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ORIGINAL RESEARCH |
From the Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, Dallas, Texas.
Address reprint requests to: Steven L. Bloom, MD, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas,TX75390-9032;E-mail: steven.bloom{at}utsouthwestern.edu.
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ABSTRACT |
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METHODS: Women with consecutive births at our hospital beginning with their first pregnancy were identified (n = 15,945). The first pregnancy was categorized as delivered between 24 and 34 weeks’ gestation or 35 weeks or beyond, singleton or twin, and spontaneous or induced. The risk of preterm delivery in these same women during subsequent pregnancies was then analyzed.
RESULTS: Compared with women who delivered a singleton at or beyond 35 weeks’ gestation in their first pregnancy, those who delivered a singleton before 35 weeks were at a significant increased risk for recurrence (odds ratio [OR] 5.6, 95% confidence interval [CI] 4.5, 7.0), whereas those who delivered twins were not (OR 1.9, 95% CI 0.46, 8.14). The OR for recurrent spontaneous preterm birth presenting with intact membranes was 7.9 (95% CI 5.6, 11.3) compared with 5.5 (95% CI 3.2, 9.4) with ruptured membranes. Of those women with a recurrent preterm birth, 49% delivered within 1 week of the gestational age of their first delivery and 70% delivered within 2 weeks. Among 15,863 nulliparous women with singleton births at their first delivery, a history of preterm birth in that pregnancy could predict only 10% of the preterm births that ultimately occurred in the entire obstetric population.
CONCLUSION: In a population-based study at our hospital, women who initially delivered preterm and thus were identified to be at risk for recurrence ultimately accounted for only 10% of the prematurity problem in the cohort.
A history of a prior preterm birth is generally accepted to be a significant risk factor for recurrence in a future pregnancy. With the recent advent of tests designed to improve the identification of women at risk for preterm delivery, the risk associated with history alone may become inappropriately minimized. In a recent multicenter investigation, for example, the odds ratios (OR) for preterm birth less than 35 weeks’ gestation associated with markers of preterm delivery such as detection of fetal fibronectin in cervical secretions (OR 5.2), ultrasonic shortening of the cervix (OR 4.1), and colonization of the genital tract with bacterial vaginosis (OR 1.3) were all lower than the risk of recurrence based solely upon a history of prior preterm birth (OR 5.8).1
Although a general, nonspecific history of preterm birth is accepted to be a risk factor for recurrence, there is little information on the recurrence risk for specific types of prior preterm deliveries.2 Moreover, given the recent increase in twin gestations,3 it is unclear if spontaneous preterm delivery of twins modifies a woman’s risk for a subsequent preterm birth. Stated differently, does a history of a spontaneous preterm twin delivery convey the same risk for recurrence as does a history of a spontaneous preterm singleton delivery? Lastly, what is the contribution of women with recurrent preterm delivery to the overall problem of prematurity in an obstetric population?
Since 1988, we have collected information on pregnancy outcomes for all women delivering at our institution. With over 10 years of computerized data involving nearly 170,000 women, many of whom with more than one delivery at our hospital, we had the opportunity to analyze the reproductive histories of a cohort of over 15,000 women beginning with their first delivery and including all subsequent consecutive pregnancies. The purpose of this analysis was to measure the risk of recurrent preterm birth based on 1) whether the first delivery was a preterm singleton or twin, 2) the labor was spontaneous or induced, 3) the timing of recurrence, and 4) the overall contribution these women made to preterm births in the study cohort.
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MATERIALS AND METHODS |
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Parkland Hospital is a tax-supported institution serving the general obstetric population of medically indigent women of Dallas County. Although a tertiary neonatal center, there are very few maternal transports for threatened preterm birth. The hospital is staffed by faculty of the Department of Obstetrics and Gynecology of the University of Texas Southwestern Medical School.
Women with a history of a prior abortion, either spontaneous or elective, were excluded, as were pregnancies complicated by fetal malformations. The outcome of the first pregnancy was categorized as either a 1) singleton delivery at or beyond 35 0/7 weeks’ gestation; 2) singleton delivery before 35 weeks’ gestation associated with idiopathic preterm labor; 3) singleton delivery before 35 weeks associated with preterm ruptured membranes; 4) singleton indicated delivery before 35 weeks for complications such as hypertension or obstetric hemorrhage caused by placenta previa or placental abruption; 5) twin delivery at or beyond 35 weeks’ gestation; or 6) twin delivery before 35 weeks associated with spontaneous preterm labor or preterm ruptured membranes. Preterm delivery (24–35 0/7 weeks) in the second pregnancy was the outcome of interest and was analyzed in relation to the outcome in the first pregnancy. The definition of preterm used in this study (24–35 0/7 weeks) was chosen because of the relative rarity of serious neonatal morbidity after 35 weeks.6
2 with calculation of OR was used for statistical analyses. Computations were performed using SAS (version 8) statistical software (SAS Institute, Cary, NC).
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RESULTS |
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ABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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is a comparison of demographic factors in women with or without singleton births less than 35 weeks in their index pregnancy (n = 15,863). African-American nulliparas as well as those women at the extremes of reproductive age were at significantly increased risk for delivery less than 35 weeks (P < .01).
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, the population of women whose first two or more consecutive pregnancies delivered at our hospital (n = 15,945) were divided into six categories based upon their first pregnancy result. The risk of delivery less than 35 weeks in the second pregnancy for women in each of these categories was then analyzed (Figure 2) using those women with first deliveries at or beyond 35 weeks as the referent group. Overall, women with prior singleton births less than 35 weeks were at a statistically significant increased risk for recurrence (OR 5.6, 95% CI 4.5, 7.0); however, this was not true for those whose first pregnancy resulted in twins delivered less than 35 weeks (OR 1.9, 95% CI 0.46, 8.14). The ORs for recurrence of a singleton birth before 35 weeks in women with a history of a spontaneous preterm birth, an indicated preterm delivery, or preterm ruptured membranes were similar (Figure 2).
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). In general, those women whose first birth was a singleton less than 35 weeks were at significantly increased risk for recurrence regardless of the timing of their initial preterm delivery. As also shown in Figure 3, a prior twin delivery, regardless of gestational age, had no significant effect on the timing of subsequent births.
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The number of women having one, two, and three prior consecutive deliveries before 35 weeks’ gestation was 579, 29, and 3, respectively. The percentage of recurrent births before 35 weeks increased incrementally in relation to the number of prior consecutive preterm deliveries. Specifically, this percentage increased from 16% recurrence for women with one prior preterm birth, 41% for two prior preterm births, and 67% recurrence when all three prior pregnancies resulted in deliveries before 35 weeks. Of those women with a recurrent preterm birth, 49% delivered within 1 week of the gestational age of the proximal delivery and 70% delivered within 2 weeks (Figure 4).
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and Figure 5, the 15,863 nulliparous women in the study cohort ultimately delivered a total of 36,365 singleton infants, commencing with their first delivery, for a mean of 2.3 births per woman. Those women whose first birth was before 35 weeks contributed, overall, to more than half of all the births less than 35 weeks that occurred when their first two deliveries were summarized. Alternatively, and as shown in Figure 5, only 10% of the preterm births in the obstetric population occurred in women who were potentially identifiable based on a history of preterm delivery in their first of two pregnancies.
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DISCUSSION |
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Unlike women with prior singleton deliveries before 35 weeks, we found that those delivered of twins before 35 weeks were not at a significantly increased risk for recurrence. Our results differed from those of Menard et al7 who were the first to examine the prognostic significance of a prior preterm twin delivery and who found that women with such a history were at significant risk for recurrence. A possible explanation for our disparate results may be that our study included only those women whose first pregnancy culminated in preterm delivery of twins whereas Menard et al7 did not describe the obstetric history preceding the index twin delivery. It is possible that some of the women studied by Menard et al7 may have had a greater risk for preterm delivery because they had a preterm birth preceding the index twin pregnancy. This would compound the risk of recurrence. Indeed, we analyzed the obstetric histories of 437 women who delivered twins preceded by another delivery at our hospital without regard to birth order. Of the 179 twins that delivered before 35 weeks’ gestation, 29 (16%) had a history of prior preterm birth compared with 14 (5%) of those who delivered beyond 35 weeks (P < .001). Thus, in our population, women with a history of preterm twin delivery were more likely to have had a history of a prior singleton preterm birth. Our results are consistent with a recent report by Rydhstroem8 who performed a population-based study using the Swedish birth registry. Rydhstroem8 found that a preterm twin delivery, regardless of etiology, did not increase the risk of recurrence in a subsequent singleton gestation; however, a prior preterm singleton delivery increased the risk of a subsequent preterm singleton and, to an even greater extent, increased the risk of a subsequent preterm twin.
The difference we observed in the risk of a future preterm birth between women delivered of singletons before 35 weeks’ gestation compared with twins suggests that women in the latter group have risk factors that do not recur, such as uterine overdistention, whereas women with a prior preterm singleton have risk factors that are unmodifiable and therefore act in subsequent pregnancies. This view is consistent with the conclusion reached by Goldenberg et al9 who stated that most of the traditional risk factors associated with singletons are not operative in twin gestations.
We found that the risk of spontaneous preterm birth increased in proportion to the number of prior preterm deliveries. Specifically, we observed that a history of one, two, and three prior consecutive spontaneous singleton deliveries before 35 weeks was associated with recurrence risks in the next pregnancy of 16%, 41%, and 67%, respectively. These findings are consistent with the results of Carr-Hill and Hall10 who, in a large cohort of Scottish women, reported that a history of one or two prior spontaneous preterm births (defined as 21 to 36 weeks’ gestation) was associated with recurrence risks of 19% and 32%, respectively. The majority of the recurrent births before 35 weeks in our study population occurred within 2 weeks of the gestational age of the prior preterm delivery. This observation that there is concordance between the gestational ages of preterm deliveries has also been reported in studies11–13 using Scandinavian birth registries as well as in a population-based study14 in the United States.
The unique finding in this analysis of the epidemiology of preterm births is that if a woman’s delivery career begins with a birth less than 35 weeks’ gestation, these women with an identifiable historical risk factor account for only 10% of the prematurity problem in the total population. Given that American women, on average, deliver 2.0 livebirths in a lifetime,15 identification of women at risk for preterm birth based solely on a history of prior preterm delivery has limited application in predicting preterm births in an obstetric population. This finding serves to emphasize the importance of developing alternative methods to identify women at risk for preterm birth.
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Footnotes |
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Received December 27, 2000. Received in revised form April 23, 2001. Accepted May 24, 2001.
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REFERENCES |
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2. Adams MM, Elam-Evans LD, Wilson HG, Gilbertz DA. Rates of and factors associated with recurrence of preterm delivery. JAMA 2000;283:1591–6.
3. Luke B. The changing pattern of multiple births in the United States: Maternal and infant characteristics, 1973 and 1990. Obstet Gynecol 1994;84:101–6.
4. Jimenez JM, Tyson JE, Reisch JS. Clinical measures of gestational age in normal pregnancies. Obstet Gynecol 1983;61:438–43.
5. McIntire DD, Bloom SL, Casey BM, Leveno KJ. Birth weight in relation to morbidity and mortality among newborn infants. N Engl J Med 1999;340:1234–8.
6. Iams JD, Goldenberg RL, Mercer BM, Moawad A, Thom E, Meis PJ, et al, for the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. The preterm prediction study: Recurrence risk of spontaneous preterm birth. Am J Obstet Gynecol 1998; 178:1035–40.[Medline]
7. Menard MK, Newman RB, Keenan A, Ebeling M. Prognostic significance of prior preterm twin delivery on subsequent singleton pregnancy. Am J Obstet Gynecol 1996; 174:1429–32.[Medline]
8. Rydhstroem H. Gestational duration in the pregnancy after a preterm twin delivery. Am J Obstet Gynecol 1998; 178:136–9.[Medline]
9. Goldenberg RL, Iams JD, Miodovnik M, Van Dorsten JP, Thurnau G, Bottoms S, et al. The preterm prediction study: Risk factors in twin gestations. Am J Obstet Gynecol 1996;175:1047–53.[Medline]
10. Carr-Hill RA, Hall MH. The repetition of spontaneous preterm labour. Br J Obstet Gynaecol 1985;92:921–8.[Medline]
11. Bakketeig LS, Hoffman HJ, Harley EE. The tendency to repeat gestational age and birth weight in successive births. Am J Obstet Gynecol 1979;135:1086–1103.[Medline]
12. Melve KK, Skjaerven R, Gjessing HK, Oyen N. Recurrence of gestational age in sibships: Implications for perinatal mortality. Am J Epidemiol 1999;150:756–62.
13. Kristensen J, Langhoff-Roos J, Kristensen FB. Implications of idiopathic preterm delivery for previous and subsequent pregnancies. Obstet Gynecol 1995;86:800–4.[Abstract]
14. Mercer BM, Goldenberg RL, Moawad AH, Meis PJ, Iams JD, Das AF, et al, for the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. The preterm prediction study: Effect of gestational age and cause of preterm birth on subsequent obstetric outcome. Am J Obstet Gynecol 1999;181:1216–21.[Medline]
15. Ventura SJ, Mosher WD, Curtin SC, Abma JC, Henshaw S. Highlights of trends in pregnancies and pregnancy rates by outcome: Estimates for the United States, 1976–96. National Vital Statistics Report, Vol. 47, No. 29. Hyattsville, MD: National Center for Health Statistics, 1999.
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