Polyspermy in in vitro fertilization of human oocytes: frequency and possible causes

In an IVF program a total of 585 oocytes (180 patients) were examined for the presence of pronuclei 16 to 20 hours after the addition of spermatozoa. The overall fertilization rate was 71%, and in 58 (10%) of the fertilized oocytes, three or more pronuclei, indicating a failure of the block to polyspermy, could be observed. The frequency of polyspermy was related to the maturity of the oocyte, determined according to morphologic criteria. Immature oocytes showed a higher percentage of polyspermic fertilization (32%) compared to that of mature oocytes (6%). Preincubation of oocytes (for 0.5-1.5, 2-4, and 5-8 hours) prior to the addition of spermatozoa increased the fertilization rate (to 67%, 70%, and 83%, respectively). The polyspermy rate, however, was not significantly different between the various preincubation intervals (13%, 14%, and 19%, respectively). The polyspermy rate was affected by the number of spermatozoa used for in vitro fertilization. Insemination with 0.5-0.8, 1.0, or 1.5-2.0 X 10(6) spermatozoa/oocyte resulted in a polyspermy rate of 6%, 20%, and 32%, respectively. The appearance of polyspermic fertilization was not related to the age of the patient (which ranged from 20 to 45 years) nor to the method of ovarian stimulation (clomiphene, hMG, or clomiphene/hMG). Because of the high incidence of polyspermy under in vitro conditions it seems to be important to routinely examine the oocytes in the pronuclear stage. Reduction of the number of spermatozoa used for in vitro fertilization and the exact timing of insemination according to the maturity of the oocyte might reduce the occurrence of polyspermic fertilization.