Chromosome analysis of human oocytes and embryos: does delayed fertilization increase chromosome imbalance?

Thirty per cent of a sample of 120 unfertilized human oocytes carried chromosome abnormalities highly correlated with maternal age (38% in patients greater than 35, as compared with 24% in younger patients). Fertilized eggs, when observed 17 h after insemination, showed in 1.6% a single pronucleus suggesting parthenogenetic activation. In 92% of the cases two pronuclei were observed and the rate of chromosome anomalies depended on the morphological aspect of the embryos. Triploidy was also encountered in 6.4% of the eggs leading to an overall rate of chromosome aberrations reaching 29.2%. Delayed fertilization drastically increased the rate of chromosome anomalies (87%) as well as the rate of mosaicism: 30% versus 10.6% in timely fertilized eggs. The high rate of chromosome disorders in early life after in-vitro fertilization (IVF) raises the ethical question of the opportunity of carrying out a genetic control of normality in human embryos at the preimplantation stage.