Twins occur about once in 80 pregnancies, triplets once in 6,000, and quadruplets about once in 500,000. Quintuplets are exceedingly rare. Such is the natural state of a?airs.
In recent years, however, the position has been altered by the introduction of the so-called fertility drugs, such as CLOMIPHENE, and human menopausal gonadotrophin which, through the medium of the PITUITARY GLAND, stimulate the production of ova (see OVUM). Their wide use in the treatment of INFERTILITY has resulted in an increase in the number of multiple births, a recognised hazard of giving too large a dose.
Twins may be binovular or uniovular. Binovular, or fraternal, twins are the result of the mother’s releasing two ova within a few days of each other and both being fertilised by separate spermatozoa (see SPERMATOZOON). They both develop separately in the mother’s womb and are no more alike than is usual with members of the same family. They are three times as common as uniovular, or identical, twins, who are developed from a single ovum fertilised by a single spermatozoon, but which has split early in development. This is why they are usually so remarkably alike in looks and mental characteristics. Unlike binovular twins, who may be of the same or di?erent sex, they are always of the same sex.
So far as fraternal, or binovular, twins are concerned, multiple pregnancy may be an inherited tendency; it certainly occurs more often in certain families, but this may be partly due to chance. A woman who has already given birth to twins is ten times more likely to have another multiple pregnancy than one who has not previously had twins. The statistical chance of a third pair of twins is 1:512,000. Identical twins do not run in families.
The relative proportion of twins of each type varies in di?erent races. Identical twins have much the same frequency all over the world: around 3 per 1,000 maternities. Fraternal twins are rare in Mongolian races: less than 3 per 1,000 maternities. In Caucasians they occur two or three times as often as identical twins: between 7 (Spain and Portugal) and 10 (Czech and Slovak Republics and Greece) per 1,000 maternities. They are more common in Afro-Caribbeans, reaching 30 per 1,000 maternities in certain West African populations.
Rarely, uniovular twins may not develop as separate individuals, being physically joined in some way. They are called conjoined or (traditionally) Siamese twins. Depending on the extent of common structures shared by the infants – this ranges from a common umbilical cord to twins with conjoined heads or a common liver – the infants may be successfully separated by surgery. (See CONJOINED TWINS.)
Parents of twins, triplets, etc. can obtain advice and help from the Twins and Multiple Births Association (TAMBA).... multiple births
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