How do we get Twins?

We have seen so many twins in our daily life! Most of us might have thought How do we get twins? Or How twins are born? Why these twins looks alike? But, why not these twins not looking same?

These days many people who face complications for becoming pregnant goes for in-vitriol fertilization. Most of cases of in-vitriol fertilization gives us twins!

Are you twins?

The idea that we normally get when we listen to the word twins is two identical people; looking alike and born by the same pregnancy.
Yes, it’s right.

So what are twins?

Twins are two offspring produced by the same pregnancy. They may be either identical or even look different. Based on the formation of zygote, twins can be classified into ‘Fraternal twins’ and ‘Identical twins’.

Identical twins are developed from the same zygote which splits and then forms into two embryos. Hence it is also known as ‘Monozygotic twins’. Fraternal twins are developed from two different eggs and are known as ‘Dizygotic twins’.

How twins are formed?

Each egg is fertilized by separate sperm cells. Since fraternal twins are separately fertilized eggs, they develop two separate amniotic sacs, placentas and supporting structures. Thus they are different from each other and are essentially two ordinary siblings who happen to be born at the same time.


Identical twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into two.

The three possible pairs of twins are:

  1. Male-male twins
  2. Female-female twins
  3. Male-female twins

Normally, same gender twins are identical twins and male-female twins are fraternal twins. But not essentially true always.

Since, same gender twins can have same DNA, they are identical twins. Male-female twins differ by the DNA and are usually fraternal twins.

Sometimes, male-female twins can also be identical twins. It’s a rare case.This is the result of an error that generally happens in the early stage of pregnancy. In this case, initially the twins will be a pair of males (sharing XY chromosome), which then change to become a male and female pair. This occurs when one half of the split fertilized egg loses a copy of its genetically encoded Y-chromosome, resulting to male-female twins.

More about Fraternal Twins

Fraternal twins happen when mom releases two separate eggs, may be at the same time or different times during her cycle and these two eggs are fertilized by two separate sperms forming two embryos. These two embryos are developed into separate babies. Thus, fraternal twins are just like any other siblings born at the same time, having only a small chance of same chromosome profile. Like any other siblings fraternal twins may or may not look similar.

How are fraternal twins conceived?

The two most common ways that fraternal twins conceived are;

  1. Superfecundation
  2. Superfetation

Superfecundation- it happens when a woman ovulates more than one egg during her cycle and the eggs are fertilized by two sperms. This may be due to heredity. Woman may have inherited a gene that causes hyperovulation (woman releases more than one egg during her regular cycle). Many fertility drugs can also cause hyperovulation.

Superfetation- it happens when mom is already pregnant when she ovulates and release another egg. The duration gap may be as long as 24 days from the release of first egg to the second egg. A different sperm will fertilize the second egg. In this instance, the babies will be delivered at the same time but with different sizes and development stages. In-vitro-fertilization is also a form of superfetation.

Fraternal twin are also more common for older mother, woman over the age of 35 with double twinning rates. Not only about twins, there are also chances of having triplets and multiples. This is quite common among animals. In humans, it’s rare. But do happen.