The risk of aneuploidy increases dramatically with maternal age. The likelihood of having a clinically recognized trisomic pregnancy is 2% to 3% for women in their 20s but increases to greater than 30% for women in their 40s. The basis for the increased risk of aneuploidy in older women is unknown. One of the most cited potential mechanisms is the production-line hypothesis, which was initially proposed in 1968. The hypothesis has 2 key components. First, it assumes that the first oocytes to enter meiosis are the first to be ovulated and that those entering meiosis last are ovulated at the end of the reproductive lifespan. Second, it assumes that the first oocytes entering meiosis have more recombination events (crossovers) than do those entering meiosis later in fetal life. Experimental evidence has been consistent with the first tenet of the production-line hypothesis: Radiolabeling studies in rodents have demonstrated that that there is a production line; the first oocytes to enter meiosis are the first to be ovulated. However, the second tenet (the predicted relationship between recombination levels and maternal age), which assumes a difference in recombination levels between oocytes entering meiosis early in fetal life and those entering late in fetal life, remains unproven. The aim of this study was to test the second tenet of the production-line hypothesis by examining meiotic recombination in human fetal oocytes. Molecular cytogenetic analysis of second-trimester human fetal ovaries was used to directly examine the number and distribution of crossover-associated proteins in prophase-stage oocytes. A total of 8518 eggs from 191 ovarian samples were collected from second-trimester elective abortions. Crossover-associated proteins in prophase-stage oocytes were examined with immunofluorescence. A high incidence of abnormal cells was found in aging women, but just as many was found in younger women. There was considerable variation in crossovers within and among women, but no evidence for a relationship of recombination-associated changes with maternal age. This study provides a direct test of the second tenet of the production-line hypothesis and demonstrates that it is not the basis for the maternal-age effect on aneuploidy.