Why Prenatal Tests Don't Always Work

A new study in mice shows that embryos can actually "correct" themselves and turn into healthy babies, throwing into question some early prenatal tests.

Prenatal Testing: The Amazing Way Embryos Fix Themselves denk creative/Shutterstock

"Something could be wrong with the baby." These are the words no pregnant woman wants to hear. But many times, these "could be"s, found as the result of prenatal tests like a CVS (chorionic villus sampling), turn out to be nothing, and the babies are born healthy—which means the expectant moms go through a lot of unnecessary worry. One such woman happened to be a research scientist and wanted to find out why it happened to her. The result? A new study published in the journal Nature Communications, which found that abnormal cells in early embryos can actually be replaced by healthy cells, leading to normal babies and inspiring hope in worried moms-to-be everywhere.

Related: Prenatal Testing Cheat Sheet

Prenatal tests not always clear

Magdalena Zernicka-Goetz, Ph.D., of the University of Cambridge in the U.K., was inspired by her own experiences with a CVS test that showed unhealthy cells but ultimately resulted in her healthy son, Simon. In her research, she and her colleagues used mice to see what happened to early cells in an embryo that were abnormal. An embryo, the stage of a baby at weeks five through ten of pregnancy, is made up of cells that will become the fetus as well as the placenta. "Our finding is that cells with a chromosome imbalance in the part of the embryo destined to become fetus are eliminated by cell death—the good cells survive and the embryo is fine," she tells Fit Pregnancy. "Cells with a chromosome imbalance in the part of the embryo destined become the placenta can survive, but these will not cause any [harmful] effect on the fetus."

This study shows that embryos have a great ability to fix themselves. On the other hand, eggs (before they are fertilized) do not. "Imbalances in chromosome make-up can happen in the egg, something that is becoming increasingly common as women are choosing to become mothers when they are older, or in the embryo," says Zernicka-Goetz, who was herself pregnant at 44. "When it happens in the egg, all cells in the embryo will be affected—repair is not possible as all cells derived from this egg will be not normal."

So when a woman has an early test like a CVS, it can be difficult for doctors to figure out how to understand the results. "CVS tests are perfectly accurate—they just need to be carefully interpreted," Zernicka-Goetz says. "If all the cells in the CVS sample are affected, this means that the chromosome imbalance most likely arose in the mother's egg. If only a proportion of the cells sampled show an abnormality, our findings would suggest that the embryo should survive and be absolutely healthy."

The same goes for the new genetic blood tests like MaterniT21. "The MaterniT21 test detects embryo DNA circulating in the mother's bloodstream," Zernicka-Goetz says. "Like CVS, the cells from which this DNA comes are derived from the placenta. So the test is great for detecting chromosome imbalances that occur in the egg and which affect the entire fetus and placenta. But if there is a chromosome imbalance in only a proportion of the placenta, then the baby can still develop properly. So the blood test has similar advantages and disadvantages to CVS—but a blood test is more convenient."

A scientist's personal connection

In Zernicka-Goetz's personal experience with her CVS, only a portion of the cells were abnormal. "In my pregnancy, CVS revealed that one-fourth of the cells sampled had an extra chromosome 2," she says. "It wasn't clear at the time what was the chance for my developing a baby to contain cells with the same genetic [abnormality]. It was a difficult time." Through an amniocentesis later in her pregnancy, she was relieved to find out her son was healthy. "After I cried for a while, I came back bigger and clearer, ready to do something to understand what had happened to me, to help future mothers like me," she says. "It was exactly through this willpower that I decided to develop a model that would allow us to determine the fate of an [abnormal] cells within the developing embryo."

Even given her experience, Zernicka-Goetz would advise expectant mothers to undergo a CVS if recommended by their doctor—but to take the results with a grain of salt, especially before making any serious decisions about the pregnancy. "I would advise older mothers, like I am, to have a test," she says. "If these tests detect chromosome imbalances affecting only a proportion of cells, it is important to remember that these early tests analyze tissue from the placenta and not the fetus. A chromosome defect in some placental cells will most likely not matter. Even if cells with that defect were inherited by the fetal part of the embryo, our work in mice shows that these cells wouldn't survive and their place would be taken by normal cells. This gives a lot of hope, as I have all reasons to predict that the same will be true for human embryos."

The next stage of research is to discover exactly what amount of healthy cells is needed to completely repair the embryo. In the meantime, Zernicka-Goetz hopes her story will inspire hope in older moms like herself. "I believe that science needs us when we cry, as we can then use not only logic but also emotions in our willpower to solve life mysteries," she says. "I felt so happy when we first observed that [abnormal] cells become eliminated by cell death and are taken over by normal cells as the embryo develops. This provided a most likely explanation for my own pregnancy result, as it is quite likely that what we observe in the mouse embryo is true for the human embryo."