Only one in three fertilizations results in the birth of a baby. Many embryos do not develop to term because they carry an incorrect number of chromosomes; they are aneuploid. We study how aneuploidy arises at the beginning of life. Aneuploidy in embryos is a main cause of pregnancy loss and infertility. It often results from chromosome segregation errors in the egg, but also frequently arises in the early embryo. Our recent work shows that aneuploidy often develops when the genetic material from both parents combines after fertilization. This is due to a remarkably inefficient process.