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Dizygotic Twins

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Updated November 22, 2011

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Fertility drugs increase your risk of having twins, triplets, or more.

Fertility drugs increase your risk of having twins, triplets, or more.

Image (c) Craig Smallish / Getty
Definition:

Dizygotic twins occur when two eggs are fertilized by two separate sperm, leading to two separate zygotes. Unlike monozygotic twins, dizygotic twins do not share the same genes. Genetically, they are like siblings. Dizygotic twins are also known as fraternal or nonidentical twins.

Dizygotic twins may occur if two or more oocytes (eggs) are released in one cycle. If each is fertilized, dizygotic twins may result. Fertility drugs are a common cause for dizygotic twins, but other factors, like family history, maternal age or weight, can also lead to dizygotic twinning.

Dizygotic Twins and Gender

There are three possibilities, in terms of gender, for dizygotic twins:

  • Male-female twins (the most common kind of dizygotic twins, occurring 50% of the time)
  • Female-female twins (the second most common occurrence of twins)
  • Male-male twins

Dizygotic Twins and Their Amniotic Sacs and Placentas

Dizygotic twins typically have their own amniotic sacs and placentas. This is also known as being dichorionic-diamniotic (sometimes called Di-Di for short). The risks for dizygotic twins who are Di-Di are low, and they have the lowest mortality rate of all twins.

There are rare instances of dizygotic twins who share one placenta, but each has his or her own amniotic sac. This is referred to as monochorionic-diamniotic (Mo-Di for short). The risks are higher for twins sharing a placenta, due to the risk of twin-to-twin transfusion syndrome. The pregnancy may be monitored more closely.

Because two placentas can fuse over the course of the pregnancy, it can be difficult to identify whether there are two placentas or one. The doctor providing prenatal care will usually send the mother for an ultrasound near the end of the first trimester, when it’s still possible to see the separation of two placentas.

Trizygotic Triplets and Quadrazygotic Quadruplets

If three eggs are fertilized by three separate sperm, this may result in trizygotic triplets. If four eggs are fertilized by four separate sperm, you may get quadrazygotic quadruplets.

It is also possible for a mother pregnant with high-order multiples to have a combination of twins. For example, triplets may start as nonidentical twins (or dizyogitic twins). Then, one of those zygotes splits, leading to a set of identical twins (or monozygotic twins). All together, you would have two identical twins and one nonidentical twin making up the set of triplets.

If you find this hard to understand, try this. Take two pens, one red and one black.

Draw a smiley face with the red pen, with a little baby hair on top.

Draw another smiley face with the black pen.

Now, from the black pen face, draw two lines down and out, each going to a separate new smiley face.

Circle in one big circle the one red face and two new black faces. Those are the triplets -- two identical twins (black pen) and one "sibling" twin (red pen).

However, the majority of high-order multiples are made up of nonidentical twins.

More about twins:

Source:

Racowsky ,Catherine; Schlegel, Peter N.; Fauser, Bart C.; Carrell, Douglas T. Biennial Review of Infertility: Volume 2, 2011. Publisher: Springer; 1st Edition. (June 9, 2011)

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