To Test or Not to Test: Zygosity in Twinsby Valorie Delp | More from this Blogger 02 Jul 2006 06:32 AM Most people assume that if twins look very much alike, the twins must be identical. But actually, determining whether or not twins are identical, (or their zygosity) can be a costly proposition. Most people know that fraternal twins are the result of two eggs being fertilized, and that identical twins are the result of one egg that was fertilized and then split. Fraternal twins share no more than 50% of their genes and are no more alike than other siblings in the family. However, identical twins are genetically. . .well, identical. Many times zygosity can be determined simply by looking at the set of twins. First of all, if they are a boy/girl set they must be fraternal. But frequently, around age 2, fraternal twins begin to look different from each other. However, one very common misconception is that if the twins look alike, they must be identical. This is not always the case. There are occasions where zygosity cannot be determined simply by looking at the twins. The most reliable way to tell if same sex twins are fraternal or identical is through DNA testing. DNA testing is costly, but painless and is reliable in determining zygosity 99.99% of the time. But should you have your twins tested to see if they're identical or fraternal? There are several good reasons to find out the zygosity of your twins: For their own interest To know the answer to the question, "are they identical?' when someone asks To assess the risks for the mother of having twins again (there is an increased risk of having twins again for mothers of fraternal twins) It could be medically relevant at some point On the other hand, some people prefer not to find out. The test can be very expensive. Many parents of twins are simply satisfied not knowing for sure, but having a good guess based on presentation and separation of the placentas, etc. While DNA testing is really the only 100% sure way to know, other factors can indicate either fraternal or identical. If it ever does become medically necessary the hospital or doctor would likely have to perform the test again to be sure anyways. In short, there are several reasons to consider having your twins tested. We chose not to have our girls tested. We have a good inkling what a zygosity test would reveal for our girls. What is your guess--fraternal or identical?
Learn more about Valorie Delp  Hello everybody! My name is Valorie and I am one busy lady! When I'm not writing or editing for families, I am busy trying to get my brood of 5 in line. Relevantbaby tags User Comments  Catherine Ipcizade (5617) 02 Jul 2006 09:06 AMThey are beautiful, Valorie---simply beautiful. Good article--my parents recently told my sister and I they don't know if we're identical or not. It was a bit shocking. We looked exactly alike when we were little. Now we're different weights and with different hair colors so it's harder to tell....I'd like the test sometimes just to know for sure! Other times, I just don't care. Thanks for the information.  Valorie Delp (49340) 02 Jul 2006 10:33 AMThank you--I'm rather partial to them myself! I think that with our parents generation, they were told based on wether or not the placentas were together or separate. But I think partly because of DNA testing becoming more available and reliable and partly because the incidence of twins is so much higher now a days. . .it's more common to find out via testing. Anyhow, I'm glad you liked the article. Catherine Ipcizade (5617) 02 Jul 2006 11:13 AMAh, that explains the confusing comment I was told about the placenta falling on the floor after we were born, causing confusion.....thanks for the clear-up! Valorie Delp (49340) 02 Jul 2006 02:05 PMOne placenta is always monozygotic (identical). However, babies can implant close to each other and the placentas can become fused giving the appearance of one placenta when there are really two. On the other hand, two placentas can be either dyzygotic (fraternal) or they can also be monozygotic--it depends on when the egg split as to whether or not identical twins have one or two placentas. Placentas are usually born (whether there are one or two) after the babies just like with a singleton. And in case any one is following this and is curious--my girls are fraternal! They have different blood types!  Kay M. (300) 29 Jul 2006 09:58 AMThanks for sending me in the direction of this article. Yes, we were called fraternal twins. Interesting story though. My mother (um the biological one... I was adopted later) realeased the first egg which became my sister. From what I understand the split off egg which was me... was not fertalized right away. My twin thrived in the womb. I did not. She was able to go home right away without any health problems. I spent nearly a month in an incubator. We had very few differences. Our birthmarks were slightly different, and when my hair came in it was more curly than hers. In childhood we looked enough alike we could switch places. Hee Hee. When we were adopted at age 10 our new parents decided we needed individual identities so they stopped the dressing alike and even had us in different classrooms. We began to get different haircuts and look slightly different. They even had us celebrate our birthdays on seperate days. I think it was good for us. Thanks again for your articles! I will continue to read everything you write. I really like what I have read so far. Valorie Delp (49340) 29 Jul 2006 05:04 PMHas anyone mentioned the term polar twinning to you? I think that's what it's called when twins share 75% of their genetic DNA but part of the egg is fertilized later. I'm obviously not a doctor so I don't know, but it sounds like that's what it could be. I am absolutely fascinated with twinning. Thanks too for the kind words.  Kara |
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