How much do you really know about how twins are conceived, born and live? Read our guide to find out all you need to know about twins.

The types of twins

There are two different types of twins, defined by how they are conceived. Most people know them as identical and non-identical twins, but they are also called maternal and fraternal twins, and dizygotic and monozygotic twins.

Fraternal and non-identical twins are most common, making up three quarters of all twins. They are conceived when two ovums are fertilised by two different sperm meaning they have different genetic profile. While they may be similar they aren't ever identical.

Maternal and identical twins come from the same fertilised ovum meaning they have exactly the same DNA. After the ovum has been fertilised, it separates into two embryos.

Conceiving twins

There is only ever a "twin gene" for fraternal twins as the affecting factor is whether the mother hyper-ovulates - meaning she releases more than one egg at one time. This means that twins are only hereditary on the mother's side, as obviously only the mother ovulates.

Mothers who conceive over the age of 35 and who have been pregnant more than once before, are much more likely to have fraternal twins. This is because older women often release more than one egg during ovulation, possibly because the body is trying to make up for the decline in the quality of the ovum.

Read up on some interesting twin facts.