There are two basic categories of twins: Identical (or monozygotic) and Fraternal (or dizygotic). There are many reasons why it's important to determine twintype, but it's not always an obvious determination. Sometimes, asking a few simple questions can help parents evaluate their twins' zygosity and come to a conclusion as to whether they are identical or fraternal.
Time Required: Variable. Sometimes it's an instant determination and sometimes it can take months or years.
- Are the twins males, females, or one of each? Monozygotic twins are always the same gender, except in rare cases where a chromosomal disorder is present. It can safely be assumed that boy/girl twins are dizygotic or fraternal.
- Do the twins have the same bloodtype? Knowing your multiples' blood type can help determine zygosity. Monozygotic (identical) twins will have the same bloodtype. Dizygotic (fraternal) twins may have the same bloodtype, or they may have different types. Therefore, it may be concluded that twins with differing bloodtypes are dizygotic, or fraternal.
- For multiples that were the result of in vitro fertilization, did the number of implanted embryos exceeds the number that were transferred? Occasionally, monozygotic multiples result after in vitro fertilization. If a single embryo is transferred, yet two embryos have implanted in the uterus, it can be assumed to be monozygotic twins.
- How many placentas were present during pregnancy? Sometimes an analysis of the placenta can reveal zygosity. Information may be revealed with an ultrasound examination, or a physical analysis of after delivery. A single, shared placenta may indicate that the twins are monozygotic. The presence of two separate placentas, or two placentas that fuse together, is less conclusive. While all dizygotic multiples will have their own placentas (with very few exceptions), monozygotics may also have individual placentas, depending on when the zygote splits.
- Were the twins diagnosed with TTTS while in the womb? TTTS (Twin to Twin Transfusion Syndrome) only effects monozygotic twins who share a circulatory connection in the placenta. Doctors have identified a few, rare cases of dizygotic twins who experienced TTTS, probably due to the reproductive technology used to conceive them. Twins diagnosed with TTTS are most likely monozygotic (identical).
- Were the twins identified as monoamnionic in the womb? If so, they are monozygotic. Only monozygotic twins develop within a single, shared amnion.
- Did you undergo prenatal testing that provided a comparable sample of DNA from each baby? It is possible that the results of prenatal testing such as amniocentesis can confirm the zygosity of the babies, so be sure to ask your doctor.
- Do the twins share physical similarities? Individuals that look remarkably alike, with incredibly similar features, are more likely to be identical. However, appearance isn't a conclusive assessment of zygosity. Many dizygotic twins share similar characteristics (family resemblance) while many monozygotic twins don't look alike due to environmental influences.
- If your answers to these questions are inconclusive, the only way to determine zygosity is with DNA testing. DNA testing is the most reliable way to confirm zygosity and know for sure. Companies like Affiliated Genetics and Proactive Genetics offer simple test kits for less than $200. If it's important for you to know, order a kit, follow the directions, and send in a sample for
- It's important to truly understand the concept of zygosity. Identical twins aren't just two individuals that look alike. The accurate term is monozygotic, describing two individuals that result from a single egg-sperm combination that splits into two. Monozygotic multiples are similar because they share the same genetic origin. Fraternal twins are correctly described as dizygotic, the result of multiple egg-sperm combinations. Their genetic similarities are the same as any siblings. They may share a family resemblance, or they may look totally different.
- Identical twins don't necessarily look alike. Even though they have genetic similarities, appearance is influenced by environmental conditions, even before birth. Identical twins may look remarkably similar due to their similar genetic makeup, or they may be different. There are many reasons why identical twins are different.
- Ultrasound or placental analysis is often inconclusive. Don't rely on your doctor's assessment during pregnancy, as many wrongly conclude that babies are fraternal simply because there are two placentas. Follow up with DNA testing after the babies are born if you want to know for sure.
- Besides curiosity, there are many reasons why parents of twins want to know their children's zygosity. It's nice to have an answer to the endless questions about twin type. But more importantly, the answer could have implications in medical situations.
- If you can't afford to purchase a DNA test kit, you may be able to get tested for free by participating in a research study. For more information, check the links in the Research About Multiples resource collection. Or contact your local mothers of multiples organization or local research university.
What You Need
- DNA test kit
- Blood test
- Placental analysis