TYPES OF TWINS AND THEIR ORIGINS

For naturally conceived multiple pregnancy, about 70% are dizygotic (DZ) twins, resulting from double or multiple ovulations in the same cycle; this is attributed to higher levels of maternal follicle-stimulating hormone (FSH) or increased FSH receptor sensitivity. Monozygotic (MZ) twinning involves postzygotic splitting, resulting in two or more embryos. This can occur at any time up to about 14 days postconception.

A third type of twin, the “polar body” twin, is very rare.

DETERMINATION OF ZYGOSITY

Zygosity determination is best carried out by analysis of restriction fragment length polymorphisms in variable number tandem repeat sequences of DNA, extracted from chorionic villus samples, amniotic fluid cells, peripheral blood white cells, placenta, cord and membranes, and tissues sampled at autopsy, including macerated stillborn fetuses (Table 23.1).

PLACENTAL ANATOMY OF ZYGOSITY

Of MZ twins, about one-third have dichorionic (DC) placentas (separate or fused disks), while two-thirds have truly single monochorionic (MC) placentas (Figure 23.1). Most MC placentas have two amniotic cavities (MC, DA), while a minority (about 1% of all twins) have a single amniotic cavity (MC, MA). Conjoined twins are MZ, and most are MC, MA.