Pregnancy and Paternity: New Fetal DNA Testing

Imagine you are a woman in a committed relationship. The worst happens and you are raped and become pregnant. What are your options?

Until recently, there was no reliable and safe way to determine paternity in the first trimester. This meant that many women chose to terminate their pregnancy without knowing who the child’s father was. Obviously this is not ideal.

If the woman could determine paternity, she could use that information to help in her decision on whether to terminate the pregnancy. Undoubtedly this would lead to fewer abortions as more women choose to carry their partner’s child to term.

The tricky part of a prenatal paternity test is obviously getting a hold of fetal DNA. One method is amniocentesis. In this procedure, a needle is inserted into the amniotic sac and fluid is withdrawn. This fluid contains a lot of fetal DNA but getting it is not only invasive, it is also normally done somewhere between the 15th and 18th week of pregnancy. This is too late for many women.

Other methods take advantage of the fact that there is a bit of fetal DNA floating around in the plasma of mom’s blood. These tests have the advantage that they are noninvasive and can be done much earlier in pregnancy…sometimes as early as 8 weeks. The disadvantage of the early versions of the test was that they weren’t always as accurate as we’d like.

There are anecdotal cases out there of women losing their partner’s child because they thought the child was the result of the rapist and keeping the rapist’s child because they thought the child was their partner’s. This is obviously unacceptable.

Recently a couple of new tests have come on line that appear to be much more accurate. Both use the fetal DNA found in mom’s blood but use novel strategies to improve accuracy.

Scientists at one testing company present results in a recent article in the New England Journal of Medicine to back up their claims of accuracy. They set up an experiment where they had the mother’s, father’s, and an unrelated man’s DNA along with DNA from the mother’s blood plasma. This was a blind experiment in that the scientists did not know which of the two tubes containing the men’s DNA was from the father. They were able to correctly pick the father in 30 out of 30 cases. They claim the odds of this happening by chance are less than one out of a billion. (For those interested, the test is available here.)

What was their secret? Well, as I talk about here, the key difference is that they focus on small bits of DNA that could have only have come from the dad. Any DNA they find in mom’s blood that doesn’t match hers had to come from the fetus. This eliminates any issues of contaminating DNA from mom. If all of this DNA matches a prospective father’s, then he is most likely the dad.

This company has competition from another testing company that is offering another high-powered test using fetal DNA found in mom’s blood. I’ve talked about the science behind this sort of test recently right here at QUEST. There isn’t a New England Journal article that focuses on their prenatal test per se, but they did manage to sequence the entire genome of a fetus so they are probably up to the task!

Even though it might seem counterintuitive, these tests will almost certainly decrease the number of abortions. And not only in cases of rape. These tests will also lead to more women keeping their children in cases of incest and even in some cases where a child might be conceived because of an extramarital affair.