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Selecting the appropriate time to deliver near term twins is critical to avoid unnecessary mortality and minimize neonatal complications.
When do you deliver twins who are at or near term? The issue of timing is often overlooked amid the greater concerns associated with prematurity among multiples. Selecting the appropriate delivery time, however, is critical to avoid unnecessary fetal mortality and minimize neonatal morbidity.
The average duration of an uncomplicated pregnancy is 280 days; a term gestation is one that has progressed to 37 weeks but not beyond 42 weeks. Notably, these criteria are based on experiences with singleton pregnancy. Most women deliver singletons between 39 and 40 weeks, a period that corresponds to the nadir in perinatal mortality for such births. Because perinatal morbidity and mortality begin to rise after 41 weeks, 39 to 40 weeks' gestation is considered the ideal for singletons. Whether these gestation and delivery criteria for singleton births are also valid for multiple gestations is open to question.
Twins mature faster
Because the lungs are one of the last organs to fully develop, lung maturity can serve as a proxy for fetal maturity. Respiratory distress syndrome (RDS) is a prematurity-related neonatal complication resulting from incomplete maturation of the fetal lungs. Evidence suggests that the fetal lungs develop functional maturity at earlier gestational ages in twins than in singletons.1
Researchers have found that twins have a fetal lung maturity value that's 22 mg/g higher than singletons matched for gestational age.2 Moreover, very-low-birth-weight twins delivered at less than 32 weeks have a lower risk of bronchopulmonary dysplasia than singleton neonates of similar size.3 The lungs of each twin develop congruently in most sets, but maturation may be asynchronous in 5% to 10% of cases.4
Another marker of accelerated maturation in twins is placental grade. Comparison of sonographically determined placental grade at different gestational ages in 158 twin pregnancies and 474 singletons found significantly more advanced placental grades throughout the third trimester in the twin group (P<.001).5 A grade III placenta and accelerated fetal lung development both suggest earlier maturation in twin pregnancies.6
Postmaturity problems occur at a younger age
Labor is generally induced between 41 and 42 weeks' gestation in singleton pregnancies because of concern about an increasing risk of complications from postmaturity. They include rising rates of meconium and meconium aspiration syndrome, intrauterine growth restriction (IUGR), oligohydramnios, fetal distress, stillbirth, and increased neonatal morbidity and mortality. If twins mature at a faster rate than singletons, it stands to reason that they may also suffer the consequences of postmaturity at earlier gestational ages.
Most studies have found reduced growth velocity in normal twins during the third trimester. Growth velocity appears to slow beyond 30 to 32 weeks' gestation whereas growth velocity in singletons increases.7
Grumbach and colleagues performed detailed fetal biometry on 103 sets of twins at 4- to 6-week intervals in the latter half of pregnancy. They found that the biparietal diameter (BPD) and abdominal circumference (AC) began to fall away from singleton norms after 31 and 32 weeks, respectively, whereas femur length (FL) remained similar to singleton norms throughout gestation.8 Similar findings were reported by Socol and colleagues, who noted that head circumference and birth length of twins were comparable to those of gestational-age-matched singletons, but BPD and AC were lower in twins.9
Serial ultrasonic examination of a cohort of discordant twin pairs by Rodis and colleagues revealed that abdominal circumference begins to diverge at 29 weeks, BPD after 33 weeks, and estimated fetal weight after 33 weeks.10 The differences in FL were not discriminatory.10 These findings suggest that the reduction in growth velocity in near-term twins is most consistent with asymmetric IUGR.
The competition for nutrients between twin fetuses can be accentuated by pathologic factors such as abnormal placentation, velamentous or marginal cord insertion, and maternal pregnancy complications such as preeclampsia-all of which occur more often in multiple gestations. Among twins, 13% have a birthweight lower than the 10th percentile by 35 to 36 weeks; the percentage increases to 23% between 37 and 38 weeks and to 38% between 39 and 41 weeks.11,12
Some investigators have argued that it is inappropriate to use fetal growth nomograms based on singleton pregnancies to evaluate twins. However, the evidence suggests that twins with birthweights lower than the 10th percentile, even if based on singleton norms, are at significantly increased risk for perinatal morbidity and mortality. In one study, the relative risk (RR) of perinatal death for twins with birthweights lower than the 10th percentile was 4.8 (95% CI, 2.74–8.42) compared to twins with weights greater than the 10th percentile.13 Increased neonatal morbidity has also been demonstrated among twins with birthweights lower than the 10th percentile.14 The literature clearly indicates that fetal growth restriction in twins is more common with advancing gestation and, along with prematurity, is a major determinant of perinatal outcome.
The effect of "relative postmaturity" on twins is significant with or without growth restriction. A study of 4,193 multiple births in England found that the risk of stillbirth increased from 1 in 3,333 at 28 weeks' gestation to 1 in 69 at 39 or more weeks.15
Kahn and colleagues studied 297,622 twin pregnancies delivered in the United States between 1995 and 1998.16 At 39 weeks' gestation, the prospective risk of fetal death in twins was 2.40 (95% CI, 1.99–2.89) as opposed to a neonatal death rate of 2.05 (95% CI, 1.54–2.72). Because the prospective risk of fetal death outweighed the risk of neonatal death at 39 weeks' gestation, the investigators recommended considering delivery at 38 to 39 weeks.16