Question: How are twins made?

Tim Downing answered on 10 Nov 2012:

Hi Julia,

There are two main types of twins – ones that are from the same original cell (“monozygotic”) and ones that are from different original cells (“fraternal”). The original cells are the sperm/egg fused together. For fraternal twins, by sheer chance the mother released two egg cells instead of the normal one, and both happened to be fertilised by two different sperm cells. So although they are born at the same time, they are effectively the same as brothers and sisters. This is why some twins look very different.

But then for monozygotic twins that are from the same cell, it’s more complex. Once the sperm/egg cells have finished fusing together, they start growing and dividing. Normally these all stay together in a ball that gets gradually bigger, but sometimes at the start the cells divide and get separated. And then these separate cells might lead to twins. This only occurs very early one, mainly in the first few days after the sperm and egg fuse, but up to 13 days.

Some things – eg a mother dog’s littler of puppies – can have lots of twins/triplets/tetrapulets/etc at the same time so it’s quite normal to have twins.

Tim