twins

twins are of two types: monovular (identical), from the union of one sperm and one ovum, and binovular (non-identical) resulting from the fertilization of two separate ova. The cell produced by fertilization is called a zygote (from the Greek for ‘yoked’), so they are also known as monozygotic and dizygotic. Dizygotic twins are physically and genetically as dissimilar as any siblings. Monozygotic twins, having resulted from the cleavage of a single ‘conceptus’ — the splitting and separation of an early embryo — are therefore, with rare exceptions, genetically identical.

The incidence of multiple pregnancies varies in different racial groups. To quote ‘Hellin's law’ (1895): ‘twins occur in 1/89 births, triplets 1/(89)², quadruplets 1/(89)³ and so on’. The formula is roughly correct, although twins occur in Caucasians 1/80 to 1/90, in Asiatics 1/150 or less, and black Africans 1/50 with the highest incidence of twinning amongst the Yoruba people of Nigeria for whom 1 in 25 births are of twins. It is the rate of non-identical (dizygotic) twinning that varies around the world: identical (monozygotic) twins occur at a similar rate of 1 in 300 births in all populations. These statistics are based on clinical findings in viable pregnancies. However the initial ‘hidden’ twinning rate is probably higher: with increasing use of ultrasound in early pregnancy it is found that before 12 weeks one of the twins may die and be absorbed leaving an apparent singleton. In Australia the rate of twinning has increased approximately 25% over the past 20 years, partly due to a significant increase in the percentage of births to women aged 35 and over, and partly to the treatment of infertility by ovulation stimulation or assisted conception by gamete intra-fallopian transfer (GIFT) or in-vitro fertilization (IVF).

Twin pregnancy is more prone to complication than single pregnancies and possible hazards of premature birth and poor growth in the womb necessitate increased antenatal surveillance. If twins are identical and they share a single placenta, one baby can steal blood from the other, causing a condition known as ‘Twin– twin transfusion syndrome’.

Multiple pregnancies carry a greater risk of losing a baby before, during, or after birth than singleton pregnancies: multiple pregnancies overall account for more than 10% of all perinatal deaths; the greater the number, the greater the risk. Cerebral palsy in survivors is six times more common in twins than singletons.

The birth of twins has been a source of fascination in many cultures throughout history and the twin image has been incorporated in myths, folklore, and religions. The Old Testament of the Bible tells of Isaac's wife, Rebekah, who eventually conceived after nineteen years of marriage. Twin boys were born. The first was red and hairy, and he was named Esau, meaning ‘red’. His brother was born holding Esau's heel and so he was called Jacob, meaning ‘he who grasps the heel’. Ancient Rome was founded, according to legend by Romulus and Remus, the twin progeny of Mars, god of war, and a mortal princess. In some African communities, twins were regarded with great favour; in others, with great suspicion. The Yoruba in Nigeria were well aware of the high mortality associated with twinning in the past, and they made small wooden sculptures, ‘ibeji’ that had spiritual significance if one of twins died.

Jim Neilson

See also assisted reproduction; pregnancy.