Chromosome anomalies in early human embryos.

A study of the chromosome constitution of an unselected group of human preimplantation embryos fertilized in vitro showed that, of 30 studied, 7 (23%) were abnormal: 6 were aneuploid, half being trisomic and half monosomic. If the increased maternal age of the oocyte donors and the lethality of autosomal monosomy is taken into account, the frequency of this kind of error is of the same order as in clinically recognized pregnancies. One oocyte had a structural abnormality. The cause of this kind of error is unknown but could be due to errors arising in spermatozoa or oocytes or the effect of environmental factors. A 'reject' group of oocytes with 3 pronuclei showed heterogeneity with respect to their chromosome complements, only one being triploid. The frequency of tripronucleate oocytes after in-vitro fertilization appears to be higher than in clinically recognized pregnancies, presumably because of the influence of superovulation on the oocytes and, under our conditions, this frequency is 9%. Therefore, if the selected and 'rejected' groups of embryos are considered together, 10 out of 33 (30%) embryos can be considered as abnormal.