On the Fifth Day
If you’re transferring day-5 embryos – called blastocysts – evaluating embryos is a completely different story. “You can’t count the cells anymore because around day 4, they compact and become like a ball,” Dr. Sakkas explains. “By day 5, that ball starts to have fluid accumulating in the center.” So embryologists first look at how much the embryo has expanded, typically giving it a rating from one to six. A score of at least three or four is ideal. In addition, these embryo specialists grade the quality of the outer-cell layer, the part of the embryo that will develop into the placenta, and the inner cell mass, which eventually becomes the baby. If these cells are tightly packed, they earn an A; if they’re more loosely grouped, they get a B or C. A high-scoring day-5 embryo, for example, might be termed a grade 4AA, which means the embryo got an “A” for both the inner- and outer-cell layers.
If a patient doesn’t have any top-drawer embryos, a fertility doctor may still opt to transfer lesser embryos. This only means the chances of implantation are lower; it does not mean that the embryo will develop into a less-than-stellar baby. “There’s absolutely no correlation between embryo morphology and a baby’s health,” Dr. Garrisi stresses.
What Fertility Specialists are Learning
Though embryologists devote a lot of time and care to grading embryos, they caution that these ratings aren’t the whole picture. In the same way that a student’s SAT score might not hint at her talent for piano, or truly gauge her potential for success in life, an individual embryo’s score doesn’t always accurately assess its chances of implanting. “We are judging the book only by its cover,” Dr. Go says. “There’s a whole universe of information that is not revealed to us.”
The universe embryologists are exploring goes well beyond what can be seen under a microscope. They’re now delving into genetics and protein synthesis, for example, to get more clues about an embryo’s potential. So far, though, the only additional tool now in use to help labs select embryos is preimplantation genetic screening (PGS), an expensive technique that involves removing one or two cells from the embryo on day 3 and screening the cells for chromosomal disorders that would lead to Down syndrome and Kleinfelter's syndrome, for example. This is the same process used in preimplantation genetic diagnosis (PGD); the difference is that PGD screens for specific diseases, such as cystic fibrosis, Duchenne muscular dystrophy, or Tay-Sachs.
In the early 1990s, when PGS first appeared on the scene, the technology seemed like it would dramatically improve pregnancy rates because it provided new information about how to select robust, healthy embryos for transfer. “But this turned out not to be a panacea for embryo selection,” Dr. Go says. Not only is PGS very costly and invasive (on rare occasions, embryos can be destroyed in the process), but it can occasionally result in a false positive or a false negative and it can’t detect as many disorders as the prenatal tests amniocentesis or chorionic villus sampling (CVS). At this point, doctors may recommend genetic screening of embryos primarily for fertility patients who have had unexplained recurrent miscarriages and/or a family history of specific heritable diseases, but not as a general screen for embryo quality.