Twins are rare and special, occurring in about 2% of all pregnancies. Of that number, 30% are identical twins. The other 70% are non-identical, or fraternal twins.
This animation will show you the differences between the development of a single baby, identical twins, and fraternal twins.
Starting with the single baby, let's go back to the beginning, when fertilization occurs. Here you see that the egg cell is fertilized by a single sperm cell to form a zygote. Over the next few days, the fertilized egg cell divides over and over to form a structure composed of hundreds of cells called a blastocyst.
During the first week after fertilization, we can look inside the blastocyst and see the mass of cells that will form the embryo. Right there. The blastocyst will continue traveling toward the uterus where it will implant in the uterine lining, and grow into a single baby.
Now let's watch the development of identical twins. Identical twins start out from a single fertilized egg cell, or zygote, which is why they're also called monozygotic twins. Like the single baby we just saw, the egg is fertilized by a single sperm cell.
Unlike the single baby, this fertilized egg cell will split into two separate embryos, and grow into identical twins. This remarkable event takes place during the first week after fertilization, and can happen at several different times:
At the two cell stage on days 1 to 3
At the early blastocyst stage on days 4 to 6
Or in the late blastocyst stage on days 7 to 9
The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. Basically, the earlier the splitting occurs, the more independently the twins will develop in the uterus. So, a pair of identical twins that split during the two-cell stage will each develop its own amnion, chorion, and placenta.
Twins that split during the late blastocyst stage will share an amnion, chorion, and placenta.
A common misconception about the conception of identical twins is that the trait for having them is passed on to future generations through the mother's genes. But the truth is science doesn't know the reason why identical twins occur. At this time, we can just say that they're examples of a nine-month double miracle.
Now let's take a look at the second type of twins. Non-identical, or fraternal, twins develop from two fertilized egg cells, or zygotes. Which is why they're also called dizygotic twins. Unlike identical twins, however, fraternal twins are definitely influenced by the mother's genes. Here's why:
When the mother of fraternal twins ovulates, sometimes her ovaries release two egg cells for fertilization. Typically, only one egg cell is released during ovulation.
During conception, both of these egg cells become fertilized by two different sperm cells, which is why fraternal twins don't look exactly alike. Sometimes they're not even the same sex.
Here in the uterus, you can see that the twin embryos develop separately each having his or her own chorion, amnion, and placenta.