twininfo.co.uk

There are two main kinds of twins: fraternal and identical. About 70% of twins are fraternal and 30% identical.  Fraternal twins come about when two eggs are fertilised and then implant in the uterus.  These twins are really just like any other siblings except that they are the same age as each other.  Fraternal twins can run in families on the mother’s side, which isn’t the case for identical twins.

Identical twins, which are less common than fraternal twins, are the result of a single fertilised egg splitting into two identical parts and continuing to grow as two separate embryos.   Because identical twins come from a single sperm and egg, they both share the same DNA, however this doesn’t mean that they will look, and be, exactly the same.  Identical twins each have their own distinct personalities and, in fact, can have markedly different temperaments and character traits. Identical twins are always same-sex sets.

In some cases, which type of twins you are carrying will be evident during pregnancy, particularly if an ultrasound shows one girl and one boy.  Some identical twins form in a single sac, sharing a placenta and amniotic membranes. However, the number of placentas isn’t always a clear indicator.  The two placentas of fraternal twins can fuse together and appear to be one and identical twins may develop with completely separate placentas and sacs.  If you have same sex babies the only way to know for sure is through genetic testing after birth.