The effect of pronuclear morphology on early development and chromosomal abnormalities in cleavage-stage embryos

Background: Pronuclear (PN) zygote morphology has been proposed as a useful tool for selecting the best embryos for transfer. Methods: PN morphology was recorded in 888 zygotes and classified according to similar/different PN size [groups A (n=816) and B (n=72)] and to the number, distribution and synchrony of nucleolar precursor bodies (NPB): subgroup I, pronuclei with 3-4 polarized NPB; subgroup II, 5-7 synchronic polarized NPB or 7-10 NPB distributed randomly; and subgroup III, morphologies other than those of groups I or II. Embryo development and chromosomal abnormalities were evaluated for each PN pattern. Results: In patients aged less than or equal to37 years, the number of zygotes reaching morula and blastocyst stage was significantly (P=0.0003) higher in group A than in group B. In group A, the incidence of chromosomal abnormalities was significantly (P=0.0247) lower than in group B, and significant differences were observed when pattern AI was compared with pattern AII (P=0.0280), AIII (P=0.0024), BIII (P=0.0077) and total B (P=0.0247). In patients aged >37 years, statistical differences among groups were not observed. Conclusions: In patients aged less than or equal to37 years, zygotes with similar PN size and with polarized NPB present the best prognosis based on embryo development and the incidence of chromosomal abnormalities, whereas in patients aged >37 years, this correlation does not exist.