Chromosome status of untransferred (spare) embryos and probability of pregnancy after in-vitro fertilisation.

Many spontaneous abortions are associated with chromosomal abnormality of the fetus. In in-vitro fertilisation (IVF) the chromosome status of untransferred ("spare") embryos and subsequent fate (pregnancy or not) of the transferred sibling embryos might be related. Since the spare and transferred embryos of a patient's cycle genetically are full siblings, the inherited chromosomal abnormalities in spare embryos have a 50% probability of also appearing in transferred embryos. We have tested whether chromosome analysis of spare embryos has predictive power for transferred embryos. 48 couples with a total of 437 embryos were selected because their spare embryos (1-4 per couple; 76 total) were successfully analysed for chromosome status. 16 patients became pregnant. These women produced a higher proportion of chromosomally normal spare embryos (9/24; 37.5%) than those who did not achieve pregnancy (1/52; 1.9%). The proportion of patients who had only normal embryos was significantly higher (p = 0.012) in the pregnant group than in the non-pregnant group, and the proportion of patients who had only abnormal embryos was significantly higher (p = 0.001) in the non-pregnant group. Patients with preclinical and clinical pregnancy losses had only chromosomally abnormal spare embryos; by contrast, 50% of spare embryos from patients with ectopic pregnancies were normal. The proportion of spare embryos that were normal (13%, 10/76), was similar to the livebirth rate of 11% per transferred embryo (19 infants from 171 transferred embryos). These results suggest that chromosome analysis of spare embryos may have predictive value for their transferred sibling embryos. We conclude that improving detection of chromosomally normal embryos for transfer should improve the success rate in IVF.