Prenatal Diagnosis

The science behind prenatal diagnosis

Prenatal diagnosis is the screening or testing for genetic diseases or other conditions before a child is born. Although there are many different techniques and tests, I will focus on a few:

  • Preimplantation genetic diagnosis (PGD): During an IVF cycle, cells from the developing embryo can be genetically analyzed for chromosomal abnormalities - usually trisomy 21 (Down Syndrome), trisomy 13 and trisomy 18. The parents can then determine which embryos, if any, to transfer into the uterus. It is also possible to determine the sex of the embryo.

  • Ultrasound: An ultrasound uses sound waves to “look” at a fetus as it develops in the uterus. Since sound waves reflect off of tissues differently depending on the density of the tissue, the sonographer can create images of the fetus’ external and internal anatomy. Ultrasound can be used to measure an embryo or fetus in order to predict the due date, detect twins, diagnose heart and other growth defects, measure heart rate, look for signs associated with Down Syndrome, and determine the sex of the fetus.

  • Chorionic villus sampling: The chorionic villi are a part of the placenta that arises directly from the embryo (as opposed to the mother). They are, therefore, genetically identical to the developing embryo. Chorionic villi can be removed from the placenta and genetically analyzed to detect Down Syndrome and other genetic disorders. CVS has a slight risk (.5-1%) of miscarriage, and can also lead to amniotic fluid leakage and/or infection.

  • Amniocentesis: A fetus develops in the amniotic sac, which is full of amniotic fluid. The amniotic fluid contains fetal cells that have naturally sloughed off. The doctor, using an ultrasound image as a guide, inserts a needle through the mother’s skin, abdominal wall, uterine wall, and into the amniotic sac, away from the fetus. Amniotic fluid is then collected, and the fetal cells contained in the fluid can be analyzed for genetic conditions such as Down Syndrome. Amniocentesis has similar risks as CVS, although CVS can be done earlier.

  • Maternal Blood Testing: A relatively recently developed technique can detect fetal DNA in the mother’s blood stream. Thus, with a simple blood draw, the fetus’ sex can be determined, and it can be screened for a variety of genetic disorders.

Societal Impacts

While the above techniques may sound like wonderful advances in medicine, they also have their downsides. First, there’s a difference between screening and testing. A prenatal genetic screen - like maternal blood testing - only gives the level of risk of a condition, but it can not with 100% certainty diagnose a condition. Prenatal genetic tests - like amniocentesis and CVS - are more diagnostic, but also have higher risks for the pregnancy. And if a non-invasive screen detects a high risk for Down Syndrome, for example, the parents are then faced with the decision of whether or not to do a more invasive, high risk test in order to more accurately determine whether Down Syndrome is present. If the results of a CVS or amniocentesis indicate a genetic disorder, then the couple could be faced with the decision to terminate the pregnancy. At the very least, these weeks of testing and waiting for results can be draining and extremely stressful. Pregnancy can already be a stressful time, and these prenatal screens and test can add to that anxiety.

On the other hand, some parents feel that they’d like to know the risks, regardless of the outcome of a test. If a screening test comes back positive for a genetic disorder, then the parents at least won’t be surprised at the birth. They’ll have time to prepare, educate themselves, and possibly arrange for special care that might be needed for their newborn.

Some people opt out of the screens and tests altogether. They might argue that what’s meant to be is meant to be, and they’d rather spare themselves the stress and anxiety surrounding these tests and their results.

And finally, with PGD it is possible to choose the sex of your child. This is illegal in some countries, including Canada - but not the US. Given that IVF and PGD could cost $15000-$20000, the opportunity to screen embryos for genetic conditions and possibly select the sex would not be affordable for everyone. Is this fair? Should people be allowed to choose the sex of their children?

Personal opinions

As someone who loves science, I’m fascinated by these medical advances, and support further research into tests and screens like these. It’s exciting when new, improved tests come out that can help people get answers to their pregnancy concerns, and hopefully allay some of their fears. However, it’s easy for me to support these tests in a general, abstract way. It becomes more complicated when we’re talking about real pregnancies in my personal life. I’ve gone through some of these discussions and decisions, and sometimes there is no easy answer. I can see why people would be in the “no testing” camp - the screenings and testing definitely can raise anxiety and stress levels during pregnancy, which can already be pretty stressful. Still, I’m glad the tests are available, and I think they should continue to be offered to pregnant women. Key to this though, is that they need to be able to make informed decisions. Through discussions with their doctor and a knowledgeable genetic counselor, the parents-to-be can weigh the pros and cons for themselves, and then decide how they want to handle the conundrum of prenatal genetic diagnosis.


Nierneberg, C. (2014). Prenatal Genetic Screening Tests: Benefits & Risks. Retrieved February 24, 2016, from

Sidhu, J. (n.d.). Women Are Paying Huge Sums To Have a Daughter Rather Than a Son. Retrieved February 24, 2016, from

I wish I hadn't known: The ups and downs of prenatal testing. (n.d.). Retrieved February 24, 2016, from