In Western populations, twins occur naturally 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 affairs.
However, the position has been altered by the introduction of 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. In the UK in 2012, over 12,000 sets of twins were born, 163 triplets and 2 sets of quads. The multiple birth rate was thus about 1 in 63.
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 two siblings. They are three times as common as uniovular, or identical, twins, who are developed from a single ovum fertilised by a single spermatozoon, but the resulting zygote splits early in development. Thus they are genetically virtually identical which is why they are usually so remarkably alike in looks and mental characteristics. Unlike binovular twins, who may be of the same or different sex, they are always of the same sex.
So far as 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.
Rarely, uniovular twins may not develop as separate individuals, being physically joined in some way. They are called CONJOINED 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.