Managing risk—to mother and fetuses—in a twin gestation
Begin by determining chorionicity. Discuss the risks of a multiple gestation, prepare parents for premature delivery, and monitor fetal growth as indicated.
IN THIS ARTICLE
The twin loss rate following amniocentesis has been evaluated in several studies. (See “Integrating evidence and experience: Does invasive prenatal testing raise the risk of miscarriage in a twin gestation?”)
A greatly elevated risk of preterm birth
Multiple gestations are at extremely high risk for premature delivery, and—like all premature newborns—these infants are at risk for a wide range of disabilities. Risk factors for premature delivery include history of second trimester pregnancy loss, preterm birth at less than 35 weeks’ gestation, more than 2 previous curettage procedures, cone biopsy, müllerian anomaly, and diethylstilbestrol exposure. Unfortunately, current yardsticks for predicting premature delivery in multiple pregnancy have serious limitations, and available interventions have not been particularly successful.
Predictors
Measurement of cervical length has been evaluated as a predictor of preterm delivery in a number of twin studies that were looking for a cutoff point that can predict which twins are at greatest risk. No such cutoff has been found.22-25
In general, studies demonstrate a low risk of preterm delivery for women who have a cervical length measurement of more than 35 mm at 24 to 26 weeks. A shorter cervical length correlates with premature delivery, but specific cutoffs have proved not to be sensitive predictors.
In the largest published series, To and colleagues evaluated 1,163 sets of twins undergoing routine care with cervical length assessments at 22 to 24 weeks’ gestation. They demonstrated a direct correlation between cervical length and preterm delivery, but were unable to define a cutoff sufficiently sensitive to be useful.25 A shortened cervix may be predictive of prematurity in general, but it does not allow the obstetrician to predict with certainty which mothers will give birth prematurely or how long a particular mother will carry.
Fetal fibronectin. The presence of fetal fibronectin (ffN) in cervicovaginal secretions is widely used as an adjunct to other potential predictors of preterm delivery. In a multistudy review that included symptomatic women, a negative ffN had a 99% negative predictive value but a poor positive predictive value (13% to 30%) for delivery within 7 to 10 days.26
Use of ffN in conjunction with cervical length has also been investigated in twin gestations. Although the negative predictive value of ffN remained high, the addition of ffN to cervical length assessment did not improve the positive predictive value of cervical length alone.27,28
Interventions
Cerclage is often used in high-risk singleton pregnancies in which a shortened cervix is seen on a sonogram. The utility of cerclage in twins is less clear. Randomized controlled trials comparing women at risk of premature delivery treated with cerclage and controls not considered at risk found no difference in the rate of premature delivery in the 2 groups.29,30 Meta-analysis of 4 randomized controlled trials also found no benefit and, in fact, detected a possibility of actual harm. Cerclage twins were more likely to deliver early (at less than 35 weeks’ gestation) and had a 2.6 relative risk of perinatal mortality. The differences found in the meta-analysis were not statistically significant, however, and the overall sample size was small (n=48).31
The best available data seem to show that cerclage based on US indications of cervical shortening is not beneficial and may even be associated with worse outcome.
17-Hydroxyprogesterone caproate (17P) has been found to decrease the rate of recurrent preterm birth in singleton gestations by almost 35%.32 Although twins are at increased risk of preterm birth, the use of 17P has not, however, been shown to be of benefit.33
Bed rest. A Cochrane Database review of 6 randomized controlled trials compared 1) patients with a multiple gestation who were offered bed rest in the hospital with 2) patients hospitalized for complications of pregnancy. The review found that bed rest did not reduce the risk of preterm birth or of perinatal mortality in the routinely hospitalized women. There was, however, a tendency to a decreased number of low-birth-weight infants born to women given bed rest.34
INTEGRATING EVIDENCE AND EXPERIENCE
The evidence for amniocentesis
Toth-Pal and colleagues compared the twin loss rate after amniocentesis in 155 twin pairs; twins who had a structural anomaly or aneuploidy were excluded. The investigators found a 3.87% loss rate at 24 weeks or less, compared with a background loss rate of 2.39% in twins who did not undergo the procedure—an insignificant difference.1
Yukobowich and colleagues compared 476 diamniotic, dichorionic twin pairs that had undergone amniocentesis with 1) 477 twin pairs undergoing routine US examination and 2) 489 singleton amniocenteses. They found a 4-week postprocedure loss rate of 2.7% in the amniocentesis twins, compared with 0.63% in twin controls who had routine US and 0.6% in the amniocentesis singletons.2 The difference is significant, but the reported loss rate is still less than, or comparable to, the reported background twin loss rate at 24 weeks or less.
Chorionic villus sampling
Only a few studies of the loss rate in twins after CVS have been published, but those that are available report a loss rate lower than, or comparable to, the background rate for twins generally. In a series of 169 twin pairs undergoing CVS at an average gestational age of 10 weeks, the risk of loss at 20 weeks or more was 1.7%.3
CVS and amniocentesis, in tandem
Although CVS and amniocentesis are not directly comparable given the difference in the timing of procedure, a few series have compared the risk of loss for the 2 procedures. Eighty-one twin pairs that underwent amniocentesis were compared with 161 twins undergoing CVS. The rate of spontaneous delivery at less than 28 weeks was 2.9% for amniocentesis, compared with 3.2% following CVS.4
To sum up
Invasive testing does not appear to increase the risk of fetal loss above the background loss rate for twins overall. Prenatal diagnosis as early as 10 weeks is a feasible option in a twin gestation, given the limitations of screening in multiple gestations.
References
1. Toth-Pal E, Papp C, Beke A, Ban Z, Papp Z. Genetic amniocentesis in multiple pregnancy. Fetal Diagn Ther. 2004;19:138-144.
2. Yukobowich E, Anteby EY, Cohen SM, Lavy Y, Granat M, Yagel S. Risk of fetal loss in twin pregnancies undergoing second trimester amniocentesis. Obstet Gynecol. 2001;98:231-234.
3. Brambati B, Tului L, Guercilena S, Alberti E. Outcome of first-trimester chorionic villus sampling for genetic investigation in multiple pregnancy. Ultrasound Obstet Gynecol. 2001;17:209-216.
4. Wapner RJ, Johnson A, Davis G, Urban A, Morgan P, Jackson L. Prenatal diagnosis in twin gestations: a comparison between second trimester amniocentesis and first trimester chorionic villus sampling. Obstet Gynecol. 1993;82:49-56.
