Twin pregnancy

What it is

Twins are either dizygotic or monozygotic (identical), and the whole topic turns on which.

• Dizygotic is more common. It is always dichorionic diamniotic (DCDA), and the twins can be of the same or opposite sex; opposite sex means dizygotic
• Monozygotic twins are always the same sex, and are always monochorionic if division occurs after 3 days of fertilization

The type follows the timing of division of the zygote: 0-3 days always DCDA; 4-8 days monochorionic diamniotic; 8-12 days monochorionic monoamniotic; 13-15 days monochorionic monoamniotic conjoint (Siamese) twin. DCDA can come from either monozygotic or dizygotic, so a different blood group means dizygotic, and with the same blood group do DNA fingerprinting. Only monozygotic can give rise to monochorionic. The outcome of twin pregnancy depends on the chorionicity. Put the other way round: dizygotic twins come from ovulation and fertilization of two oocytes, which results in dichorionic diamniotic placentation and two separate placentas, while monozygotic twins come from a single oocyte with subsequent division of the zygote, and the timing of that division generally determines the placentation, so a monozygotic pair may have two separate placentas or one placenta that is monochorionic monoamniotic or monochorionic diamniotic. Rare cases of dizygotic twins with monochorionic placentation after assisted reproductive technology have been reported, as have atypical forms: chimeric twins, mirror image twins, discordant monozygotic twins and polar body twins. Sesquizygotic multiple pregnancy is the term for the rare intermediate between monozygotic and dizygotic twinning.