What Are Identical Twins?
It’s important to accurately understand twin type. 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 appearances. 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.
How Do You Know If Your 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. An ultrasound can help determine some signs, including the gender of the babies and the evaluation of the placenta. 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. 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 fluid-filled sac that contains the fetus. It is made up of two membranes, the amnion and the chorion. 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.
Twin-to-Twin Transfusion Syndrome (TTTS)
TTTS is a disease of the placenta 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 5 to 15 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.
Conjoined Twins
Conjoined twins occur when the embryo splits late, more than 12 days after conception, and does 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.
