Luteinizing hormone (LH) is the hormone that directly triggers ovulation. A sudden spike in LH, often called the “LH surge,” causes the mature egg to break through the ovarian wall and release into the fallopian tube. The peak of this surge typically occurs 10 to 12 hours before the egg is actually released, which is why ovulation predictor kits measure LH levels in urine.
But LH doesn’t work alone. Ovulation is the result of a carefully timed chain of hormonal signals that begins weeks before the egg is released. Understanding each player in that chain helps explain why ovulation sometimes doesn’t happen on schedule, or doesn’t happen at all.
How the LH Surge Actually Works
For most of the menstrual cycle, LH circulates at relatively low levels. Then, about midway through the cycle (around day 12 to 14 in a typical 28-day cycle), LH levels spike dramatically. This surge does something very specific: it ramps up the activity of enzymes inside the dominant follicle that break down the ovarian wall. Think of it as a controlled weakening of the tissue surrounding the egg, creating an opening for the egg to pass through.
The surge itself is brief. LH levels rise sharply, peak, and then fall over roughly 24 to 48 hours. The egg is released about 10 to 12 hours after that peak. Once released, the egg survives for about 12 to 24 hours in the fallopian tube, which is why the fertile window is relatively narrow each cycle.
The Hormones That Set the Stage
LH pulls the trigger, but several other hormones load the gun. The process starts in the brain, in a region called the hypothalamus, which releases a signaling hormone called GnRH in rhythmic pulses. The speed of these pulses matters enormously. Early in the cycle, slower pulses favor the release of follicle-stimulating hormone (FSH) from the pituitary gland. As the cycle progresses toward ovulation, the pulses speed up, shifting the balance toward LH production instead.
FSH does exactly what its name suggests. During the first half of the cycle (the follicular phase), it stimulates a group of ovarian follicles to begin maturing. Each follicle contains an immature egg. Over the course of about two weeks, one follicle outpaces the others and becomes the “dominant” follicle. As this winning follicle grows, it produces increasing amounts of estrogen.
Here’s the clever part: rising estrogen initially suppresses FSH, which prevents the remaining follicles from catching up. But once estrogen reaches a critical threshold, it flips from suppressing the brain’s signals to amplifying them. This positive feedback loop is what ultimately triggers the massive LH surge. The GnRH pulses from the hypothalamus become so rapid they’re nearly continuous, flooding the pituitary with the signal to release LH. That same continuous signaling eventually desensitizes the pituitary, which is likely how the surge shuts itself off once ovulation is complete.
What Happens After the Egg Is Released
Once the egg leaves the ovary, the empty follicle transforms into a temporary structure called the corpus luteum. This structure starts producing progesterone, a hormone that prepares the uterine lining for a potential pregnancy. Progesterone levels rise significantly in the days after ovulation. Doctors sometimes use a blood test around day 21 to 23 of the cycle to check whether ovulation occurred: a progesterone level above 10 ng/mL generally confirms it did, while levels below that suggest either no ovulation or a problem with the timing of the test.
If the egg isn’t fertilized, the corpus luteum breaks down after about 10 to 14 days, progesterone drops, and the uterine lining sheds as a period. If fertilization does occur, signals from the early embryo keep the corpus luteum alive and progesterone flowing until the placenta takes over.
Why Ovulation Sometimes Doesn’t Happen
Because ovulation depends on such a precise hormonal sequence, disruptions at any point in the chain can prevent it. The most common example is polycystic ovary syndrome (PCOS), which affects roughly 1 in 10 women of reproductive age.
In PCOS, the normal balance between LH and FSH gets thrown off. GnRH pulses run too fast, which pushes LH levels chronically high while FSH stays relatively low. Without enough FSH, follicles begin to develop but stall before any one of them matures enough to ovulate. These stalled follicles accumulate on the ovaries, producing the “polycystic” appearance on an ultrasound. Meanwhile, the excess LH drives the ovaries to produce too many androgens (male-type hormones like testosterone), which further interferes with follicle growth.
Insulin resistance, which often accompanies PCOS, makes this worse. Elevated insulin stimulates the release of a brain chemical called kisspeptin, which revs up GnRH signaling even more, pushing LH higher and amplifying androgen production. It becomes a self-reinforcing cycle: high androgens disrupt GnRH pulsing, which raises LH, which drives more androgen production.
Other common reasons ovulation fails include extreme stress, significant weight loss or gain, thyroid disorders, and elevated levels of prolactin (the hormone associated with breastfeeding). Each of these disrupts the hypothalamic signals that start the whole cascade.
How Ovulation Predictor Kits Use This Biology
Home ovulation tests work by detecting the LH surge in urine. When you see a positive result, it means LH levels have started to rise, and ovulation is likely within the next 24 to 36 hours. This makes the day of the positive test and the following day the most fertile points in the cycle.
These kits are generally reliable, but they have limits. In PCOS, where baseline LH levels run high, the tests can produce misleading positives. Some women also experience an LH surge without the follicle actually releasing an egg, a phenomenon called luteinized unruptured follicle syndrome. A positive ovulation test confirms the hormonal signal was sent. It doesn’t guarantee the egg made it out.
For a more definitive confirmation, tracking basal body temperature can help. Progesterone released after ovulation raises resting body temperature by about 0.5 to 1°F. A sustained temperature shift over several days, combined with an earlier positive LH test, gives a more complete picture of whether ovulation actually occurred.