Women who conceive less than 18 months after giving birth are about 10-50% more likely to have a very or moderately premature infant than are women whose interpregnancy intervals are between 18 and 59 months. Women for whom the interval between a delivery and the next conception is 60 months or more have a similarly elevated risk of giving birth prematurely. Fifty-five percent of women in the study on which these findings are based fell into one of these two categories of risky interpregnancy intervals.1
To explore the relationship between interpregnancy interval and prematurity, researchers used information from a nationwide data set linking births and infant deaths. They analyzed 289,842 singleton infants born between January 1, 1991, and September 1, 1991, to Hispanic women of Mexican origin and white non-Hispanic women who had had at least one previous live birth. Women from these ethnic groups were chosen because they have similar prematurity rates. The researchers accounted for possible community and temporal influences on birth outcomes by matching Hispanic and white women who lived in the same county and delivered in the same month in a one-to-one ratio.
Infants were defined as extremely premature if they had a gestational age of less than 23 weeks, very premature if they were born at 23-32 weeks' gestation, moderately premature at 33-37 weeks, term at 38-42 weeks and postterm at more than 42 weeks. After comparing maternal characteristics and prematurity rates according to interpregnancy interval (as calculated from birth certificate data), the researchers conducted logistic regression analyses to assess the independent effects of various factors on the risk of having a very or moderately premature infant.
Thirty-seven percent of women had interpregnancy intervals of less than 18 months (which were further categorized as less than six months, 6-11 months and 12-17 months). Forty-six percent had intervals of 18-59 months (categorized in the bivariate analyses as 18-23 months, 24-29 months, 30-35 months, 36-47 months and 48-59 months). The remaining 18% had intervals of 60 months or longer.
Women with short interpregnancy intervals had more risk factors than women whose interpregnancy intervals were 18-59 months. Those with less than 12 months between a delivery and the next conception were significantly more likely to be Hispanic (45-63%) than were women whose intervals were in the 18-59-month range (43-53%). Women whose interpregnancy intervals were less than 18 months were younger, were less educated, had higher parity and had received less prenatal care than women whose intervals were 18-59 months. For example, 50-67% of those with the shortest intervals were aged 26 or younger, compared with 33-46% of those with 18-59-month intervals; 21-26% and 17-20%, respectively, had had more than three children. The proportions who received inadequate prenatal care were 24-42% among women with short intervals, compared with 18-20% among those with intervals of 18-59 months.
The risk profile of women with interpregnancy intervals of more than 59 months differed little from that of women with intervals of 18-59 months. These women were significantly more likely to be Hispanic and were older than women with 18-59-month intervals.
At the bivariate level, women whose interpregnancy intervals were less than 18 months were significantly more likely to give birth prematurely than women whose intervals were 18-59 months. Some 13-15% of births to women with short interpregnancy intervals were moderately premature, and 1-2% were very premature. Among women with 18-59-month intervals, 11-13% of births were moderately premature and 1% were very premature.
In the logistic regression analysis, the investigators took into account all factors that were significant at the bivariate level, plus the woman's birthplace (United States vs. elsewhere), whether she had previously had an infant who was premature or small for gestational age, and the sex of the most recent baby. Women with short interpregnancy intervals were significantly more likely to have had a very premature infant (odds ratios, 1.3-1.5) and to have had a moderately premature infant (1.1-1.2) than were women with intervals of 18-59 months. Those with interpregnancy intervals longer than 59 months were also at increased risk of having had a very premature infant (1.5) or a moderately premature infant (1.1).
The risk of having very and moderately premature infants was also somewhat elevated (odds ratios, 1.1-1.3) for Hispanic women, women with 9-11 years of education (as compared with 12 years of schooling), women who had borne more than three children (as opposed to two) and those who had a boy. Somewhat higher risks (1.5-4.9) were found for women 15-17 years of age, as compared with ages 18-26, and for women who had previously had an infant who was premature or small for gestational age. Women who had had inadequate prenatal care (as classified according to the Kotelchuck Adequacy of Prenatal Care Utilization Index) were significantly more likely than those with adequate care to have a very premature (4.0) or moderately premature infant (2.2); the risks of these outcomes were sharply higher among women who had had "adequate plus" care (11.4 and 6.9, respectively). By contrast, women who were 27-34 years old, were foreign-born or had more than 12 years of education had significantly reduced odds of having had a premature infant (0.6-0.9).
The investigators point out that while short interpregnancy intervals are associated with an elevated risk of low birth weight, findings regarding their effect on prematurity have been inconclusive. Although the researchers acknowledge that the results of their study do not prove causality and that other factors were associated more strongly with risk, they conclude that both short and long intervals are associated with an elevated risk of prematurity. They suggest that since "interpregnancy intervals are a potentially modifiable risk factor for low birth weight, ...childbearing women and health care providers should be informed about the importance of family planning and pregnancy spacing."--L. Gerstein
1. Fuentes-Afflick E and Hessol NA, Interpregnancy interval and the risk of premature infants, Obstetrics & Gynecology, 2000, 95(3):383-390.