Association of pronuclear Z score with rates of aneuploidy in in vitro-fertilised embryos

The present study aimed to correlate the Z-scoring system to outcomes in in vitro fertilisation (IVF) cycles performed in conjunction with preimplantation genetic diagnosis. In 178 cycles, a total of 468 pronuclear embryos was scored prospectively using the Z score before culture and biopsy on Day 3 to detect chromosomes 13, 16, 18, 21, 22, X and Y. The results showed significant differences between Z1 and Z4 scoring embryos, with Z1 giving an increased biopsy rate (83.3% v. 51.1%), embryo quality (> or = fair: 83.3% v. 57.8%) and embryo growth rate (> or = six cells: 87.0% v. 44.4%). The incidence of chromosomally normal embryos decreased from Z1 to Z4 (Z1: 40%; Z2: 29.7%; Z3: 22.7%; Z4: 13.6%; P < 0.04). Thus, the Z scoring, mainly Z1 and Z4, correlated significantly with the expected embryo outcomes and preimplantation genetic diagnosis findings. Further improvement of pronuclear scoring with the use of a two-stage assessment is proposed.