This picture shows an embryo biopsy.
Each embryo is between three and four cells which are expected on the day after fertilization. Embryo testing is possible with pulling a single cell out of the embryo.
By removing one cell from the embryo we are able to test for single gene disorders (e.g., cystic fibrosis, beta thalassemia) or balanced translocations. Then, disease-free embryos can be selected for transfer and pregnancy.