Objective
The purpose of this study was to evaluate the effect of earliest previous spontaneous preterm birth (SPTB) gestational age on cervical length, pregnancy duration, and ultrasound-indicated cerclage efficacy in a subsequent gestation.
Study Design
Planned secondary analysis of the National Institute of Child Health and Human Development–trial of cerclage for cervical length of <25 mm. Women with at least 1 previous SPTB between 17-33 weeks 6 days of gestation underwent serial vaginal ultrasound screening between 16 and 23 weeks 6 days of gestation; cervical length at qualifying randomization evaluation was used.
Results
We observed a significant correlation ( P = .0008) between previous SPTB gestational age and qualifying cervical length. In a linear regression model that was controlled for cervical length and cerclage, neither previous SPTB gestational age nor the interaction between cerclage and previous birth gestational age was significant predictor of subsequent birth gestational age.
Conclusion
Although there is an association between previous SPTB gestational age and cervical length in women with a mid-trimester cervical length of <25 mm, there does not appear to be a disproportionate benefit of cerclage in women with earlier previous SPTB.
The rate of preterm birth (PTB) is increasing; in 2007, it reached 12.3%. Previous spontaneous PTB (SPTB) is considered 1 of the strongest historic risk factors for recurrent SPTB. Previous reports have linked the gestational age at previous SPTB to the gestational age at subsequent SPTB and have determined that the risk of SPTB at <37 weeks in the current gestation increased with decreasing gestational age of the earliest previous spontaneous preterm delivery. In 1 large observation study, gestational age strata were defined as <28, <30, <32, <35, and <37 weeks. An early previous SPTB between 23 and 27 weeks was associated with recurrence in the current gestation with relative risks of 3.1, 2.7, and 2.4 for earliest previous delivery at 23-27, 28-34, and 35-36 weeks, respectively. The strongest association between history and current outcome was found for early previous SPTB and early SPTB in the current gestation with 22.1-fold increase (95% confidence interval, 4.6–106.9) in the relative risk for early delivery (5.1% vs 0.23%; P = .001.) Other studies have confirmed the inverse relationship between increasing risk of recurrent SPTB and decreasing age of previous SPTB.
We conducted a multicenter randomized trial to evaluate the utility of cerclage for shortened cervical length in women with a history of SPTB. Cervical length shortening has been shown to increase the risk of SPTB in high-risk women with a previous spontaneous SPTB. Cervical length in a subsequent pregnancy also correlates well with gestational age at previous preterm delivery. We have shown that cerclage prevents recurrent PTB in women with both a previous spontaneous PTB at 17 weeks to 33 weeks 6 days of gestation and a short cervical length of <25 mm that was identified between 16 weeks and 22 weeks 6 days of gestation. The findings of this clinical trial support the conclusion of a recent metaanalysis of 4 previous randomized trials: cerclage placed for cervical shortening significantly reduced preterm birth at <35 weeks of gestation by 39% in women with a previous PTB.
In a secondary analysis from the same randomized, controlled trial, we examined the natural history of cervical length shortening in women who had at least 1 SPTB between 17 weeks and 33 weeks 6 days of gestation. We compared the rate of cervical length shortening and the time to shortening for women whose earliest previous PTB was in the mid trimester, which was defined as <24 weeks of gestation, vs those at 24-33 weeks. Similar comparisons were performed on the basis of each patient’s most recent birth history. In both cases, we found that women with a previous SPTB at <24 weeks of gestation are at a higher risk of cervical shortening and do so at a higher rate and at an earlier gestational age than do women with a later PTB history.
We sought in this investigation to estimate whether information regarding earliest gestational age at previous PTB would be informative for clinical management, because surveillance of cervical length in women with previous PTB is advantageous for risk assessment for recurrent PTB. Our null hypothesis is that there are no significant interaction effects between gestational age at previous SPTB and cerclage efficacy on subsequent gestational age at delivery. This analysis addressed whether cerclage intervention that is based on degree of cervical shortening and whether consideration of previous birth gestational age would offer relevant information once cervical shortening had been identified.
Materials and Methods
This was a planned secondary analysis of the National Institute of Child Health and Human Development–sponsored randomized trial that evaluated cerclage for women with singleton gestations, previous SPTB (17 weeks to 33 weeks 6 days of gestation), and cervical length <25 mm that had been measured with serial transvaginal ultrasound (TVU) evaluations between 16 weeks and 22 weeks 6 days of gestation. This trial was performed at 15 US Clinical Centers between January 2003 and November 2007. Each Center obtained Institutional Review Board approval. The methods and materials are described in detail in the report of the parent trial. Importantly, healthy, multiparous women with at least 1 previous SPTB between 17 weeks and 33 weeks 6 days of gestation were recruited. Our process included confirmation of the obstetric history by a review of the subject’s medical records. When efforts to retrieve the records of the previous birth were unsuccessful, we accepted women as eligible if the events surrounding the previous birth included spontaneous causes (such as preterm labor or preterm membrane rupture) and the reported birthweight was <2 kg. Exclusion criteria were fetal anomaly, planned history-indicated cerclage, and clinically significant maternal-fetal complications. Gestational age was always confirmed by standard sonographic biometric measurements at <20 weeks’ gestation. Sonologists underwent a uniform certification process by a single investigator (J.O.) to ensure uniformity in sonographic measurements of TVU cervical length screening.
Women with previous SPTB were screened with TVU cervical length starting at 16 weeks to 21 weeks 6 days of gestation, then every 2 weeks until 22 weeks 6 days of gestation, unless the cervical length was observed to be 25-29 mm, after which the scan frequency was increased to weekly. Women who had a cervical length <25 mm at 16 weeks to 22 weeks 6 days of gestation were assigned randomly to cerclage or no cerclage after having given informed consent.
We examined the relationship between previous PTB gestational age and the shortest cervical length to be observed before randomization. The associations between previous PTB gestational age and the gestational age of this delivery were examined with linear regression. The effects of cervical length, cerclage group, and the interaction between previous birth gestational age and cerclage group were also considered in multivariable linear regression models. χ 2 and Student t tests were used to evaluate demographic characteristics. We selected an alpha level of < .05 to represent statistical significance.
Results
Of 1014 women with previous SPTB who were screened with TVU cervical length at 16 weeks to 22 weeks 6 days of gestation, 318 women had a cervical length <25 mm, of which 302 women agreed to the randomization. Of these, 301 women with birth outcomes were available for this analysis: 148 women were assigned randomly to cerclage, and 153 women were assigned randomly to no cerclage. The demographic and sonographic characteristics for these subjects can be found in the Table .
| Variable | Group | ||
|---|---|---|---|
| Cerclage (n = 148) | No cerclage (n = 153) | P value | |
| Maternal age, y a | 26.4 ± 5.5 | 26.6 ± 5.1 | .75 |
| Previous births, n b | 2 (1–4) | 2 (1–4) | .66 |
| Race/ethnicity, n (%) c | .36 | ||
| Black (non-Hispanic) | 80 (54) | 93 (61) | |
| White (non-Hispanic) | 25 (6.9) | 28 (18) | |
| Hispanic | 27 (8.2) | 17 (11) | |
| Asian | 1 (0.7) | 0 | |
| Other | 15 (0.1) | 15 (9.8) | |
| Gestational age of earliest previous preterm birth, wk a | 24.2 ± 4.8 | 24.5 ± 4.7 | .58 |
| Gestational age of most recent birth, wk a | 26.4 ± 6.7 | 27.1 ± 6.5 | .37 |
| Gestational age at randomization, wk a | 19.4 ± 1.9 | 19.5 ± 2.0 | .56 |
| Cervical length at randomization, mm a | 18.6 ± 6.3 | 19.5 ± 5.3 | .21 |
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