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  • The presence of 2 or more embryonic or fetal heartbeats seen on ultrasound


The incidence of multiple gestations is 33.7 per 1000 births. Multiple gestations in general are complicated by increased maternal morbidities (hypertensive disorders, diabetes, hemorrhage) and fetal morbidities (preterm birth, twin-twin transfusion syndrome, growth discrepancies). With a 6 times increased rate of preterm delivery in twins, there is a 5 times higher rate of stillbirth and 7 times increased rate in neonatal death, primarily due to prematurity. Pregnancy outcomes correlate with the number of fetuses present and the chorionicity/amnionicity of the pregnancy.


A. Zygosity & Chorionicity

Zygosity refers to the genetic makeup of a multiple gestation, and chorionicity refers to the placental composition. Zygosity cannot be determined antenatally unless twins are of different sex or zygosity testing is performed via invasive diagnostic testing (eg, amniocentesis or chorionic villus sampling). It is vital to determine chorionicity early in gestation due to associated risks to the pregnancy.

1. Monozygotic

The rate of monozygotic twinning is 1 in 250 pregnancies, and 30% of all twins are monozygotic. Monozygotic twins are the result of cleavage of a fertilized single ovum by a single sperm. The process remains uninfluenced by race, age, and parity, but is affected by infertility treatment interventions thought to occur due to injury to the zona pellucida that influences splitting. Timing of division of the embryo dictates placentation (Figure 17–1).

Figure 17–1.

Mechanism of twinning. A. At 0–4 days after fertilization, an early conceptus may divide into 2. Dichorionic diamniotic. Placentas may be separate or fused. B. Division between 4 and 8 days leads to a blastocyst with 2 separate embryoblasts. Monochorionic diamniotic. C. Between 8 and 12 days, the amnion and amniotic cavity form above the germinal disc. Embryonic division leads to 2 embryos with a shared amnion and chorion. Monochorionic monoamniotic. D. Incomplete splitting or partial embryo fusion. Monochorionic monoamniotic; conjoined twins.


If the zygote divides within 72 hours of fertilization prior to the morula phase and trophoblast differentiation, a dichorionic diamniotic twin pregnancy occurs with separate or fused placentas, 2 chorions, and 2 amnions. This accounts for one-third of monozygotic twinning. Dichorionic gestations have the lowest rate of overall complications.


Monochorionic diamniotic pregnancy results from division on day 4–8, after differentiation of the trophoblast has occurred but prior to the formation of the amnion. A single placenta is found with a common chorion and 2 amnions. This accounts for two-thirds of monozygotic twinning. Monochorionic pregnancies have an increased risk of adverse outcomes including fetal structural malformations, ...

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