How to Identify Twins at Low Risk of Spontaneous Preterm Delivery
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationship to this field of study.
Synopsis: Cervical length measurement at 23 weeks of gestation is a good screening test for predicting twins at low risk of preterm and very preterm delivery, especially in DC twins. The present results suggest that a cut-off of 25 mm should be recommended.
Source: Sperling L, et al. How to identify twins at low risk of spontaneous preterm delivery. Ultrasound Obstet Gynecol. 2005;26:138-144.
The incidence of twins is now about 1 in 40 pregnancies, up from the 1 in 80 figure over a decade ago. Infertility treatments are almost exclusively responsible for this doubling effect. This means that about 100,000 twin pregnancies will need to be cared for in the United States this coming year, and since these pregnancies are associated with higher morbidity and mortality rates, they require diagnostic and therapeutic attention. In addition to the obvious inconvenience, discomfort and, sometimes, tragedy experienced by patients with twins, we have found that the cost generated by one set of twins is 6 times that of a singleton pregnancy. A simple but excellent Danish study has surfaced that deals with how to predict the largest problem for twins—prematurity.
Sperling and colleagues evaluated 383 twin pregnancies appearing for care prior to 15 weeks. Each twin had a cervical length (CL) assessment by transvaginal ultrasound at 23 weeks of gestation. The managing physicians were only provided the results of the test if the CL was less than 1.5 cm. Eighty-nine percent had dichorionic-diamniotic (di-di) placentation and 58% of these were contributed by patients having had assisted reproduction technologies.
The overall rate of delivery before 28 weeks was 2.3% (1.5% di-di and 9.1% monochorionic) When using a cutoff of 33 weeks, the preterm rate in the overall twin sampling was 18.5% (17.1% for di-di and 29% for monochorionic) Five percent of patients had a CL of less than 2.0 cm, and a sensitivity of this measurement was 33.3% for delivery at less than 28 weeks. If the CL exceeded 2.5, the chances of delivering after 32 weeks was 95.3% and of delivering after 28 weeks was 98.6%.
Unlike other studies suggesting a higher rate of PTB in twins conceived through ARP, this study found no statistically significant difference in outcome between these pregnancies and those naturally conceived. Sperling et al did find a huge difference in preterm birth between monochorionic and dichorionic twin pregnancies.
Commentary
This study and others, while focusing on the use of CL in identifying those pregnancies at high risk for PTB, has shown that the method can label, with very reasonable accuracy, those at very low risk for delivery before 32 weeks, a critical threshold for neonatal morbidity.1-5 One of the major reasons for increased cost of twin pregnancies is the tendency to over-test and over-observe all twin pregnancies. A simple CL should allow us to loosen up on those at extremely low risk for preterm labor, thereby inviting them to enjoy a singleton-like pregnancy.
References
- Goldenberg RL, et al. The preterm prediction study: risk factors in twin gestations. National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Am J Obstet Gynecol. 1996;175(4 Pt 1):1047-1053.
- Heath VC, et al. Cervical length at 23 weeks of gestation: prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol. 1998;12:312-317.
- Souka AP, et al. Cervical length at 23 weeks in twins in predicting spontaneous preterm delivery. Obstet Gynecol. 1999;94:450-454.
- Shulman A, et al. Vaginal sonography of the cervix for the prediction of "time to delivery" in ART twins gestations. Twin Res. 2002;5:255-259.
- Soriano D, et al. The role of sonographic assessment of cervical length in the prediction of preterm birth in primigravidae with twin gestation conceived after infertility treatment. Acta Obstet Gynecol Scand. 2002;81:39-43.
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