LIF and Infertility: When the quantity of cytokine secretion was compared on the same day of the cycle between the two groups of women, LIF production in fertile women on days 18–21 of the menstrual cycle was 3.5 times higher than that in infertile women with RIF and 2.2 times higher than LIF production in women without RIF (p < 0.01 and p < 0.05, respectively) [55].