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Formation

Women are born with several hundred thousand primary oocytes (from the Greek "oon" meaning "egg" and "kytos" meaning "cell") in the ovaries. Primary oocytes are the cells that eventually develop into mature eggs which, if fertilized, become embryos. Most of them degenerate during childhood and only several hundred mature during the course of a woman's reproductive lifetime.

The eggs are released by the ovaries and are available for fertilization about once a month (except during pregnancy). The release of the egg from the ovary is called ovulation. When it is released, it is caught by the fingerlike ends of the fallopian tubes, through which it travels for about 4 days to the uterus.

Fertilization can occur any time during those 4 days following ovulation. If it is not fertlized during that time, the egg degenerates and flows out of the body with the menstrual blood.

If the egg is fertlized, it becomes a zygote (from the Greek "zygotos" meaning "yolked"). During the first week after fertilization, the zygote divides exponentially (i.e., splits into 2 cells, then 4, then 8, 16, 32 and so on). By the end of the first week, it is a few hundred cells that form a hollow ball with a swelled inner cell mass (embryonic disc) that eventually develops into an embryo and, later, a fetus. It is during this very early stage of development that monozygotic, or identical, twins develop: the embryonic disc splits into 2 or more separate discs that develop into 2 or more embryos.

About one-third of monozygotic twins result from a division that occurs within the first 2 or 3 days following conception. If the embryonic cells divide much later than about 9 days, the twins are rarely (only 4% of the time) delivered alive.

Sometimes, two eggs are released during a single menstrual cycle and, if both eggs are fertilized, lead to dizygotic twins. If more than 2 eggs are released, as often occurs when an infertile women is treated with superovulation therapy, a higher-order pregnancy can result.

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