Selection pressure for the factor-V-Leiden mutation and embryo implantation.

The factor-V-Leiden mutation is seen in high frequencies in white people, despite its contribution to second-trimester abortion, preterm birth, and deep-vein thrombosis. The reason for its high frequency is not known. We investigated 102 mother-child pairs who had had successful in-vitro fertilisation by intracytoplasmic sperm injection as a model for human implantation. In 90% (9 of 10) of mother-child pairs who carried factor-V-Leiden mutation, the first embryo transfer was successful, compared with 49% (45 of 92) in factor-V-Leiden negative pairs (p=0.018, Fisher's exact test). Furthermore, the median number of unsuccessful transfers was lower in pairs who were positive for the mutation (0, range 0-2) than those who were negative (1, 0-8) (p=0.02, Mann Whitney U test) suggesting that improved implantation rate is an important genetic advantage of the factor-V-Leiden mutation.