Fraternal Twins
With fraternal twins (also known as 'non-identical twins'), the most common type, the mother contributes two eggs that each are fertilized in the womb by two different sperm cells from the father. The two eggs form two zygotes, and these twins are therefore also known as 'dizygotic' as well as 'biovular' twins.
Fraternal twins, like any other siblings, may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be a different sex or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal twins are simply brothers and/or sisters who happen to have the same age. They are genetically as similar as any ordinary siblings.
This type of twin is more common if there is a family history of twins, also more common for older mothers, with twinning rates doubling in mothers over the age of 35.
Identical Twins
With identical twins, one egg (zygote) from the mother is fertilized by one sperm from the father, and then very early in development the embryo splits and two fetuses grow.
If the zygote splits very early (in the first 2 days after fertilization) they may develop separate placentas and separate sacs. These are called dichorionic, diamniotic (or 'di/di') twins. While all fraternal twins are 'di/di', this occurs 20 - 30% of the time in identical twins.
Most of the time in identical twins the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ('mono/di') twins. These twins are very similar genetically, and share a single afterbirth.
Very occasionally, twins will also share the same sac (fluid cavity). In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac. These are called monochorionic, monoamniotic ('mono/mono') twins.
Finally, the zygote may split extremely late, resulting in conjoined twins.
Semi-identical twins
If a mother's egg splits and then both go on to be fertilized it can
result in semi-identical twins, who would be identical on their
mother's side but fraternal on the father's side.
