Luteinizing hormone (LH) is the hormone directly responsible for triggering ovulation. A sudden spike in LH, often called the “LH surge,” causes a mature egg to break free from its follicle in the ovary roughly 24 to 36 hours later. But LH doesn’t work alone. Ovulation depends on a carefully timed sequence involving at least three other hormones, each playing a distinct role in preparing the egg, triggering its release, and supporting what comes after.
How the LH Surge Triggers Egg Release
For most of the menstrual cycle, LH circulates at relatively low levels. Then, around the middle of the cycle, it surges dramatically. In the days just before ovulation, LH levels can climb from a baseline of around 6 to 17 IU/L to several times that amount. This spike sets off a cascade of activity inside the dominant follicle, the fluid-filled sac in the ovary that houses the maturing egg.
The surge activates signaling pathways that do two critical things. First, it severs the microscopic connections between the egg and the surrounding support cells, freeing the egg from its anchoring network. Second, it triggers the production of enzymes that break down the structural proteins holding the follicle wall together. Collagen, the main scaffolding protein in the follicle, gets cleaved by specialized enzymes and essentially dissolved. Inflammatory-like chemicals called prostaglandins also ramp up, thinning the follicle wall at its weakest point. The result is a controlled rupture: the follicle breaks open, and the egg is swept into the fallopian tube. The whole process resembles a localized inflammatory response, which is why some people feel a twinge of pain (called mittelschmerz) on one side during ovulation.
The Hormones That Set the Stage
LH pulls the trigger, but other hormones load and aim it.
Follicle-stimulating hormone (FSH) kicks things off at the start of each cycle. As its name suggests, FSH stimulates several follicles in the ovaries to begin growing. Over the first week or so of the cycle, one follicle becomes dominant while the others fade. FSH drives the maturation of the egg inside that winning follicle, getting it ready for eventual release.
As the dominant follicle grows, it produces increasing amounts of estrogen. For most of the follicular phase, estrogen actually suppresses LH, keeping it in check. But once estrogen climbs high enough for a sustained period, something flips. The brain’s hormonal control center switches from suppressing LH to amplifying it. This is called positive feedback: rising estrogen signals the pituitary gland to unleash a massive burst of LH. In all spontaneously ovulating mammals, this estrogen-driven switch is what generates the LH surge.
There’s also emerging evidence that a small, early rise in progesterone, reaching roughly 0.5 ng/mL in the blood before the LH surge begins, plays a role in initiating the surge itself. This pre-ovulatory progesterone bump appears to act independently of LH, serving as an additional physiological trigger that helps coordinate the timing of ovulation.
What Happens Right After Ovulation
Once the egg escapes, the now-empty follicle doesn’t simply disappear. The remaining cells transform into a temporary structure called the corpus luteum, a saffron-yellow mass that seals the break in the ovary and begins producing hormones of its own. Its most important job is making progesterone, which thickens the uterine lining and prepares the body for a potential pregnancy. If the egg isn’t fertilized, the corpus luteum breaks down after about 10 to 14 days, progesterone drops, and menstruation follows. If pregnancy occurs, the corpus luteum keeps producing progesterone until the placenta takes over.
How Ovulation Predictor Kits Work
Because the LH surge reliably precedes ovulation by about a day to a day and a half, over-the-counter ovulation predictor kits (OPKs) are designed to detect LH in urine. When the test reads positive, it signals that ovulation is likely imminent, making the next 24 to 36 hours the most fertile window.
One thing worth knowing: not all kits use the same sensitivity threshold. Available tests trigger a positive result at LH concentrations ranging from 20 to 40 mIU/mL. A kit with a lower threshold (like 20 mIU/mL) will detect the surge earlier but may also pick up pre-surge fluctuations, while a higher threshold (like 40 mIU/mL) is more specific but could miss a shorter or lower-amplitude surge. If you’ve been getting inconsistent results, the sensitivity of your particular brand may be a factor worth considering.
When the Hormonal Sequence Breaks Down
Ovulation depends on each hormonal step happening in the right order and at the right level. When that sequence is disrupted, the result is anovulation, cycles where no egg is released.
The most common hormonal cause is polycystic ovary syndrome (PCOS), which affects a significant number of women of reproductive age worldwide. In PCOS, higher-than-normal androgen levels disrupt the normal hormonal signaling that leads to follicle maturation and the LH surge. Follicles may start growing but never reach the stage where they can respond to LH properly, leading to irregular or absent ovulation. Other causes of anovulation include thyroid disorders, chronically elevated stress hormones, very low body weight, and conditions affecting the pituitary gland’s ability to produce FSH or LH in the first place.
In all of these cases, the underlying issue traces back to the same hormonal relay: something prevents the normal buildup of estrogen, the proper FSH-driven follicle maturation, or the LH surge itself. Treatments for anovulation typically aim to restore or mimic one or more of these hormonal steps, helping the body complete the sequence that ends with an egg being released.