ABSTRACT
Monozygosity occurs in naturally conceived multiple pregnancies as well as those higher-order multiple pregnancies (HOMPs) conceived by assisted reproductive technologies (ART). (For zygosity in artificially conceived HOMPs, see Chapters 29 and 30.) Because many monozygotic (MZ) components of HOMPs are also monochorionic (MC), obstetric risks are very high, particularly in NC multifetal pregnancy. Whereas the frequency of MZ twinning in ART is surprisingly high, few NC triplets are trizygotic. No large body of information on the zygosity of NC quadruplets and quintuplets exists. This author has studied two sets of NC quadruplets. One set was MC, tetra-amniotic, and therefore MZ. The other consisted of a surviving pair who were MC diamniotic (DA) as well as an MC monoamniotic (MA) pair which had undergone multifetal reduction. At most, this quadruplet set was dizygotic (DZ), and they may all have been MZ. The Dionne quintuplets appear to have been MZ.
