There are two basic categories of zygosity when it comes to twins: identical (or monozygotic) and fraternal (or dizygotic). Identical twins aren’t just two individuals that look alike. They result from a single egg-sperm combination that split 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 born separately. They may share a family resemblance, or they may look totally different. There are many reasons why it’s important to determine twin type, but it’s not always an obvious determination. While some twins can be identified immediately, others may not be determined for months or years. Sometimes, a few simple clues can help parents evaluate their twins’ zygosity and come to a conclusion as to whether they are identical or fraternal.
The Sex of the Twins
All babies possess either XX or XY chromosomes that distinguish whether they are female or male, respectively. A male has XY chromosomes and a female has XX chromosomes. The occurrence of female-male twins happens when one X egg is fertilized with an X sperm and the other X egg is fertilized with a Y sperm. Monozygotic twins are always the same sex, except in rare cases. It can safely be assumed that male/female twins are dizygotic or fraternal. But male/male and female/female twins may also be fraternal.
Blood Type
Knowing your multiples’ blood type can help determine zygosity. Monozygotic (identical) twins will have the same blood type, with a few very rare exceptions. Dizygotic (fraternal) twins may have the same blood type, or they may have different types. Therefore, it may be concluded that twins with differing blood types are dizygotic, or fraternal. However, twins with the same blood type can be either fraternal or identical.
How They Were Conceived
For multiples that were the result of in vitro fertilization, did the number of implanted embryos exceed 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 a case of monozygotic twins.
Number of Placentas
Sometimes an analysis of the placenta can reveal zygosity. Information may be revealed with an ultrasound examination or a physical analysis 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.
Prenatal Diagnoses
If your twins were diagnosed with twin-to-twin transfusion syndrome (TTTS) while in the womb, that is a clue that they are identical. TTTS only affects 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). Similarly, if twins were identified as monoamniotic in the womb, they are monozygotic. Only monozygotic twins develop within a single, shared amnion. If you underwent prenatal testing that provided a comparable sample of DNA from each baby, it is possible that the results could confirm the zygosity of the babies. Amniocentesis, for example, can confirm zygosity, so be sure to ask your doctor.
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. There are many reasons why identical twins are different.
Testing for Zygosity
Zygosity can be determined through blood testing, placental analysis or with an at-home DNA testing kit. 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 for medical situations. Ultrasound or placental analysis is often inconclusive. While a doctor can make an assessment during pregnancy, 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.
A Word From Verywell
If the clues 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. If it’s important for you to know, order a kit, follow the directions, and send in a sample for analysis. 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. Or contact a “parents of multiples” organization (such as Multiples of America) or local research university.
