The hypothesis regarding the role of lipids in human fertility suggests that cholesterol is a major substrate for steroid hormone synthesis and that it has been shown to affect the hormonal milieu and steroid hormone synthesis in both men and women. However, it remains unclear whether human lipid concentrations can affect time to pregnancy (TTP). For this reason, the study from Eunice Kennedy, USA, was published. Kennedy? Schisterman and his team at the National Institute of Child Health and Human Development conducted a study to assess the relationship between lipid concentrations and TTP in men and women. The study was a population-based prospective cohort study that recruited couples from 16 counties in Michigan and Texas between 2005 and 2009, using a sampling frame to identify couples who were planning to become pregnant in the future. The 501 couples who planned to become pregnant and discontinued contraception were followed for 12 months, or until pregnancy was detected based on human chorionic gonadotropin. The study found that couples with higher mean serum free cholesterol levels in both sexes did not become pregnant during the follow-up period and that serum free cholesterol levels in the female partner were significantly associated with prolonged TTP in both individual and couple-based cholesterol concentration models. Only in the model based on the cholesterol concentrations of both partners, the free cholesterol concentration of the male partner was associated with TTP. Sensitivity analysis suggested that the above correlations could not be explained by potentially unmeasurable confounding factors such as diet. This study suggests that serum free cholesterol concentrations in both men and women can influence TTP, highlighting the importance of cholesterol and lipid homeostasis in male and female fertility.