The menstrual cycle is driven by four main hormones: gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and the ovarian hormones estradiol and progesterone. These hormones rise and fall in a coordinated sequence, each triggering the next stage of the cycle. A few supporting hormones, like inhibin, fine-tune the process along the way.
The Brain Starts the Cycle
Everything begins in a small region at the base of the brain called the hypothalamus, which releases GnRH in rhythmic pulses. The speed of those pulses acts like a control dial for the entire cycle. During the first half of the cycle (the follicular phase), GnRH fires roughly once every 60 to 90 minutes. After ovulation, progesterone slows it down dramatically, to about once every four hours. This shift in pulse speed determines which downstream hormones get produced and in what quantities.
Each pulse of GnRH signals the pituitary gland, a pea-sized structure just below the hypothalamus, to release FSH and LH into the bloodstream. Faster GnRH pulses favor LH production, while slower pulses favor FSH. This simple frequency change is one of the key ways the body shifts gears between the two halves of the cycle.
FSH and the Follicular Phase
FSH does exactly what its name suggests: it stimulates the growth of follicles in the ovaries. Each follicle is a tiny fluid-filled sac containing an immature egg. At the start of each cycle, rising FSH levels wake up a group of follicles and push them to grow. As these follicles develop, they begin producing estradiol, the most potent form of estrogen.
Here’s where it gets competitive. As estradiol rises, it sends a signal back to the pituitary to dial down FSH production. This creates a narrowing window: FSH levels start to drop, and only the follicle that has become most sensitive to FSH can keep growing. That follicle becomes the “dominant” follicle. The rest, unable to thrive on diminishing FSH, stop developing and break down. This selection process explains why most cycles release just one egg. The dominant follicle gains its edge not just because FSH drops, but because it develops more receptors for FSH than its neighbors, effectively grabbing more of a shrinking supply.
How Estradiol Triggers Ovulation
For most of the follicular phase, estradiol acts as a brake on the brain. Higher estradiol means less GnRH and less LH. This is negative feedback, the same kind of thermostat logic your body uses everywhere. But once the dominant follicle pumps out sustained high levels of estradiol for long enough, something remarkable happens: the feedback flips from negative to positive. Instead of suppressing GnRH and LH, estradiol now amplifies them.
This switch triggers a massive surge of LH from the pituitary. The LH surge is the direct cause of ovulation. It begins about 36 hours before the egg is released, peaks about 10 to 12 hours beforehand, and causes the dominant follicle to rupture and release its egg into the fallopian tube. Ovulation prediction kits work by detecting this LH surge in urine.
Progesterone and the Luteal Phase
After the egg is released, the emptied follicle transforms into a temporary hormone-producing structure called the corpus luteum. Its primary job is to produce progesterone, and it does so in large quantities. Progesterone levels jump from roughly 0.2 ng/mL in the follicular phase to as high as 25 ng/mL during the luteal phase, a hundredfold increase.
Progesterone reshapes the uterine lining to prepare for a potential pregnancy. During the first half of the cycle, estradiol made the lining grow thicker. Progesterone now stops that growth, switches the glands in the lining to a secretory mode (producing nutrients), and triggers changes in the surrounding tissue that would support an embryo burrowing in. It also slows down GnRH pulses, which suppresses further LH and FSH release and prevents another follicle from developing mid-cycle.
One effect you can actually measure at home: progesterone raises your basal body temperature by about 0.5 to 1 degree Fahrenheit after ovulation. Before ovulation, morning temperature typically hovers between 97.0 and 98.0°F. That post-ovulation rise stays elevated throughout the luteal phase and is the basis of temperature-based fertility tracking.
What Happens When No Pregnancy Occurs
If the egg isn’t fertilized, the corpus luteum has a built-in expiration date of roughly 10 to 14 days. As it breaks down, progesterone and estradiol levels plummet. This withdrawal of progesterone is the direct trigger for menstruation. Without progesterone to maintain it, the thickened uterine lining destabilizes. The specialized spiral blood vessels in the lining constrict due to a surge in local prostaglandins and other vasoactive compounds. Immune cells flood the tissue, inflammatory mediators increase, and the tissue breaks down and sheds.
Those same prostaglandins are responsible for menstrual cramps. They cause both the blood vessel constriction and the uterine muscle contractions that help expel the lining. As hormone levels bottom out, the pituitary is freed from suppression, FSH begins to rise again, and the next cycle starts.
Inhibin: The Fine-Tuning Hormone
Beyond the four major players, a protein hormone called inhibin helps regulate the cycle with more precision. Produced by the ovarian follicles (and later the corpus luteum), inhibin selectively suppresses FSH without affecting LH. This is part of the mechanism that ensures FSH drops at just the right time during follicle selection. There are two forms: inhibin B, which peaks during the follicular phase as follicles grow, and inhibin A, which is highest in the luteal phase. Together they act as a fine-tuning signal, making sure FSH stays within the narrow range needed at each point in the cycle.
How Hormones Affect What You Notice
These hormonal shifts produce changes you can observe without any lab work. Cervical mucus is one of the most reliable indicators. During the first half of the cycle, rising estradiol makes cervical mucus thin, clear, and stretchy, sometimes stretching several centimeters between your fingers. This consistency helps sperm travel. After ovulation, progesterone transforms the mucus into something thick, sticky, and opaque, forming a barrier that’s far less permeable.
Mood, energy, and appetite also shift across the cycle, though these vary widely between individuals. Estradiol tends to have mood-boosting effects, which is one reason many people feel their best in the days leading up to ovulation. The progesterone-dominant luteal phase, by contrast, is when premenstrual symptoms like bloating, breast tenderness, and irritability are most common. These symptoms intensify as both progesterone and estradiol drop sharply in the final days before a period begins.
Understanding this hormonal sequence gives you a framework for interpreting what your body is doing at any point in the cycle, whether you’re tracking fertility, managing symptoms, or simply curious about why you feel different from one week to the next.