In a normal menstrual cycle, a woman releases a mature egg from a follicle that develops in the ovary. Luteinizing hormone (LH) matures the egg inside the follicle and causes the follicle to burst, releasing the egg. The rise in LH, called an LH surge, is triggered in response to rising estradiol (a form of estrogen) levels, and normally precedes ovulation, the release of the egg, by 24 to 48 hours, the American Pregnancy Association explains. If no LH surge occurs, ovulation won't take place. Several factors prevent an LH surge.
No Follicular Development
Sometimes, no follicular development takes place during a given month. Because the LH surge is caused by rising estradiol levels, and rising estradiol levels come from developing follicles, lack of follicular development means that estradiol remains low and LH isn't triggered. Typical causes for no follicular development include elevated follicle stimulating hormone (FSH) levels, which occur in peri-menopause or menopause and often prevent an egg from developing.
Diana Hamilton-Fairley and Alison Taylor report in an article entitled "Anovulation", published in the September 2003 edition of the British Medical Journal, that polycystic ovary syndrome, where abnormally high levels of male hormones may prevent follicle recruitment, accounts for as many as 70 percent of cases of anovulation. Women who are severely overweight or underweight often do not ovulate because their hormone levels are out of balance. Stress, illness or weight changes can also interfere with follicle development in a given month.
Poor Follicle Growth
Sometimes, a follicle will begin to grow normally and then stop. Estrogen is produced but not in sufficient quantities to trigger the LH surge, and the egg is not released.
LH Suppression with Medication
Several medications are used in assisted reproductive technology to suppress the release of LH, which can prematurely release follicles before an egg retrieval in in vitro fertilization. These medications, which include Leuprolide acetate, Ganirelix and Cetrotide, are called Gonadotropin Releasing Hormone (GnRH) antagonists or agonists, the Advanced Fertility Center of Chicago states. These medications are given subcutaneously, under the skin, until approximately 36 hours before a scheduled egg retrieval to prevent early ovulation and loss of the follicles.