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
Heart defects. The incidence of congenital heart defects is 4 times greater in monochorionic twins, even in the absence of twin-to-twin transfusion syndrome (TTTS).12 Cardiac malformations may occur secondary to abnormal lateralization during embryogenesis or result from an abnormal vascular distribution in the shared placenta.9,10 The presence of abnormal vascular communications may also cause limb reduction defects and the rare acardiac twin.
Long-term neurologic morbidity. In one series, the incidence of cerebral palsy was 8%, compared with 1% among dichorionic twins. In twins followed to 2 years of age, rates of minor neurologic morbidity were 15% in monochorionic twins and 3% in dichorionic twins. The overall rate of neurologic disorders in monochorionic twins was 23%, regardless of fetal weight.13 At 4 years, long-term neurologic morbidity was particularly high in single survivors of a monochorionic pair; the incidence of cerebral palsy has been reported to be as high as 50% in single survivors, compared with 14.3% in cases in which both twins survived.14
Twin-to-twin transfusion syndrome. In this condition, abnormal vascular connections arise in the shared placenta, allowing blood to be shunted from one fetus to the other. The syndrome is unique to monochorionic gestations and occurs in 15% to 20% of cases.15 A significant percentage of neurologic morbidity is probably the result of TTTS. To evaluate for TTTS, include a detailed anatomic survey and serial US every 2 weeks beginning in the second trimester as part of the surveillance of monochorionic twin gestations. (See “TTTS: Diagnosis, staging, treatment.”)
Twin-related morbidity and mortality are directly related to chorionicity. Twin embryos in a single chorion (monochorionic twins) have a higher rate of perinatal morbidity and mortality than do twins in separate sacs (dichorionic twins). To some extent, the higher risk faced by monochorionic twins—of twin-to-twin transfusion syndrome and certain structural and chromosomal abnormalities, for example—is the result of complications uniquely related to having a single placenta. But recent evidence also suggests that the higher risk of adverse outcomes is associated with monochorionicity itself, independent of the complications attributable to the single placenta.1
When twins develop in separate chorionic sacs, the risks are not as great. All fraternal twins (approximately 2/3 of all twins) are dichorionic and, therefore, at lower risk of an adverse outcome. The situation is more complex with identical (monozygotic) twins, however: Most (70%) are monochorionic, but approximately one third (30%) have separate chorionic sacs and are therefore dichorionic.
Reference
1. Leduc L, Takser L, Rinfret D. Persistence of adverse obstetric and neonatal outcomes in monochorionic twins after exclusion of disorders unique to monochorionic placentation. Am J Obstet Gynecol. 2005;193:1670-1675.
Down syndrome and other chromosomal abnormalities
Estimating odds
Assessing the likelihood of a chromosomal abnormality (aneuploidy) in a multiple gestation is complicated by differences in twinning mechanisms (chorionicity versus zygosity) and by the increasing rate of dizygotic twinning with advancing maternal age. The risk is greater in dizygotic twin gestations than in age-matched singleton gestations. The definition of advanced maternal age (AMA) in a twin pregnancy has ranged from 31 to 33 years of age in reports in the literature.2,16,17
The probability that a twin gestation contains a fetus with a chromosomal abnormality is directly related to zygosity. Each twin in a dizygotic gestation carries an independent risk, so the composite risk for the pregnancy is a summation of the independent risk for each fetus. For monozygotic twins, the risk is similar to the age-related risk in a singleton gestation. Presumptions about zygosity are based on chorionicity: Almost all (90%) dichorionic twins are dizygotic and all monochorionic twins are monozygotic.
What is the utility of noninvasive screening?
Multiple gestations can be screened for aneuploidy using maternal age, maternal serum markers, and nuchal translucency (NT) on US, or combinations of these assessments.
When first-trimester serum markers (free β-human chorionic gonadotropin and pregnancy-associated plasma protein A [PAPPA]) are combined with NT and maternal age, a pregnancy-specific risk can be calculated that includes the individual contribution of each fetus, thus yielding an improved detection rate. In monochorionic twins, the NTs are averaged to calculate a single risk for the entire pregnancy. In dichorionic twins, the risk for each fetus is calculated independently and then summed to establish a pregnancy-specific risk. The combined test has a reported detection rate of 84% for monochorionic twins and 70% for dichorionic twins, compared with detection rates of 85% to 87% for singletons at a 5% false-positive rate.18,19 The integrated test (combined test plus measurement of second-trimester serum analytes) has a 93% detection rate for monochorionic twins and a 78% detection rate for dichorionic twins, compared with 95% to 96% for singletons at the same 5% false-positive rate.18,19 Second-trimester screening has a lower detection rate in both singleton and twin gestations.
