To know the formation of a twin baby, you must first know how a baby is forming. Every month a single egg cell comes out from the female ovary. When that egg cell attaches to the sperm and develops a new zygote cell, this process is known as fertilization.

         Stages of cleavage in the zygote

Then a single zygote cell divides into 2, 2 into 4, 4 into 8, 8 into 16...like this in the process of mitosis division; it forms the blastocyst cell. This blastocyst will look like a ball - it includes more than a hundred cells. This is the cell needed for the baby's development. When blastocyst attaches to the endometrium, which is the inner wall of the uterus, then it's called pregnancy. This process happens after seven days of fertilization.

A new cell embryoblast forms in the blastocyst, and the embryo grows like a baby inside the embryoblast.

Let's see how twin babies are forming.

         Monozygotic and Dizygotic

Twins may be identical (monozygotic) or non-identical (dizygotic). It is based on zygote. Identical twins occur when a single egg cell is fertilized to form one zygote, divided into two separate embryos. After the growth of these embryos, the babies are identical in appearance; even their genetic background will be the same.

Blood groups and health conditions will be the same, and both of their sex must be the same. If one is male and another one also will be male, or one is female, and another one also will be female. Their character and IQ level also will be the same. Except the fingerprints, everything will be the same as identical twins.

In identical twins, four categories are there. It's based on the time duration of the cleavage process in the fertilized egg. If it's within three days, they will have a different placenta and different chorion (outer membrane) & amnion (inner membrane). If between four and seven days, they will have the same placenta and different chorion & amnion. When the blastocyst divides after seven to thirteen days, they will have the same placenta and same chorion & amnion. If it's after thirteen to fifteen days, they should be conjoined twins.

In some females, in a month at the same time, two egg cells are released from the ovary. When that two eggs are fertilized by two separate sperm simultaneously, then non-identical twins occur. Their appearance, skin color, sex, and character, everything will be different. In this case, only 25 percent of possibilities are to be the same sex. Their genetic background is only 50 percent will match each other.

When the level of estrogen is high in women, it produces more egg cells (hyperovulation). Based on the number of egg cells released from the ovary, many babies (multiple pregnancies) will be formed. The process of test-tube baby treatment follows the same procedure to get (baby) twin babies.

When a woman does not have enough eggs for fertilization, they are injected with the required hormone to produce the eggs. After the eggs are received from the ovary, they combine with the sperm samples for fertilization. Till the fertilized eggs turn into an embryo, this process goes under lab care. Once the embryo forms, it is transferred to the woman's uterus.

I hope this article helps to understand the formation of twin babies.