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  • In multiple gestations, how is chorionicity determined, and why is it important?

  • What potential complications occur in multiple gestations?

  • What antepartum assessment is used for multiple gestations?

  • What considerations are there for the route and timing of delivery?

CASE 39-1

A 24-y.o. G1 at 35 4/7 weeks gestation with diamniotic-dichorionic twins presents to L&D complaining of contractions. Vital signs include blood pressure 124/67, heart rate 98, and oxygen saturation of 100% on room air. A sterile vaginal exam shows that the cervix is 5 cm dilated, 80% effaced, and ‒1 station. Bedside ultrasound reveals both twins are in cephalic presentation. Prenatal records are available, and there are no complications during this pregnancy. A recent growth ultrasound from 1 week ago shows estimated fetal weights of 1800 g for twin A and 1680 g for twin B. The patient is admitted in labor, and the route of delivery is discussed.

Multiple births have increased over the last 30 years, with the most recent data from 2016 showing the twin birth rate to be 33.4 twins per 1,000 total births. The rate of twin births has risen 77% from 1980 to 2016 (18.9 to 33.4 per 1,000 births). However, higher-order multiple birth rates have decreased, down 46% from its 1998 peak at 193.5 births per 100,000 to 101.4 births per 100,000 in 2016.1 The increase in the number of twin births has been associated with delayed childbearing. Both older maternal age at the time of conception and the use of assisted reproductive technologies increase the probability of multifetal gestations.24

Multiple gestations are, unto themselves, one of the most common high-risk conditions in obstetrics. As we continue to see an increase in multiple infant deliveries, it is important that we recognize the higher risk of adverse birth outcomes compared with singletons, with the risk increasing with plurality.

This chapter will concentrate predominantly on twin gestations, given its consistent incidence.


Zygosity refers to the genetic makeup of the multiples, and chorionicity refers to the placental composition. Multiple gestations are either monozygotic or dizygotic. Monozygotic twins (i.e. identical) result from the division of a zygote derived from a single sperm and single ovum. Dizygotic (i.e. fraternal) twins are a result of the fertilization of two separate ova by two separate sperm. Spontaneous dizygotic twins are more common than monozygotic twins, occurring at a rate of 70% and 30%, respectively.5

The placental composition, or chorionicity, looks at the chorion and amnion layers surrounding the developing embryo. It is determined by the mechanism of fertilization and by the timing of embryo division. Early ultrasound is recommended to determine chorionicity and amnionicity. Determining chorionicity is critical to pregnancy outcome and management.68 The most reliable identifier of dichorionic twins is the presence of two separate placentas ...

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