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How Twins are Formed


How Twins are Formed

All who would win joy, must share it; happiness was born a twin. Lord Byrone

The news that twins are on their way is usually met with surprise and a fair amount of shock. Suddenly, a whole host of carefully made considerations must be multiplied by two. However, when the initial shock wears off, appreciation of the double miracle growing within takes hold. This is when you will more than likely become very interested in exactly what takes place when twins are conceived.

Twins are either identical or fraternal:

Identical Twins

Identical – or monozygotic – twins result when one egg is fertilised but later splits to form two embryos. Both embryos carry the same DNA and are always the same gender. The two embryos then develop into two foetuses sharing the same womb. Identical twins do not always share a placenta – this depends entirely on when the egg split. If the split occurred early enough each embryo will have its own placenta. Approximately one quarter of identical twins are mirror images of each other.

Fraternal Twins

Fraternal – or dizygotic – twins occur when two eggs are fertilised by two separate sperm cells. Each embryo develops with its own placenta, but fraternal twins may be different sexes and may have differing blood types. Fraternal twins are twice as common as identical twins and are as genetically diverse as siblings born at different times.

Annelise at 12 weeks with her twins

twins formed


Johan Theron Nel
M.B.Ch.B (Stellenbosch) M.Med. (O.& G.) (Stellenbosch) F.C.O.G. (S.A.), M.R.C.O.G. (London), F.R.C.S. (Edinbourgh)
Core Obstetrics and Gynaecology with Examination Guidelines for M.B.Ch.B. Published 1995

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