When you're trying to get pregnant, knowing that you've ovulated is crucial in knowing whether or not you've been successful in your quest. The LH surge, or a sudden increase in luteinizing hormone, is a fairly reliable indicator that ovulation will occur within 24 to 48 hours. As your menstrual cycle progresses and an egg follicle matures, estrogen levels increase, eventually causing a surge of LH. It's this surge that causes the egg to be released from your ovaries, a process more commonly known as ovulation. There are some cases, however, in which you can detect an LH surge and not actually ovulate.
Just because you detect an LH surge with an ovulation predictor kit (OPK), it doesn't ensure that you will ovulate. Some women will have a number of small surges or mini-surges before the final surge that triggers ovulation. These mini-surges are the body's way of gearing up to ovulate. To confirm that the surge you detected was a final surge, continue taking OPKs until they are negative or until you've confirmed ovulation by seeing a marked increase in your basal body temperature, the temperature taken at the same time first thing every morning.
It is possible to have an LH surge and not ovulate within the next few days. According "Taking Charge of Your Fertility," there are are a number of things that can delay ovulation even once your body has shown the signs. Stress, illness, medications, exercise and drastic weight changes can cause your body to change its mind (so to speak) and put ovulation on hold.
Polycystic Ovarian Syndrome
Polycystic Ovarian Syndrome (PCOS), a condition in which numerous cysts develop inside the ovaries, can cause you to have an LH surge and not ovulate. The cysts are caused by trapped egg follicles that don't release and fill with fluid. Woman with PCOS frequently experience hormonal imbalances caused by these cysts, including excess production of testosterone and luteinizing hormone. This imbalance can trigger a positive result on an OPK, regardless of whether or not ovulation will occur.
Luteinized Unruptured Follicle Syndrome
Occasionally, you can experience all the classic signs of ovulation, including an LH surge and change in basal body temperature and still not have ovulated. In a condition called, Luteinized Unruptured Follicle Syndrome (LUFS), the egg reacts to the LH surge but is unable to pass through the ovarian wall. The follicle then forms a cyst which, in most cases, dissolves on its own. Since your body reacts as though ovulation has occurred, the only way to confirm LUFS is by seeing the cyst on an ultrasound.
Many fertility drugs, including Clomid, can invalidate tests designed to detect an LH surge. These drugs increase the levels of LH in your system, which makes the test positive even if a true LH surge (and subsequent ovulation) has not occurred.