Diagnosis of multiple gestation is confirmed by sonogram most accurately in the first trimester, when separate gestational sacs are easily seen to determine chorionicity: the “twin-peak” sign (also known as the “lambda sign”) for dichorionic twins and the “T sign” for monochorionic diamniotic twins. Early determination of chorionicity is critical for guidance of further clinical care. Dichorionic chorionicity can be confirmed at later gestational age based on two placentas or differing fetal sex. Twin fetuses most commonly result from fertilization of two separate ova (dizygotic or fraternal twins). Monozygotic or identical twins arise when a single ovum divides after fertilization (Figure 10-1).

Dizygotic dichorionic/diamnionic twins (70%-80% of all twins) result from the fertilization of two ova. Each fetus has its own placenta and a complete and separate amnion-chorion membrane. Before 8 weeks' gestation, separate gestational sacs surrounded by a thick echogenic ring is suggestive of dichorionicity.

Monozygotic twins (20%-30% of all twins) result from cleavage of a single, fertilized conceptus. The timing of cleavage determines the placentation. This is more likely if separate echogenic rings are not visible before 8 weeks' gestation.

• Dichorionic/diamnionic monozygotic twins (8% of all twins) result from cleavage in the first 3 days after fertilization. They will have separate amnions and chorions, just like dizygotic twins. They have the lowest perinatal mortality rate of all monozygotic twins.

• Monochorionic/diamnionic twins (14%-20% of all twins) are produced by cleavage between days 4 and 8 after fertilization. They share a single placenta but have separate amniotic sacs. Two fetal poles with two yolk sacs suggest diamnionicity. The mortality rate for monochorionic/diamniotic twins is approximately 3 times higher than for dichorionic gestation.

• Monochorionic/monoamnionic twins (<1% of cases) are produced by cleavage after the eighth day. The fetuses share a single placenta and a single amnionic sac because both amnion and chorion were formed before cleavage. Later cleavage is even more rare and results in conjoined fetuses. Two fetal poles with only one yolk sac suggest monoamnionic gestation. Monoamnionic gestations have historically been reported to have mortality up to 50% to 60%; however, more recent reports suggest mortality rate to be much lower.

Higher order multiples have more frequent placental anomalies. Monochorionic and dichorionic placentation may both be present.