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Ever wonder how twins are created?

The average length of a woman’s menstrual cycle is 28 days, although it is not uncommon for a woman to have a cycle that that is shorter or longer. During the first half of the menstrual cycle, levels of estrogen begin to rise, which causes the lining of the uterus to thicken. Typically, one of the eggs in a woman’s ovaries also begins to mature. Halfway through the cycle – around day 14 of a 28 day cycle – ovulation occurs. The mature egg is released from the ovary and travels down the fallopian tube. Pregnancy occurs if the egg is fertilized and implants in the uterus. If the egg is not fertilized, hormone levels decrease and the egg and lining of the uterus are shed during a woman’s menstrual period.

However, sometimes two eggs are released from a woman’s ovaries within a 24 hour period during the same cycle. If both eggs are fertilized and implant in the woman’s uterus, this leads to the development of fraternal (polyzygotic, non-identical) twins. It is more common for women over 35 to have fraternal twins, and for women using fertility drugs. In the general population, estimates of polyzygotic twins is approximately 12 in 1000 births.

Identical (monozygotic) twins are created when a single, fertilized egg divides into two separate embryos. Monozygotic twins look “identical” because they share the same genes (DNA structure). In contrast to fraternal twins, identical twins do not run in families. In the general population, estimates of monozygotic twins is approximately 4 in 1000 births.

The use of in-vitro fertilization has significantly increased the incidence of multiple births, due to the transfer of more than one embryo during treatment. However, given the considerable risks and adverse health conditions associated with twin and triplet pregnancies, considerable emphasis is being placed by professional associations (e.g., European Society of Human Reproduction and Embryology; Canadian Fertility and Andrology Society) to supporting single embryo transfers during IVF.

Read more about the menstrual cycle here.

To read about some of the risks involved with multiple pregnancies, click here.

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