How do twins happen?
Twins are two babies from only one mother at the same day of birth. [1] Twins can born in two possible ways. One way is that only one egg fertilizes and later splits into an identical twin. The other way is that two eggs are release and met one sperm: this gives a non-identical twin. Generally, a third of twins are identical twins and second-third of twins are non-identical twins.
Non-identical twins are become pregnant with at the same time. Rarely, in this type, the twins can have different fathers. [2] First, one egg meet with one sperm and they fertilize. Later, one sperm met the fertilized cell, so one egg meets two sperms. So, this make the twins can have two different fathers but one same mother. Non-identical twins share only half of their genes, so their faces can might be very similar or very different.
Identical twins are formed by splitting eggs. They split for no reason. But when they split, they share exact same genes. Based on today’s research, the egg splits because of the ovulation-including drug, the treatment can cause more splitting system. [3]
We can determine twins whether they are identical or non-identical by number of the placenta. If they have only one placenta, they are identical twins. If they have two divided placenta, they are non-identical twins.
[1]http://www.babyworld.co.uk/information/pregnancy/twins/why_twins_identical.asp
[2]http://www.babyworld.co.uk/information/pregnancy/twins/why_twins_identical.asp
[3]http://www.babyworld.co.uk/information/pregnancy/twins/why_twins_identical.asp
Pictures: http://pregnancy.about.com/cs/twinsmore/a/twinbasics.htm
http://www.pregnancy-bliss.co.uk/howtwinsoccur.html
Figure 1

- four kinds of twins
Figure 2

- Twins with two different sacs and separated placentas.




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