How Do Multiple Births Occur?

The numbers of multiple births have increased over the last twenty years. This increase is primarily due to the fact that one-third of women who are having babies are over the age of thirty, and these women are more likely to conceive multiples. Another reason for the increase in multiple births is due to fertility drugs and fertility procedures. There are two basic types of multiples: identical/monozygotic and fraternal/dizygotic.

What are identical/monozygotic multiples?

Identical/Monozygotic multiples come from a single fertilized egg and are genetically identical. Monozygotic multiples are often referred to as identical twins/multiples. These multiples would all be the same gender and look alike.

How do identical/monozygotic multiples occur?

The occurrence of identical multiples is considered a random event and is not influenced by age, race, or heredity. Identical multiples occur in 3 to 4 of every 1,000 live births.

What are fraternal/dizygotic multiples?

Dizygotic multiples or fraternal multiples, result from multiple eggs that are fertilized by different sperm. These multiples may be different genders. This form of multiples would be no more genetically identical than normal siblings.

How do fraternal/dizygotic multiples occur?

The most common type of multiples is fraternal/dizygotic multiples. Dizygotic multiples occur commonly with infertility treatments. The increase of fraternal multiples is also due to the fact that older women are more likely to have multiples, and many women today are conceiving at a later age.
Other women who have an increased chance of fraternal multiples include those who have a higher body mass index (BMI), have recently discontinued hormonal birth control, and those who have had more babies.

Which category do higher-order multiples (triplets, quadruplets, etc.) fall into?

Higher-order multiples (triplets, quadruplets, etc.) are all a variation of identical or fraternal multiples. For example, triplets can result from one zygote (an egg fertilized by one sperm) dividing into three. This means that the triplets would be genetically identical.
Triplets could also occur if one of two zygotes divide in half. This would mean that two of the triplets would be genetically identical while the third child is considered a fraternal multiple. Similar scenarios occur in quadruplets and other forms of multiples.

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