Other than performing preimplantation genetic screening, the embryologist cannot assess with the naked eye which embryos are chromosomally normal. What we do have is years of research in morphometrics which outlines which embryonic markers are more likely to result in a live birth. Embryos are grown in culture media under controlled conditions to mimic the human body (temperature and pH). The longer the embryos stay in culture, the more information we have to differentiate each embryo. Embryos are evaluated on Day 1,2, 3, and 5 and every score is taken into consideration when choosing embryo(s) for transfer. 1 embryo = 1 baby is the ultimate goal. One paramount expanded blastocyst to transfer on Day 5 that results in a healthy baby with a small cohort to cryopreserve for subsequent frozen embryo transfers indicates a successful IVF cycle.
Recent data shows that the trophecopderm (the outer cells of the blastocyst which make the placenta) are the most important embryonic marker for implantation success. The cells should appear scalloped and cohesive. The inner cell mass (ICM, which will form the fetus) is also a very important to score and contributes to the overall grade of the embryo. This group of cells should be prominent in the cavity. The blastocele (fluid filled center) should be expanded and the zona should be thin. Thinning of the zona is extremely important for the hatching of the blastocyst; as the blastocyst expands, the zona thins, allowing hatching out, which must occur for implantation into the endometrium.