[Semen quality and fertility in a population-based follow-up study].

Little is known about the importance of semen quality in male infertility. We followed 430 first pregnancy planners with no previous reproductive experience recruited among more than 50,000 cohabiting trade-union members. The couples were consecutively enrolled into the study as they discontinued contraception and were followed up for six month menstrual cycles or until a pregnancy was verified. Each man provided a semen sample at enrollment. Women kept daily record of vaginal bleeding and sexual activity. The probability of conception increased with increasing sperm concentration up to 40 x 10(6)/ml, but any higher sperm density was not associated with additional likelihood of pregnancy. The proportion of sperm with normal morphology was strongly related to likelihood of pregnancy, independently of sperm concentration. In conclusion, sperm concentration and morphology are valuable biological markers of male fecundity. Reference values for semen quality based on biological knowledge can be established.