How do twins or multiple births occur?

Twins or multiple births occur when more than one embryo or foetus is carried in a single pregnancy.

Twins can be categorised into two types: identical (monozygotic) and non-identical (dizygotic). Identical twins occur when a single egg is fertilised to form one zygote which then divides into two separate embryos. Each shares exactly the same genetic material, hence the term 'identical'. This process is a random event and is not influenced by age, race, or family history.

Non-identical twins, on the other hand, occur when two separate eggs are fertilised by two different sperm, resulting in two zygotes in the same pregnancy. These twins share about 50% of their genes, just like any other siblings. The likelihood of having non-identical twins is influenced by factors such as maternal age, family history, and certain fertility treatments.

Multiple births can also occur in the form of triplets, quadruplets, and so on. This happens when more than two eggs are fertilised or when an already split embryo splits again. The same principles and factors apply to these multiple births as to twins.

The process of multiple births is a fascinating example of human reproduction and genetics. It involves complex biological processes and is influenced by a variety of factors. Understanding these processes can provide valuable insights into human biology and genetics, and can also have practical applications in areas such as fertility treatments and genetic counselling.