Heredity of twin births and different twin types

“Do twins run in your family?” This has probably been one of the most asked questions – by friends as well as strangers – since the very moment we learned we were expecting twins. What follows is usually me giving a brief explanation of why it doesn’t really matter if there are twins in our families or not, as identical twins (like our girls) are never hereditary. People then usually stop asking, but most of the times I can see by the look on their face that either they didn’t quite understand or they just don’t (want to) believe it. So I’ll take a moment here to explain a bit more about the heredity of twins and the differences between identical and fraternal twins.

First of all, there’s no point in asking if twins run in Dad’s family, as twins can only be hereditary through the mother’s side and the father has no influence on this at all (sorry guys). Besides, the only type of twins that could occur because of twins “running in the family”, is fraternal (non-identical) twins. Not that all fraternal twin pregnancies happen only due to hereditary factors – there are other possible reasons as well, like maternal age, fertility treatments or nutrition. With identical twins, however, things are different. Identical twins do not run in families and fertility treatments do also not seem to have any influence on the probability of having identical twins. Identical twins ‘just happen’.

Let me explain a bit on the differences between fraternal and identical twins, as well as the medical terms used to identify the different types of twins during pregnancy. I’m including an image as well – I know my drawing abilities are not great but hopefully it’ll give you an idea.

Fraternal twins:

Fraternal twins occur when two eggs are fertilized by two sperms. They are also called dizygotic twins, or non-identical twins. Fraternal twins can be two girls, two boys or a boy and a girl. Dizygotic twins develop in two separate amniotic sacs (diamniotic) and have separate outer membranes and placentas (dichorionic). However, in some cases the two placentas can fuse together into what seems to be one single placenta, and many times this cannot be seen on ultrasounds during pregnancy and only be confirmed after birth of the twins. Fraternal twins are the most common type of twins and the amount of fraternal twin births has been increasing, among other things due to increased use of fertility treatments as well as increased maternal age.

Identical twins:

Identical twins, also known as monozygotic twins, occur when one egg is fertilized by one sperm and then splits in two in the first days or weeks after conception. Identical twins are either two boys or two girls. Depending on how many days have passed since conception when the splitting of the egg occurs, monozygotic twins can develop in two separate amniotic sacs with two outer membranes and placentas (diamniotic, dichorionic), or they can develop in two separate amniotic sacs but share one outer membrane and one placenta (diamniotic, monochorionic – this is the most common type of identical twins), or they can share one single amniotic sac as well as one outer membrane and placenta (monoamniotic, monochorionic – this is the rarest type of twins, with higher risks during pregnancy). Identical twin births are not as common as fraternal twin births and so far there have not been discovered any factors that could cause or trigger identical twin pregnancies. As said before, they ´just happen´ - a wonderful gift of nature! 

So next time you talk to a parent of twins, keep this brief explanation in mind before asking if twins run in their families, as it might save both you and the parent a possibly uncomfortable situation – especially in those cases where there is no history of twins in the family and fraternal twins were conceived after using fertility treatments, as that might not be something the parent wishes to share with anyone, especially strangers!

And talking about uncomfortable (or odd, silly, …) questions: as a parent of twins you’ll hear plenty of those whenever you go somewhere with your twins. In the beginning you might be annoyed or even feel offended by some of the questions people ask; later on, you’ll get used to it and with a bit of humor it can even be fun to be creative with your answers – a silly question deserves a sillier answer, right? I think this will make a great topic for one of my following blog posts: ‘Questions to parents of twins (the odd, the silly, the uncomfortable) and how to respond to them’. Coming soon!