Extra or missing chromosomes can happen in any conception or pregnancy, simply by a random error in how the egg, sperm, or early embryo formed. However, the chances increase as the mother’s age increases. Based on embryo biopsies performed at the blastocyst stage (day 5/6), the chance of producing an embryo with an abnormal number of chromosomes is up to 37% in women under 35 and over 75% for women 40 or older.* *Data from 104,788 Trophectoderm (TE) samples screened at Natera.