How Multiple Births Occur

Ever wonder how multiple births occur, and why some women are more likely to have them than others? A multiple birth is when more then one fetus is carried to term. Multiple births can be twins, triplets, quadruplets or even higher numbers.

All multiple births are considered high-risk pregnancies. This doesn't mean that the multiples are doomed to be unhealthy, it simply reflects the higher chance of complications in a multiple pregnancy.

Types of Multiples
The most common multiple birth is twins. Twins can be either fraternal or identical. Multiples occur naturally in many families. 

In the case of fraternal twins (Dizygotic), two sperm fertilize two different eggs. These two babies have separate amniotic sacs and placentas. Genetically they are no more similar then any other siblings.

For identical twins (Monozygotic), a single fertilized egg splits in two sometime around the second week after conception. These twins share an amniotic sac and placenta. Identical twins will have the same sex, same hair and eye color and the same blood type.

Triplets can be a combination of Monozygotic, Dizygotic or Trizygotic. Trizygotic is when three sperm fertilize three separate eggs. Trizygotic triplets are fraternal and they are no more genetically similar than any other siblings.

When four sperm fertilize four different eggs, the sibilings are Quadrazygotic. As with fraternal twins and triplets, these quadruplets are no more similar genetically then regular siblings.

For anything more then four babies, like quintuplets, the combinations can be two monozygotic children and three trizygotic children. Identical quadruplets and quintuplets are exceptionally rare.

Frequency of Multiples
Approximately 4 sets of identical twins are born for every 1,000 births. The rate for fraternal twins varies because it is dependent on ovulation. African Americans have the highest fraternal twin birth rate, at 10 to 40 per 1,000 births. Caucasians deliver 7 to 10 sets of fraternal twins per 1,000 births. An older woman is more likely to have fraternal twins then a younger woman.

The chance of naturally occurring triplets is 1 per 10,000 births; naturally occurring quads are 1 per 600,000 births.

Your chances of conceiving multiples increases if you have a family history of twins or other multiple births.

What Triggers Multiple Births?
Most multiples are conceived when a fertilized egg splits and forms two separate babies, or if a woman has multiple ovulations during a single menstrual cycle. The second ovulation can occur after a woman is pregnant and produce a second, simulataneous pregnancy.

Receiving fertility treatments may also cause multiple births. In vitro fertilization has a higher chance of producing multiples, because several fertilized eggs are implanted at once. Hormone therapy can also encourage multiples by increasing the number of eggs released

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