How does multiple pregnancy occurs?
The number of
multiple pregnancies has increased over the past twenty years. This increase is
mainly due to the fact that a third of women who have children are over thirty
years of age, and these women are more likely to conceive twins.
Another reason for
increased multiplicity has to do with infertility medications and infertility
treatments. There are two main types of multiples: identical/monozygotic and
fraternal/dizygotic.
What are identical/monozygotic multiples?
Identical/monozygotic multipliers arise from a single fertilized egg and are
genetically identical. Monozygotic twins are often referred to as identical
twins/sets. All these multiples would be of the same gender and would look the
same.
How do identical/monozygotic multipliers arise?
The appearance of
identical multiples is considered a random event and does not depend on age,
race or heredity. Identical multiples occur in 3-4 for every 1000 live births.
What are fraternal/dizygotic multiples?
Dizygotic multiples
or fraternal multiples result from the fertilization of several eggs by
different spermatozoa. These multiples can be of different genders. This form of
twins will not be more genetically identical than normal siblings.
How do fraternal/dizygotic multiples occur?
The most common type
of plural is the fraternal/dizygotic plural. Multiple dizygotes often occur
during infertility treatment. The increase in fraternal twins is also due to
the fact that older women are more likely to have twins, and many women today
become pregnant at an older age.
Other women at
increased risk of twins include women with a higher body mass index who have
recently stopped taking hormonal birth control, as well as women who have had
more children.
Which category do higher-order
multiples fall into?
All higher order
multiples are variations of identical or fraternal multiples. For example,
triplets can result from the division of a zygote (an egg fertilized by a
sperm) into three parts. This means that the triplets will be genetically
identical.
Triplets can also occur if one of the two zygotes splits into two. This would mean that two of the triplets would be genetically identical and the third child would be considered multiple siblings. Similar scenarios occur in quadruplets and other forms of twins.