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A DNA test can prove that two people are related and the degree to which they are related. If they are identical twins DNA will show this. If they are fraternal, their DNA will show only that they are siblings.
Identical twins are two different people who grew from a single fertilized egg. Once the sperm reaches the egg, the egg divides into two identical cells, which eventually grow into physically identical and nearly genetically identical people.
Fraternal twins are two different eggs fertilized by two different sperm. They are not identical and aside from being in utero and born at the same time, they are biologically no closer than any other siblings.
Not only can a DNA test help you determine if you and another person are twins, but it can also help an expectant mother determine if her twins are fraternal or identical while she is pregnant.
Can a DNA Test Give a False Positive?
A DNA test can give a false positive reading. A test performed early in pregnancy can show that a mother is carrying twin babies. But due to “vanishing twin syndrome,” the test can produce a false positive. There is a second baby when the test is administered, but a miscarriage causes only one baby to grow to term and be born.
Additionally, undetected tumors and mosaicism might also trigger a false positive in a DNA test.
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Can Twins Have Different Fathers?
Yes, but it is extremely rare. In most cases, nobody will ever know that there are two different fathers. If two different sperm fertilize two different eggs at approximately the same time, it can result in two babies that appear to be fraternal twins. DNA testing would show them to be only half-siblings.
How Early Can Twins Be Detected?
Twins are detectable very early in a pregnancy. In the case of fraternal twins, there will always be two fertilized eggs. With identical twins, they would not be detected until the egg splits, but this happens very early in the development process.
Di-di twins, which are twins with their own separate inner and outer sacs and placentas, are detectable at week five of the pregnancy. Heartbeats might not be discoverable yet, but the sacs have formed and are visible.
Mo-di or mo-mo twins aren’t detectable until after six weeks. These types of twins share a single sac and identifiable only by their two heartbeats.
Di-di, mo-di, and mo-mo is not always an indication as to whether twins are identical or fraternal. For instance, dichorionic diamniotic (di-di) have their own inner and outer sacs and placentas. This type of twins can be fraternal or identical, though identical di-di twins are sometimes mistaken for fraternal twins. All fraternal and a third of all identical twins are di-di.
Mo-di and mo-mo twins are always identical. Monochorionic diamniotic (mo-di) share one placenta and one outer membrane, but each has its own inner membrane. This is the most common type of identical twins. Monochorionic monoamniotic twins share a single placenta and outer and inner membranes. This is a very rare occurrence and accounts for only about one percent of all identical twins.
Keep in mind, two placentas sometimes fuse into one, which makes it seem as if fraternal twins are identical. A DNA test would prove otherwise.
How are After-Birth Twin DNA Tests Performed?
If it was impossible to tell for sure during the pregnancy that twins are identical, a test is performed after birth. This can be done using a blood sample from the placenta(s) or using by swabbing the cheek cells from each baby. Samples can be taken from the comfort of your own home and sent to a lab for analysis.
Why Take an Identical Twin DNA Test?
Sometimes the reason for taking a DNA test to prove twins are identical is mere curiosity. Twins can look very much alike without being identical – the same as siblings born at different times. Confirming that twins are indeed identical, if it was not determined in utero, lets them know they are of a rare variety.
Another reason to take an identical twin DNA test is that the twins want to participate in a scientific study. Twin studies are very common, but participants must prove they are identical or fraternal before they are admitted into most studies.
Some mothers choose to undergo testing during pregnancy to determine if their twins are identical because this increases the risk of complications. Approximately 15 percent of monochorionic twins experience twin-to-twin transfusion syndrome, in which one twin gets too much blood and the other does not get enough. Mothers pregnant with monochorionic twins see their doctor more often than they would in other types of pregnancies.
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“Twins with Different Fathers: Is It Possible?” Flo.Health - #1 Mobile Product for Women’s Health, flo.health/pregnancy/pregnancy-health/twins-with-different-fathers. Accessed 1 Dec. 2020.
JoNel Aleccia. “UW Experts Shed Light on False Positives in Prenatal Tests.” The Seattle Times, Apr. 2015, www.seattletimes.com/seattle-news/health/uw-experts-shed-light-on-false-positives-in-prenatal-tests. Accessed 1 Dec. 2020.