Sometimes during the ovulation process, the ovaries are prompted to release multiple eggs. There are many reasons for this occurrence. A normal female body has two ovaries, one on the left side and one on the right. Both contain thousands of eggs. Generally the ovaries take turns ovulating, but occasionally they will each release an egg in a single cycle. Some women have a genetic tendency to hyperovulate, or release numerous eggs every month. Other women might release multiple eggs on an occasional basis due to hormonal fluctuations. Sometimes hormones are influenced by medication, such as birth control pills or fertility enhancements like Clomid or Pergonal. But they can also be impacted by a woman's age (older women tend to release multiple eggs more often than younger women) or by lifestyle.
Dizygotic twins are conceived if two eggs are fertilized by sperm (di meaning two and zygotic referring to the fertilized egg, or zygote). If both zygotes implant in the uterus, you've got twins! Likewise, three zygotes produce triplets and four would equal quadruplets. Generally, even higher order multiples that are born of separate zygotes are all referred to as dizygotic, although a more accurate term would be multizygotic.
Dizygotic twins are essentially siblings that happen to be born at the same time. They are each formed from a separate egg and a separate sperm. Just as related children within a family have a variety of appearances and characteristics based on the combination of genetic traits they inherit from their parents, dizygotic twins can also be very alike or very different. They can be boys, girls, or one of each.
When people talk about twins being hereditary or skipping a generation, they are referring to dizygotic twins. Women with a gene for hyperovulation are more likely to conceive twins since they routinely release multiple eggs. Their daughters have an increased chance of having twins as well, if they inherit the gene. However, if a woman gives birth to a son, his impact on the twinning process is irrelevant, as men do not have ovaries or ovulate. However, his daughter may inherit the hyperovulation tendency and produce twin grandchildren, thus causing the gene to “skip” a generation.
Causes of Dizygotic Twinning
Many other interesting characteristics influence dizygotic twinning. A woman's chances of conceiving dizygotic twins increase when:
She's had a previous multiple pregnancy
She's over age forty-five
She's already had several other children
She is a dizygotic twin herself, or her mother or maternal grandmother was a twin
She has irregular sexual intercourse
She consumes a lot of dairy products (not including those from organic sources)
She has a BMI (body mass index) higher than 30
She conceives shortly after discontinuing birth control pills. (This is not recommended due to the risk of birth defects.)
FIGURE 2-1 Egg-Splitting Chart
▲ (a.) Monozygotic (identical) twins form from a single egg that splits after fertilization. (b.) Dizygotic (fraternal) multiples result when more than one egg is fertilized. (c.) Polar body twins are the result of single ova that splits prior to fertilization.