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Home › Articles › Multiples › Twins

Twins

The Science Behind Those Dynamic Duos

By Carma Haley Shoemaker

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According to Hanson, another reason for the development and birth of fraternal twins is the use of fertility medications. These medications are used to stimulate the release of the egg from the ovaries, and it is quite common for more than one egg to be released at a time. "Fertility drugs are responsible for at least 35 percent of all multiple births – especially the birth of fraternal twins," says Hanson. "As fertility drugs are used for periods of time to achieve the desired result – pregnancy – the body may become over-stimulated and release more than one egg. If both or all of the eggs released are fertilized, a multiple birth results."

"I have a boy and a girl, so my twins are fraternal," says Trina Lambert, a freelance writer from Englewood, Colo. "Although I ovulated regularly, I was unable to stay pregnant. My doctors gave me [a fertility drug] for three months in a row before I was able to sustain a pregnancy. I don't know if I would have conceived twins anyway."

Identical Twins
Identical twins – medically termed as monozygotic twins – occur when an ovum splits into equal halves during early embryonic development, resulting in two offspring of the same pregnancy. Identical twins are always of the same sex, have the same genetic makeup, possess identical blood groups and will resemble each other physically. Unlike fraternal twins, identical twins occur equally in all races and age groups.

"What happens when identical twins form is that the egg is fertilized and it starts going through the cell divisions during the early part of fetal development," says Hanson. "For some reason, which we don't totally understand, the cells separate during this division, giving way to two fully-formed ova, which will develop into identical twins."

Elisa Ast All, mother of three children including identical twin girls, was shocked when she learned she was having twins. "I always assumed you had to have a family history of twins in order to conceive them naturally, as I did," she says. "But with identical twins, it's just a random act of nature – not handed down through families – and I was blessed to get them." The girls look so much alike that even close friends can't always tell them apart.

Twins with popsicles Other Twin Types

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