HOW A MULTIPLE PREGNANCY
OCCURS
The numbers of pregnancies that are multiples 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 multiples is due to fertility
drugs and fertility procedures. Even with this, there are still 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 the same gender or 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 does 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 be 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.
