In a nutshell, the two most common types of twins are identical and fraternal. Sometimes there are obvious ways to tell which type is what, but there are some complexities with determining the zygosity of same-sex twins - let's get into it.
Twins and multiples are usually described according to:
- how many eggs they develop from – one or two
- if they share a placenta in their mother’s womb
The most common types of twins are fraternal twins and identical twins. There are other types of unusual and atpyical twins such as chimaeric twins, mirror image twins, polar body twins, fetus papyraceus and more! Check out this article written by our Patron, Professor Mark Umstad, on some of the rarer types of twins.
We know that a pregnancy starts when a sperm fertilises an egg. The fertilised egg is called a zygote. Occasionally, a woman’s ovaries release more than one egg, and when two of those separate eggs are fertilised by two separate sperm, twins are formed. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.
We know that a pregnancy starts when a sperm fertilises an egg and that the fertilised egg is called a zygote. When one egg is fertilised by one sperm and then splits, twins are formed (sometimes the egg even splits into three, forming identical triplets!). These twins or multiples are called identical twins / triplets or monozygotic twins / triplets (meaning one zygote). In the traditional model of twinning, the timing of splitting of a monozygotic embryo determines the number of placentas and amniotic sacs. This complexity with egg split timing is why it can be tricky to determine zygosity of twins while they're still in the womb, and why health professionals can still get it wrong and inform their patients that they are having dizygotic, fraternal twins! The only way to be sure on identical vs fraternal is to conduct a genetic test on your multiples (or risk your family and friends telling you for years "but they look the same!").
There is a lot of information to take in when you're reading about twins and multiples. Medical terms, common names, how many eggs and sperm cause what types of twinning, genetic (DNA) similarity, whether there's a hereditary predisposition that results in twinning, what impact fertility treatment has on twinning and the likelihood of birth defects. Take a moment to reference this summary matrix that spells it all out.
Are they fraternal?
The science of fraternal twinning
Are they identical?
The science of identical twinning
The genetics of twin types
