The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 4

MULTIPLE PREGNANCY

Multiple pregnancy results from the ovulation and subsequent fertilisation of more than one oocyte. In such case the fetuses are genetically different (polyzygotic or non-identical). Multiple pregnancy can also result from splitting of one embryonic mass to form two or more genetically identical fetuses (monozygotic). In all cases of polyzygotic multiple pregnancy each zygote develops its own amnion, chorion and placenta (polychorionic). In monozygotic pregnancies, there may be sharing of the same placenta (monochorionic), amniotic sac (monoamniotic) or even fetal organs (conjoined or Siamese).

When the single embryonic mass splits into two within three days of fertilisation, which occurs in one-third of monozygotic twins, each fetus has its own amniotic sac and placenta (diamniotic and dichorionic) (Figure 1). When embryonic splitting occurs after the third day following fertilisation, there are vascular communications within the two placental circulations (monochorionic). Embryonic splitting after the 9th day following fertilisation results in monoamniotic monochorionic twins and splitting after the 12th day results in conjoined twins.

Figure 1. In monozygotic twins embryonic splitting within the first three days of fertilisation results in a diamniotic and dichorionic pregnancy, splitting between days 3 and 9 results in a diamniotic monochorionic pregnancy, splitting between days 9 and 12 results in a monoamniotic monochorionic pregnancy, and splitting after the 12th day results in conjoined twins.