Introduction
Incidence of twins and higher multiples
Multiple pregnancy is not the normal situation in humans. Naturally conceived twins account for only 1% of pregnancies, while only 0.1% are triplets and higher multiples. Since the advent of assisted conception and in vitro fertilization (IVF) the rate of multiple pregnancies has increased substantially. In the UK the multiple pregnancy rate has increased from 9.6 per 1000 in 1976 to 15.7 per 1000 in 2010 [1]. In the United States the situation is similar, with an increase from 18.9 per 1000 in 1980 to 32 per 1000 in 2005 [2].
Types of multiple pregnancy (MZ/DZ) plus etiology
Multiple pregnancies may be derived from the same or separate fertilized ova. Approximately 28% of twins in Western countries are from a single ovum (monozygous) and result in identical twins, while 72% are from separate ova (dizygous), resulting in non-identical or fraternal twins. The rate of monozygotic twinning appears to be constant worldwide, whereas dizygous twinning is more common in Black Africans (1 in 24 pregnancies) and least common in those of Far Eastern origin (1 in 140 pregnancies) [3].
Monozygous twins are thought to arise as the result of a random event in the conceptus, leading to splitting of the early embryo into two halves. Occasionally it is thought that an abnormal clone arising in the early conceptus leads to splitting into twins. However, in general, monozygous twins appear to be genetically and physically identical.
Dizygous twinning shows a familial tendency, which is maternally inherited. Sisters of dizygous twins have a 2.5-fold increased rate of having dizygous twins [4]. Dizygous twins are also more common in older mothers [4].
Exogenous hormones given to enhance fertility are associated with multiple ovulations and thus multiple fertilized ova and dizygous twins. The vast majority of twins resulting from assisted conception (AC) and IVF are the result of implantation of more than one fertilized egg into the mother’s uterus. However, 0.8–5% of pregnancies following AC and IVF turn out to be monozygous, compared to the expected rate of 0.45% [5].