Adolescent women having their second birth are at elevated risk of giving birth either moderately or very prematurely, as well as of experiencing a stillbirth, compared with women aged 20-29. According to results from an analysis of Scottish hospital discharge data for the period 1992-1998, adverse outcomes for second teenage births were generally found both among current smokers and among nonsmokers. However, there was no relationship between age and birth outcomes among nonsmokers having a first birth. Teenage mothers, regardless of their parity or smoking status, were significantly less likely than older women to require an emergency cesarean section.¹

Past research has suggested that teenagers are more likely to experience complications when having their first birth than are older women, although it is unclear if this situation reflects a biological effect or is caused by differences between younger and older women in socioeconomic status or health behavior. Similarly, while studies have indicated that second births are also more likely to be problematic for younger than for older women, inability to control for important confounding variables has left the true extent of an association unclear.

Researchers in Scotland utilized a database of morbidity records to identify all singleton births (live births and stillbirths) that occurred between 1992 and 1998. For their main analysis, they selected women aged 15-29 who had never smoked, had a first or second birth at 24-43 weeks and bore an infant weighing more than 500 g. A second sample was then drawn, comparable to the first, from among women who were current smokers at the time they commenced prenatal care.

Six birth outcomes were studied. The researchers categorized babies whose birth weight was in the lowest 5% of newborns at their gestational age (according to values calculated for Scottish live-born infants during the study period) as small for gestational age. They classified live births occurring at 24-32 weeks of gestation as very premature, and those occurring at 33-36 weeks as moderately premature. Stillbirths were babies born dead at or after 24 weeks, while neonatal deaths comprised live-born babies who died within 28 days of delivery. Finally, all unplanned cesarean sections were classified as emergency cesareans.

The analyses were based on 9,699 first births and 1,225 second births to women aged 15-19, and on 59,315 first births and 39,994 second births to 20-29-year-olds. The investigators used multivariate analysis to control for the effects of maternal height, degree of socioeconomic deprivation, previous spontaneous or induced abortion, and year of delivery. (Analyses of second births also controlled for the effect of a previous perinatal death.)

Among nonsmokers having a first birth, there were no statistically significant differences in most outcomes between women aged 15-19 and those aged 20-29: The two groups had comparable odds of having an infant with a birth weight in the lowest 5%, a very premature or moderately premature delivery, a stillbirth and a neonatal death. The odds of an emergency cesarean section were significantly reduced for women aged 15-19, however (odds ratio, 0.5).

In contrast, among nonsmokers having a second birth, the odds of several complications were increased. Compared with women giving birth at ages 20-29, women aged 15-19 had significantly higher odds of a very premature delivery (odds ratio, 2.5), a moderately premature delivery (1.6) or a stillbirth (2.6). As with the women having a first birth, teenage mothers having a second birth had significantly lower odds than older mothers of needing an emergency cesarean section (0.7).

The results for roughly 70,000 smokers also showed different odds of adverse outcomes depending on women's age and parity: Relative to women aged 20-29, teenagers giving birth for the first time had slightly higher odds of a moderately premature delivery (odds ratio, 1.1) and lower odds of bearing a baby who was small for gestational age (0.8) or of needing an emergency cesarean delivery (0.5). In contrast, teenage smokers having a second birth had significantly elevated odds of having a very premature birth (2.1), a moderately premature delivery (1.5) or a neonatal death (2.5). Teenagers' odds of having an underweight baby or of needing a cesarean delivery were significantly reduced relative to the odds among older women (0.8 and 0.7, respectively).

The researchers suggest that given their findings of no elevated risks of poor birth outcomes among nonsmoking teenage mothers having their first birth, it is possible that previous investigators who found a relationship between age and birth outcomes did not adjust adequately for the effects of cigarette smoking. In contrast, they note, their findings among both nonsmokers and smokers having second births "suggest a causal relation between [a] second teenage birth and adverse pregnancy outcome."

The authors observe, though, that this association is not necessarily attributable to a short interval between births among the teenage mothers, since the size of the effect is greater than that seen in past studies of short birth intervals. Moreover, the outcome most strongly linked to short intervals in other studies--an infant born small for gestational age--was not significantly related to age at birth in their analysis. They conclude that only a prospective study will be able to determine whether biological or social factors are behind the associations they found.--M. Klitsch

REFERENCE

1. Smith GCS and Pell JP, Teenage pregnancy and risk of adverse perinatal outcomes associated with first and second births: population based retrospective cohort study, BMJ, 2001, 323(7311):476-479.