Finding out for sure if a twin is fraternal or identical is called zygosity determination. It is often important to know what type of twins you have. It can help in understanding their development, explaining some of their social behaviors, and, of course, satisfying curiosity. It can also be important if medical illnesses occur, for such reasons as determining the heredity of the illness or whether organ transplantation is needed. There are several methods that determine twin type, or zygosity. They vary in accuracy and expense. Many times, insurance companies do not cover the cost of this determination. The methods include: SIMILARITY METHOD This is perhaps the most obvious method and the one used by the common person. If they look alike, they are identical or monozygotic; if they look different, they are fraternal or dizygotic. Experienced observers can be quite accurate in labeling the twin type using things such as hair; eyes; skin color; shapes of nose, lips, and eyes; fingerprint analysis; and tissue grafts between twins. These experienced observers, usually researchers, can claim a 94-96 percent chance of correctly identifying the twin type, compared to blood tests. PLACENTA METHOD Analyzing the placenta(s) has been the common way that obstetricians have determined twin type in the past, and placental analysis is still performed today. Unfortunately, this method can be quite inaccurate and depends on the expertise of the doctor. In the past it was assumed that one placenta meant that the twins were monozygotic, or identical, and that two placentas meant that they were dizygotic, or fraternal. This has proved to be untrue. Parents can often be misled when given the twin type based only on examination of the fetal tissues by an inexperienced clinician. More important than the placenta itself are the fetal membranes, which can be helpful in determining twin type. They are the thin membranes filled with fluid that surround the fetus and are attached to the placenta. The amnion is the inner membrane, and the chorion is the outer membrane. A shared amnion always means monozygotic twins, but is very rare. Two amnions and one chorion also means monozygotic twins. If there are two amnions and two chorions the twin type can be either mono- or dizygotic. Fraternal or dizygotic twins will always have two placentas, as the two eggs implant separately into the uterus. However, if the eggs implant close together the placentas can become fused and take on the appearance of one placenta. Twins can then be mislabeled monozygotic. However, identical twins do not always have one placenta. Monozygotic twins come from an egg that splits into two. The timing of the split will determine the number of placentas. If the split happens after the egg has already implanted into the uterus, there will be one placenta. If the split happens earlier, before implantation, then the two eggs can implant separately into the uterus and form two separate placentas. The twins will still be identical. BLOOD TESTING Using blood tests can give an accurate determination of twin type. There are many other blood groups besides the usual ABO blood type. There are also serum proteins and enzymes. The more blood groups studied, the more accurate the identification will be. Using eight blood groups, there is a 97 percent chance of correctly identifying identical twins. With the use of more blood groups, the accuracy can climb to nearly 99 percent. HLA typing is a similar measure of many blood proteins and can be 100 percent accurate. This is the type of testing that is done for matching donors for organ transplants. It must be performed in a specialized laboratory. Blood typing tends to be much less expensive than HLA typing. DNA TESTING This is called DNA fingerprinting. This method of twin typing is virtually 100 percent accurate and is performed only at specialized laboratories. DNA contains the genetic building blocks of an organism. In DNA fingerprinting, the DNA is separated into smaller segments and compared. The chance of two individuals having the same DNA sequence at the same time would be extremely rare unless they were monozygotic twins. The test can be performed from blood specimens or from scrapings of cells from the inner cheek, called a buccal smear. This method, while being one of the most accurate, is also the most expensive. Original text: http://www.nomotc.org/library/twinning_facts.html