Multiple births and twins

Usually one baby per birth; sometimes multiple births (up to six). The most common are twins — about 1 in 86 births worldwide.

(a) Fraternal (dizygotic, non-identical) twins:

• two ova are released together (from one or both ovaries) and fertilised by two sperms; each develops as an independent embryo with its own amnion/sac and placenta.
• They differ genetically (as ordinary brothers and sisters), may be of different sex.

(b) Identical (monozygotic) twins:

• one ovum fertilised by one sperm; the dividing zygote (in the early cleavage or blastocyst stage) separates into two masses of cells, each developing as an embryo.
• They are genetically identical (same sex, same characteristics), usually share one placenta and may share the same amniotic sac.
• Incomplete separation gives conjoined (Siamese) twins, which can often be separated surgically.