Question: What causes twins or triplets to be born?

  1. Fertilisation of one egg which splits in two or three (very rare), or fertilisation of several eggs.


  2. @thepaleone: There can be a couple of reasons for multiple births.

    The first is that multiple eggs get fertilised and successfully implant and develop into babies. This is where you can get twins (2 eggs) or triplets (3 eggs) that can be of different genders. This is called ‘fraternal’ twins/triplets.

    The second is when the fertilised egg splits into two (or three) embryos, not just one – and then two (or three) babies form. These will all be the same gender and are what we know as ‘identical’ twins/triplets. The biological terms for this is ‘monozygotic’ (meaning from one zygote – the zygote is the initial cell formed when the sperm and egg join).