Twins and… Not twins?

Twins and... Not twins?

A few days ago I ran into two people in the same cafe that I usually go to every morning before work. I realized they were born to the same mom and at the same time, I mean, twins. But I noticed a lot of differences between them (physically) and I think I had been staring at them for a long time because one of them asked me if something was wrong. I apologized for my “little harassment” and asked them if they were twins, to which they answered no, yes, they were born on the same day and almost at the same time but they weren’t twins. Then my head started asking a lot of questions and I ran back to work looking for information because I thought it would be a good title for an article. Differences between twin boys and “not twin” boys.

It is very difficult to refer to people who are born on the same day and at the same time because it is not a topic that usually differs, mostly people classify these births in the same way (twins). Therefore, it cannot be said differently in all languages, such as English. So I will refer to them as “not twins”. Many are often confused (like me) between the differences of twins and not twins. What are they? How does a twin pregnancy occur and how does a twin pregnancy occur? Well I will explain how they differ, what types of twin pregnancies can occur and how you can know if you are expecting twins or not twins.



In search of information, I found that the word twin and twin have the same etymological origin. Both mean “two alike, duplicate” even though they are used to refer to two different types of pregnancy. When a woman is pregnant with twins, the obstetrician may not identify them that way, but will refer to them as twins, this happens because the distinction is made “popularly” but at the medical level, they are all called twins.


Twins occur when the two babies are gestated in the same pregnancy, that is, they come from the same egg fertilized by a sperm. The egg ends up dividing after fertilization, giving rise to two genetically identical embryos. Twins share 100% of their genes, for this reason they are always the same sex and identical.


Not twins occur in the same pregnancy but not from the same fertilized egg, in this case, they are two sperm that fertilize two different eggs. Yes, they are two brothers who are born at the same time but they are not identical, they share only 50% of their genes. That is why they can be of different sex and their physical characteristics are not identical.


It is not always easy to distinguish during pregnancy if it is twins or not twins, in certain cases, the difference can be detected in an ultrasound during the first months. If it can be seen that they share the chorion (membrane that surrounds the amniotic sacs), it will be known that they are twins. If the two embryos are in the same sac, they will also be twins, and if they are of different sex, well, they are not twins.

It’s great to have two instead of one. Well, I think so.


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