Incidence
Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide (about 0.3% of the world population).The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world. This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India and up to over 20 in some Central African countries. The exact cause for the splitting of a zygote or embryo is unknown. IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000. Genetic and epigenetic similarity
Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same fingerprints however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique.Monozygotic twins always have the same genotype.
More than half of twins are born weighing less than 5.5 pounds (2.5 kg), while the average birth weight of a healthy baby should be around 6–8 pounds (3–4 kg). This is largely due to the fact that twins are typically born premature. Premature birth and low birth weights, especially when under 3.5 pounds (1.6 kg), can increase the risk of several health-related issues, such as vision and hearing loss, mental disabilities, and cerebral palsy. There is an increased possibility of potential complications as the birth weight of the baby decreases. Twin-to-twin transfusion syndrome
Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the donor twin, is small and anemic, the other, the recipient twin, is large and polycythemic. The lives of both twins are endangered by this condition. Stillbirths
Stillbirths occurs when a fetus dies after 20 weeks of gestation. There are two types of stillbirth, including intrauterine death and intrapartum death. Intrauterine death occurs when a baby dies during late pregnancy. Intrapartum death, which is more common, occurs when a baby dies while the mother is giving birth. The cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. Caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time.