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Twin pregnancy: Prenatal issues

Stephen T Chasen, MD
Frank A Chervenak, MD
Section Editors
Charles J Lockwood, MD, MHCM
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD, FACOG


Twin pregnancy is associated with higher rates of almost every potential complication of pregnancy, with the exceptions of postterm pregnancy and macrosomia. The most serious risk is spontaneous preterm delivery, which plays a major role in the increased perinatal mortality and short-term and long-term morbidity observed in these infants. Higher rates of fetal growth restriction and congenital anomalies also contribute to adverse outcome in twin births. In addition, monochorionic twins are at risk for complications unique to these pregnancies, such as twin-twin transfusion syndrome (TTTS), which can be lethal or associated with serious morbidity.

This topic will provide an overview of the antepartum care of women with twin pregnancy. Intrapartum management is reviewed separately. (See "Twin pregnancy: Labor and delivery".)


Ultrasound examination is the only safe and reliable method for definitive diagnosis of twin gestation. Early ultrasound assessment also provides accurate estimation of gestational age, which is important in all pregnancies, but particularly important in management of twin pregnancies because of the higher risks for preterm delivery and growth restriction. In addition, chorionicity and amnionicity can be determined by ultrasound examination (see 'Assessment of chorionicity' below). This is critical because monochorionic twins have a shared fetoplacental circulation, which puts them at risk for specific serious pregnancy complications, such as twin-twin transfusion syndrome and twin anemia-polycythemia sequence [1-5]. These complications increase the risk for neurologic morbidity and perinatal mortality in monochorionic twins compared with dichorionic twins [2,3,6-9]. In addition to the complications associated with monochorionic twinning, monoamniotic twins also are at risk for cord entanglement and conjoined twins.

Most pregnant women in resource-rich countries undergo routine screening ultrasound examination. Randomized trials comparing routine ultrasound examination with ultrasound performed only for clinical indications have proven that a significant number of twin pregnancies are not recognized until the third trimester or delivery in women who do not undergo routine ultrasound examination [10,11]. As an example, the RADIUS (Routine Antenatal Diagnostic Imaging with Ultrasound Study) study of over 15,000 pregnant women reported that 38 percent of twin pregnancies remained unrecognized until after 26 weeks of gestation in women who did not have a routine second trimester ultrasound examination, and 13 percent of twins were not diagnosed until delivery [10]. The Helsinki Ultrasound Trial reported similar findings: Approximately 25 percent of twin pregnancies were not identified until after 21 weeks of gestation [11]. In both trials, no twin pregnancies were missed on ultrasound examination.

A policy of routine first or second trimester ultrasound examination would diagnose twin gestations at a time when amnionicity and chorionicity are easily determined [12]. Prenatal ultrasound screening guidelines vary worldwide. In the United States, the American College of Obstetricians and Gynecologists (ACOG) does not endorse routine ultrasound examination because, in a population of women with low-risk pregnancy, routine diagnostic sonography has not resulted in a reduction in perinatal morbidity and mortality or a lower rate of unnecessary interventions in randomized trials [10,11,13]. This may be related to the small numbers of twins in these trials and a lack of a standardized protocol for management of multiple gestations [14]. ACOG endorses ultrasound examination when there are specific indications for imaging, such as when twins are suspected because uterine size is greater than expected for menstrual dates. (See "Routine prenatal ultrasonography as a screening tool".)

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Literature review current through: Nov 2017. | This topic last updated: Oct 12, 2017.
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