How a multiples pregnancy occurs

How a multiples pregnancy occurs is based on cell division at conception.

The numbers of pregnancies that are multiples have increased over thelast twenty years. This increase is primarily due to the fact that onethird of women who are having babies are over the age of thirty, andthese women are more likely to conceive multiples. Another reason forthe increase in multiples is due to fertility drugs and fertility procedures.Even with this, there are still two basic types of multiples: identical/monozygoticand fraternal/dizygotic.


What are identical/monozygotic multiples?

Identical/Monozygotic multiples come from a single fertilized eggand are genetically identical. Monozygotic multiples are often referredto as identical twins/multiples. These multiples would all be thesame gender and look alike.

How do identical/monozygotic multiples occur?

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

What are fraternal/dizygotic multiples?

Dizygotic multiples or fraternal multiples, result from multipleeggs that are fertilized by different sperm. These multiples may bethe same gender or different genders. This form of multiples wouldbe 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. Theincrease of fraternal multiples is also due to the fact that olderwomen are more likely to have multiples, and many women today are conceiving at a later age. Other women who have an increasedchance of fraternal multiples include those who have a higher bodymass index (BMI), have recently discontinued hormonal birth control, andthose 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 variationof identical or fraternal multiples. For example, triplets can beresult from one zygote (an egg fertilized by one sperm) dividing into three. Thismeans that the triplets would be genetically identical. Triplets couldalso occur if one of two zygotes divide in half. This would mean that two of the triplets would begenetically identical while the third child is considered a fraternalmultiple. Similar scenarios occur in quadruplets and other forms ofmultiples.

Last Updated: 7/2007