Abstract
The average incidence of twin pregnancy is approximately 1 in 80 although the figure shows a wide variation geographically and is greater in older patients and those with a family history of multiple pregnancy. The variation is due to the differing incidence of binovular (non-identical or dizygous) twins in which two mature ova are shed and fertilized at the same time. Both ova may arise from one Graafian follicle or, less commonly, two follicles maturate in 1 month. The incidence of uniovular (identical or monozygous) twins appears to be more constant, but the reason for this is not clear. In this type only one ovum is fertilized and the resultant zygote divides into two blasto-meres which separate and develop individually. The early blastomeres are totipotent — in other words, each is capable of developing into a complete organism and twinning may occur at various stages in the initial cell divisions. Uniovular twins are always of the same sex. Careful examination of the placenta and membranes may reveal the type of twins born, although, in view of the different stages at which twinning is possible, this is not always the case.
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