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Clinical Reviews

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.

July 2007 · Vol. 19, No. 07


Multiple gestations are far more common today than they once were—up 70% since 1980.1 Today, every 1,000 live births include 32.3 sets of twins, a 2% increase in the rate of twin births since 2004. Why this phenomenon is occurring is not entirely understood but, certainly, the trend toward older maternal age and the emergence of assisted reproduction are both part of the explanation.

Multiple gestations are of particular concern to obstetricians because, even though they remain relatively rare, they are responsible for a significant percentage of perinatal morbidity and mortality.

The difficulties that twins encounter are often associated with preterm birth and occur most often in identical twins developing within a single gestational sac. Those difficulties include malformation, chromosomal abnormalities, learning disability, behavioral problems, chronic lung disease, neuromuscular developmental delay, cerebral palsy, and stillbirth. Women pregnant with twins are also at heightened risk, particularly of gestational hypertension, preeclampsia, and gestational diabetes.2

Your task is to manage these risks so that the outlook for mother and infant is as favorable as possible.

Determining chorionicity in the first trimester

Ultrasonographic determination of chorionicity should be the first step in the management of a twin gestation. The determination should be made as early as possible in the pregnancy because it has an immediate impact on counseling, risk of miscarriage, and efficacy of noninvasive screening. (See Is there 1 sac, or more? Key to predicting risk.”)

The accuracy of ultrasonography (US) in determining chorionicity depends on gestational age. US predictors of dichorionicity include:

  • gender discordance
  • separate placentas
  • the so-called twin-peak sign (also called the lambda sign) (FIGURE 1), in which the placenta appears to extend a short distance between the gestational sacs; compare this with FIGURE 2, showing monochorionic twins with the absence of an intervening placenta
  • an intertwin membrane thicker than 1.5 mm to 2.0 mm.

US examination can accurately identify chorionicity at 10 to 14 weeks’ gestation, with overall sensitivity that is reported to be as high as 100%.3-5


The twin-peak sign on US

This dichorionic–diamnionic twin gestation demonstrates the so-called twin peak, or lambda, sign (arrow), in which the placenta appears to extend a short distance between the gestational sacs.


Monochorionic–diamnionic twin gestation

This US scan of a monochorionic twin gestation reveals the absence of an intervening placenta

What can go wrong

Monochorionic twins

Twins who share a gestational sac are more likely than 2-sac twins to suffer spontaneous loss, congenital anomalies, growth restriction and discordancy, preterm delivery, and neurologic morbidity.

Spontaneous loss. In 1 comparative series, the risk of pregnancy loss at less than 24 weeks’ gestation was 12.2% for monochorionic twins, compared with 1.8% for dichorionic twins.6 Spontaneously conceived monochorionic twins may have the highest risk of loss.7 However, monochorionic twins occur more often in conceptions achieved by assisted reproductive technology—at a rate 3 to 10 times higher than the background rate of monochorionic twinning.8


Managing a multiple gestation with minimal risk

  • A multiple gestation involves a higher level of risk than a singleton pregnancy
  • Chorionicity is the basis for determining risk. Twins within a single sac (monochorionic) are at higher risk of malformation, Down syndrome, and premature birth
  • The risk of Down syndrome can be estimated by noninvasive screening in the first trimester and by chorionic villus sampling or amniocentesis later in the pregnancy
  • A detailed anatomic survey at 18 to 20 weeks’ gestation should be done to detect possible malformations
  • Assessment of cervical length, performed every 2 weeks from the 16th to the 28th week, may help predict premature delivery—but is not definitive
  • Assessment of fetal growth every 4 weeks in dichorionic twins and every 2 weeks in monochorionic twins can alert you to potential problems. This is particularly important for detecting signs of twin-to-twin transfusion syndrome

Congenital anomalies. These occur 2 to 3 times as often in monochorionic twins, and have been reported in as many as 10% of such pregnancies. Reported anomalies include midline defects, cloacal abnormalities, neural tube defects, ventral wall defects, craniofacial abnormalities, conjoined twins, and acardiac twins.9-11 In light of these risks, a detailed anatomic survey is suggested for all twins.

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.”)

Is there 1 sac, or more? Key to predicting risk

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.


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.

TTTS: Diagnosis, staging, treatment

The diagnosis of twin-to-twin transfusion syndrome (TTTS) depends on the presence of a single monochorionic placenta and abnormalities in the volume of amniotic fluid (the polyhydramnios–oligohydramnios sequence). The syndrome may have an abrupt or gradual onset, heralded by discordancy and restriction in the growth of the 2 fetuses.

The natural history of the syndrome and treatment outcome are based on a staging system described by Quintero and colleagues1:

Stage I is characterized by polyhydramnios–oligohydramnios with the bladder still visible in the donor twin

Stage II The donor bladder is no longer visible

Stage III is defined by abnormal Doppler studies showing absent or reversed flow in the umbilical artery, reversed flow in the ductus venosus, or pulsatile umbilical venous flow

Stage IV is indicated by hydrops in either twin

Stage V One or both twins die.

The prognosis for TTTS grows poorer with increasing stage and is poor if the condition goes untreated, with a reported survival rate of only 25% to 50% for 1 twin when the diagnosis is made in the second trimester.2,3 Treatment options include removal of excess amniotic fluid through serial amniocenteses (amnioreduction), fetoscopic laser coagulation of communicating vessels, selective fetocide, and perforation of the membrane that separates the twins (septostomy).

Serial amnioreduction is the most common procedure for treating TTTS. When Senat and colleagues compared the efficacy of serial amnioreduction with fetoscopic laser occlusion in a randomized control trial, however, they found that the laser group had a significantly higher likelihood of survival of at least 1 twin (76%) than the amnioreduction group (56%).4

Septostomy. A recently published randomized trial in which amnioreduction was compared with septostomy found no difference in survival between the 2 treatments.5 Septostomy often has the advantage of requiring only 1 procedure to be successful, whereas repeated amniocenteses are necessary in serial amnioreduction. Septostomy does carry the risk of creating a single amnion, as the size of the membranous defect created by the perforation is difficult to control.

Selective fetocide using US-guided cord occlusion or radiofrequency ablation has been described when there is a coexisting fetal anomaly, growth restriction, or a chromosomal abnormality in 1 twin (heterokaryotypia).6,7 Use of bipolar coagulation in this setting has been associated with a liveborn in 83% of cases and intact neurologic survival in 70%.7 Radiofrequency ablation has also been described for selective fetal termination in monochorionic placentation with an abnormality in 1 twin.6 Data presented at the 2006 annual meeting for the Society for Maternal–Fetal Medicine showed no difference in the overall complication rate between these 2 techniques of selective fetocide.8



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