It works by preventing the effects of progesterone thus stopping ovulation. In severe cases, malnutrition disrupts ovulation and causes a lack of menstruation. Luteinizing hormone also stimulates gene expression leading to ovulation . Infertility due to lack of ovulation is common. Weight loss or anorexia can also cause hormonal imbalance, leading to irregular ovulation (dysovulation). A surge of Luteinizing Hormone (LH) triggers ovulation. In many cases, metformin also supports ovarian function and return to normal ovulation. Females are slightly larger than the males and exhibit high annual ovulation and pregnancy rates. When someone is infertile due to lack of ovulation, PCOS is the most common cause. They work primarily by preventing ovulation or fertilization. Initiation of ovulation after menarche is not inevitable. Inhibition of follicular development and the absence of an LH surge prevent ovulation. Inhibition of follicular development and the absence of a LH surge prevent ovulation. During ovulation, this thickens for implantation. The absence of ovulation is called "anovulation".