Why identical twins are always of the same gender

And as those cells divide and multiply, at some point very early in embryonic growth they split into two individuals. So that you can imagine that the genetic material in these two identical twins is identical to each other, hence the name.

The contrast to this would be fraternal twins, which are twins born at the same time. However, they are actually the result of fertilization of two different eggs, and thus can be different genders and look different, and have different characteristics.

What's becoming increasingly clear is even though we call identical twins identical, and we think of their genetic material as being identical, we need to be reminded that there are environmental influences that may change, actually, the characteristics of identical twins, reminding us, as we already know, that even though identical twins come from the same genetic material, in characteristics they may actually have differences as well.

More detailed information on DNA testing and how to find out whether twins or triplets are identical is available on the Multiple Births Foundation website. Page last reviewed: 21 May Next review due: 21 May Home Pregnancy Finding out you're pregnant Back to Finding out you're pregnant. Pregnant with twins. Different types of twins One-third of all twins will be identical and two-thirds non-identical.

Identical twins Identical monozygotic twins happen when a single egg zygote is fertilised. The egg then divides in 2, creating identical twins who share the same genes. Non-identical twins Non-identical dizygotic twins happen when 2 separate eggs are fertilised and then implant into the womb uterus. These non-identical twins are no more alike than any other 2 siblings. Non-identical twins are more common. The babies may be of the same sex or different sexes. Fraternal twins may be of the same or different sexes.

They share half of their genes just like any other siblings. In contrast, twins that result from the fertilization of a single egg that then splits in two are called monozygotic, or identical, twins. Identical twins share all of their genes and are always the same sex.