Our preceding studies have already demonstrated the advantage of intracytoplasmic morphologically selected sperm injection (IMSI) over the conventional IVF-ICSI procedure in terms of pregnancy rate. This study was undertaken to determine whether the increased pregnancy outcome was attributable to the preferred nuclear morphology of the selected spermatozoa, and not to the special sperm preparation technique modified by IMSI. Comparison between two matched IMSI groups, i.e. negative comprising 38 cycles, where no spermatozoa with intact nuclei were available for microinjection; and positive, involving ovum microinjection by spermatozoa with strictly defined morphologically normal nuclei. Implantation and pregnancy rates were significantly higher, and abortion rates significantly lower, in the positive group compared with the negative group (25.0+/-25.9 versus 5.9+/-12.9%, F=15.8, P< or =0.01; 52.6 versus 18.4%, chi2=9.7, P< or =0.01; and 10.0 versus 57.1%, chi2=7.1, P< or =0.02, respectively). Implantation and pregnancy by ICSI is associated with morphological nuclear normalcy of sperm. Sperm with a morphologically abnormal nucleus usually have low fertility potential, but some with certain nuclear abnormalities may still be able to produce pregnancy following ICSI.
