Hopefully, most parents have a good understanding of where babies come from. But what about twins?
Do you know why you are expecting or have had twins?
Was it is surprise?
Did you have fertility treatments?
Does it run in the family?
That last one is a common question when people discover that you are expecting twins.
Does it run in the family?
Expecting parents often become indifferent to that question after being asked it the first 40-50 times, but after you actually have your twins and you discover how much work it is, you start thinking about it again.
Do twins run in the family? If so, then those family members are partly responsible and should come on over and help out!
The other big question you get is 'are they identical or fraternal twins?' And answering this question can help you answer the other one about the genetics of twin births.
In general, it is believed that having fraternal (dizygotic) twins can be genetic and that this predisposition can cause women to be more likely to release more than one egg at a time, offering the chance for more than one egg to be fertilized at a time. Other factors that can increase your chances of having fraternal twins include using fertility treatments and a mother being over 35-40 years old when she becomes pregnant.
The chance of having identical (monozygotic) twins is usually not thought to be genetic.
Although it is possible to do genetic testing to determine if you have had identical or fraternal twins, you can sometimes tell by the pattern of development of the placenta(s) and its inner (amnion) and outer (chorion) membranes.
Fraternal twins are the result when two different eggs (ova) are fertilized by two different sperm. This leads to the development of two separate placentas, each with its own chorion and amnion.
Fraternal twins are more common than identical twins and account for about 2/3 of twin pregnancies.
Identical twins develop when a fertilized egg splits. Depending on when the split occurs will determine if the twins share a placenta, with either one or two chorions and amnions, or if they each develop their own placentas. In general, the later the spit occurs, the more likely that the twins will share one placenta.
Fraternal vs. Identical Twinning
Even after they are born, it is sometimes difficult to know whether twins are identical or fraternal. It can be easier if they:
- share one placenta (identical)
- are different sexes (fraternal)
- have different blood types (fraternal)
It is harder to know if they are the same sex, have the same blood type, or if there are two placentas, since they could then be either fraternal or identical twins.
Don't be fooled by a fused placenta (fraternal twins), which can look like it is just one placenta, or if the twins don't look alike. Sometimes, factors during the pregnancy, especially twin to twin transfusion syndrome, can lead to identical twins that have very different birth weights and are mistakenly thought to be fraternal. And sometimes, fraternal twins can look enough alike to be confused with identical twins.
If you or your doctors aren't sure of the zygosity of your twins, you can consider having DNA testing done.
Higher Order Multiples
The same factors occur with higher order multiples, with either multiple eggs being fertilized or one or more fertilized eggs splitting.
For example, in the case of triplets, you could have:
- three separate eggs being released and fertilized by three different sperms (fraternal triplets)
- two separate eggs being released, with one splitting after it is fertilized (2 of the triplets will be identical and the other fraternal)
- the last type is the most rare, with one egg being released and splitting three times after it is fertilized (identical triplets)
Dr. Iannelli is the proud father of identical twin boys and a somewhat helpful stepson who is still wondering why we had to bring home two babies at the same time.