

Pregnancy Outcomes

In the Utah study, the investigators examined vital statistics data for 134,088 white women aged 13-24 who delivered singleton, first-born infants between 1970 and 1990. Mothers in Utah generally are white and married, receive adequate prenatal care and tend not to smoke or use alcohol or illicit drugs; consequently, they tend to be at low risk for adverse pregnancy outcomes. Therefore, the researchers suggest, data on underweight, premature and undersized infants born to this population may provide insights into the biological risks associated with teenage pregnancy.

The investigators categorized infants as low-birth-weight if they weighed less than 2,500 g at delivery, as premature if their gestational age was less than 37 weeks and as undersized if their birth weight was below the 10th percentile for their gestational age and gender. They compared data on infants born to women aged 13-17 and aged 18-19 with data on those born to women 20-24 years old.

To offset the potentially confounding effects of socioeconomic variables, the analysts classified the data according to the mother's marital status, educational level (defined as appropriate of inappropriate for her age) and adequacy of prenatal care (as determined by the trimester in which she first sought care and the number of visits she made for care). They assumed that women with the most favorable background (those who were married, had an age-appropriate level of education and received adequate prenatal care) would be the least likely to have adverse pregnancy outcomes; poor outcomes among this group, they suggest, could be attributable to underlying biological factors.

Within the study sample, 11% of births were to women aged 13-17, 21% were to older teenagers and 67% were to women aged 20-24. Young teenage mothers were the least likely to be married (62%, compared with 79% of older teenagers and 94% of those in their early 20s) and to have received adequate prenatal care (52%, as against 62% and 76% of those aged 18-19 and 20-24, respectively). Mothers aged 13-17 and those aged 20-24 were more likely than 18-19-year-olds to have obtained an age-appropriate level of education (95-96% vs. 85%).

Young teenagers had the highest incidence of poor outcomes: They were the most likely to have had a low-birth-weight baby (7%), a premature delivery (10%) or an undersized infant (14%). Women aged 20-24, on the other hand, were the least likely to have experienced these outcomes (4%, 5% and 10%, respectively). In each age-group, the frequency of these outcomes was highest among women who were unmarried, whose educational level was not age-appropriate and, particularly, who had received inadequate prenatal care: Those who had had inadequate care were at least twice as likely as those who had gotten adequate care to bear a low-birth-weight or premature infant.

The high risk for young teenagers was apparent even among those with the most favorable socioeconomic characteristics. In this group, mothers aged 23-17 had 1.7 times the risk of those aged 20-24 of bearing a low-birth-weight infant, 1.9 times the risk of delivering prematurely and 1.3 times the risk of having an undersized infant; for older teenagers, the relative risks were 1.2 1.5 and 1.1, respectively. The researchers also found that women aged 13-15 were significantly more likely than those aged 16-17 to have adverse outcomes.

Analysis of data on smoking during pregnancy (which were available for women who gave birth in 1989 and 1990) showed that even among nonsmokers, teenagers had a higher risk of adverse outcomes than women aged 20-24; however, only young teenagers' risk of having a low-birth-weight infant was significantly elevated.

The researchers note that their data did not include information on psychosocial characteristics that may increase the risk of adverse pregnancy outcomes or on illicit drug use, but they add that in this sample, biases due to these data limitations are likely minimal.

In discussing their results, the analysts suggest that two "general features of biologic immaturity" may help explain the apparent elevated risk of adverse pregnancy outcomes among teenagers: Young gynecologic age may predispose a woman to infection, increased prostaglandin production and thus an elevated risk of premature delivery; also, a pregnancy that occurs while the mother is still growing may cause her to compete with the fetus for nutrients. Therefore; the investigators conclude, reducing the incidence of poor pregnancy outcomes among teenagers will require identifying the biological factors that contribute to these outcomes and taking steps to minimize their effects.