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Pregnant with Identical Twins

What You Need to Know About Identical Twins Pregnancy

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Updated June 02, 2014

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A pregnancy with identical twins is different than a pregnancy with just one baby. There are some unique medical issues and complications specific to identical twins. If you are pregnant with identical twins, these are some things you should consider.

What are identical twins?

It's important to accurately understand twintype. The term identical is used to describe >monozygotic multiples -- twins that form from a single fertilized egg that splits into two. Because they originate from the same sperm/egg combination, these twins have the same >genetic origins. With the same DNA, they often have very similar appeareances. Because they look alike, the term "identical" has become synonymous with monozygotic twins. However, the correct description is monozygotic, indicating that the twins formed from one (mono) zygote (fertilized egg). Dizygotic twins, which form from two separate eggs fertilized by two different sperm, are also known as "fraternal" twins.

For more information about zygosity, please visit these resources: Determining Zygosity in Twins and Multiples How to Tell if Your Twins are Identical or Fraternal Twin Pregnancy FAQ: Are My Twins Identical or Fraternal?

How do I know if my twins are identical?

It is not always possible to confirm zygosity during pregnancy, but your medical provider may be able to assess whether the babies are identical or fraternal. Ultrasound can help determine some signs, including gender of the babies, and evaluation of the placenta. Monozygotic twins are always the same sex (two boys or two girls), and may share a single placenta, the organ that nourishes the babies within the womb. However, some monozygotic twins, those that split early after conception, may develop with two placentas, and a quick ultrasound scan is not always conclusive. Women who are undergoing additional prenatal testing, such as amniocentesis or Chorionic Villus Sampling (CVS) may also have an opportunity for conclusive zygosity testing.

Special Considerations for Identical Twins

If your medical provider confirms that you are pregnant with identical twins, there are some special questions that should be answered to ensure a successful outcome. Monozygotic twins are susceptible to certain conditions which can threaten the health of one or both babies. Be sure to discuss these risks with your medical caregiver if you are pregnant with identical twins.

  • Monoamniotic-Monochorionic(MoMo) Twins: When a fertilized egg splits shortly after conception, the two embryos will implant and develop with separate placentas and sacs. However, when the split is delayed, the twins may share a placenta, as well as an amniotic sac. The amniotic sac is the bag of waters that contains the fetus; it is surrounded by the chorion, an outer membrane. When the twins share both an amnion and a chorion, they are described as Monoamniotic-Monochorionic twins, or "MoMo" twins. This is a fairly rare condition; only about 1 percent of twins develop with this situation. However, it can pose risks for the fetuses. MoMo babies are at risk for cord entanglement or cord compression. Learn more about MoMo Twins.

  • Twin-to-Twin Transfusion Syndrome (TTTS): TTTS is a disease of the placent that impacts identical twins. The majority of monozygotic twins split between four and eight days after conception, and they may develop with a single, shared placenta. However, each baby has its own amnion (bag of waters), within a shared chorion (outer membrane). While they do not face the complications of MoMo twins mentioned above, about twenty percent of monochorionic twins develop abnormal blood vessels within the shared placenta that causes blood flow to be transfused unequally between the babies. One twin essentially becomes a blood donor to the other, causing problems for both babies. The donor twin has decreased blood flow, slow growth, and insufficient amniotic fluid, while the recipient twin's excess blood strains its heart and has too much amniotic fluid. TTTS can be managed during pregnancy to minimize the risks to the babies; in more severe cases, laser surgery can correct the blood flow in the placenta. Learn more about Twin-to-Twin Transfusion Syndrome.

  • Conjoined Twins: Conjoined twins occur when the egg splits late, eight or more days post conception, and the cells do not fully divide. Conjoined twins are connected at some point on the body, and may share tissue, organs or limbs. Conjoined twins are extremely rare, and are generally discovered by ultrasound. A pregnancy with conjoined twins requires careful management and close monitoring. Learn more about conjoined twins.

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