The hormones, step by step

The cycle runs on a handoff between the brain and the ovaries.

The brain starts it

A region at the base of the brain called the hypothalamus releases a hormone called GnRH in pulses. travels to the pituitary gland, which responds by releasing and into the blood. FSH and LH travel to the ovaries and tell them to make and .¹

A diagram of the signal chain. The hypothalamus releases GnRH, which tells the pituitary gland to release FSH and LH. These reach the ovaries, which make estrogen and progesterone. Estrogen and progesterone signal back to the pituitary.

The ovaries answer back, and the message changes during the cycle:

Early on, rising estrogen tells the pituitary to ease off. With less FSH, only the most advanced follicle keeps growing, and the others stop.
When estrogen gets high enough, the message flips. The pituitary releases a big burst of LH, with a smaller rise in FSH. That burst triggers ovulation. It only comes once a follicle has grown enough to make that much estrogen.
After ovulation, the message goes back to "ease off."¹

The month at a glance

A simplified chart of FSH, estrogen, progesterone and LH across a cycle. FSH is moderate early and falls, estrogen peaks just before ovulation, LH spikes at ovulation with a small FSH rise, and progesterone rises afterward and falls before the next period.

Watch for the order: starts the follicles growing, climbs, spikes to trigger the egg's release, then takes over. The steps below walk through it.

Step by step

Follicles start to grow. The releases follicle-stimulating hormone (FSH), which makes follicles in the ovaries grow.
Estrogen rises and builds the lining. As the follicles grow, the hormone estrogen rises. makes the lining of the uterus, called the endometrium, grow and thicken. That way, if a pregnancy happens, an egg can attach to it.
A surge of LH triggers ovulation. Luteinizing hormone (LH) is released by the pituitary gland and tells an ovary to release an egg. It starts to surge about 36 hours before ovulation and peaks about 12 hours before. Some people test their levels to find out when they ovulate.²
The empty follicle becomes the corpus luteum. After it releases an egg, the main follicle turns into a structure called the corpus luteum. It makes progesterone, along with some estrogen. matures the lining for a possible pregnancy. Glands in it develop and store sugar for energy, and the blood vessels that supply it expand.¹ It also makes cervical mucus thicker and less stretchy, which makes it harder for sperm to pass, and it raises body temperature slightly.¹
If there's no pregnancy, hormones fall. The dissolves and hormone levels drop. Very low estrogen and progesterone tell the body to start a period, and the lining sheds.
If there is a pregnancy, the corpus luteum stays. A fertilized egg releases a hormone called hCG, which tells the corpus luteum to keep making progesterone. Progesterone stays high, so the lining is not shed and no period starts.¹

A lot of what people notice during the cycle follows these rises and drops. The next two lessons explain how.

Recap

The cycle starts in the brain. The hypothalamus signals the pituitary gland, which releases FSH and LH. Estrogen and progesterone feed back to the pituitary.
FSH makes follicles grow, and rising estrogen thickens the lining of the uterus.
A surge of LH triggers ovulation.
After ovulation, the corpus luteum makes progesterone, which matures the lining so it could support a pregnancy.
When estrogen and progesterone both drop, a period starts.
In a pregnancy, hCG keeps the corpus luteum making progesterone, so no period starts.

Words to know 10

corpus luteum	What the follicle turns into after it releases an egg. It makes progesterone.
endometrium	The lining of the uterus. It thickens each cycle and is shed during a period.
estrogen	A hormone that rises in the first half of the cycle and thickens the lining of the uterus.
FSH (follicle-stimulating hormone)	The hormone that makes follicles in the ovaries grow.
GnRH (gonadotropin-releasing hormone)	A hormone from the hypothalamus that tells the pituitary gland to release FSH and LH.
hCG (human chorionic gonadotropin)	A hormone released by a fertilized egg. It tells the corpus luteum to keep making progesterone.
hypothalamus	A small region at the base of the brain that controls the pituitary gland.
LH (luteinizing hormone)	A hormone from the brain that tells an ovary to release an egg.
pituitary gland	A gland in the brain that makes and releases hormones.
progesterone	A hormone that rises after ovulation and matures the lining of the uterus for a possible pregnancy.