Several hormones shift dramatically in the days before your period, with progesterone and estrogen undergoing the most significant drops. These two hormones peak about a week after ovulation, then fall sharply in the final three to five days before menstruation begins. That rapid decline is what triggers both the physical shedding of the uterine lining and many of the symptoms you associate with PMS.
Progesterone: The Biggest Drop
Progesterone is the dominant hormone of the second half of your cycle, known as the luteal phase. After ovulation, a temporary structure on the ovary called the corpus luteum produces large amounts of progesterone to prepare the uterine lining for a potential pregnancy. During the mid-luteal peak (roughly a week after ovulation), progesterone levels can reach 2 to 25 ng/mL, compared to just 0.1 to 0.7 ng/mL during the first half of the cycle.
When no pregnancy occurs, the corpus luteum breaks down in a process called luteolysis. Progesterone production drops first, followed by structural breakdown of the corpus luteum itself. This decline is rapid, not gradual, and it’s the primary signal that tells your body to shed the uterine lining and start your period.
Estrogen Falls Too, But Differently
Estrogen follows a slightly different pattern. It has two peaks during a typical cycle: one just before ovulation and a second, smaller peak during the mid-luteal phase. Both estrogen and progesterone then decline sharply in the late luteal phase, roughly three days before your period starts. The withdrawal of estrogen contributes to the cascade of changes that lead to menstruation, and it also plays a role in premenstrual symptoms like headaches and mood shifts.
Estrogen also influences your body’s fluid balance. It activates a hormonal system (the renin-angiotensin-aldosterone system) that promotes fluid retention. As estrogen levels fluctuate in the days before your period, you may notice bloating or puffiness that resolves once menstruation begins.
How Progesterone Withdrawal Affects Your Brain
Progesterone doesn’t just act on your reproductive organs. Your body converts it into a compound that works directly on the brain’s calming system, enhancing the effects of GABA, the brain’s main “slow down” signal. This progesterone byproduct has anti-anxiety and mildly sedative properties, similar in mechanism to certain prescription sedatives.
When progesterone drops rapidly before your period, your brain loses that calming influence. Research in animal models shows that a fast progesterone decline increases anxiety-like behavior, while a gradual decline does not. This helps explain why some people experience irritability, anxiety, or low mood in the premenstrual window. It’s not the absolute level of these brain-calming compounds that matters most, but how quickly they change.
The connection between progesterone withdrawal and mood is also linked to serotonin, the neurotransmitter most associated with emotional stability. Specific serotonin receptor pathways become more sensitive during progesterone withdrawal, which is one reason why SSRIs (a common type of antidepressant) can be effective for severe premenstrual mood symptoms.
Prostaglandins Rise as Hormones Fall
While progesterone and estrogen are dropping, another group of chemical messengers is doing the opposite. Prostaglandins, which are produced in the uterine lining, increase substantially during the luteal phase. One type of prostaglandin rises from about 10 to 25 ng per 100 mg of tissue during the first half of the cycle to 65 to 75 ng in the luteal phase. That’s roughly a threefold increase.
Prostaglandins cause the uterus to contract, which helps shed the lining during your period. But when levels are high, those contractions become strong enough to cause cramping. The buildup begins before your period actually starts, which is why cramps can show up a day or two early. Higher-than-average prostaglandin levels are directly linked to more painful periods.
FSH Starts Rising Before Your Period Ends
Your body doesn’t wait for one cycle to fully finish before preparing for the next. Follicle-stimulating hormone (FSH) begins to rise about 24 hours before your period starts, reaching a peak about 24 hours after the onset of menstruation. This early FSH rise is what recruits the next batch of follicles in your ovaries, setting the stage for the cycle ahead. Luteinizing hormone (LH) follows a similar pattern but rises slightly later, peaking the day after your period begins.
This transition happens specifically because progesterone and estrogen are low. When those hormones are elevated during the luteal phase, they suppress FSH. Once they drop, that brake is released and FSH climbs.
Blood Sugar and Insulin Sensitivity Shift
The hormonal changes before your period also affect how your body handles blood sugar. During the luteal phase, insulin sensitivity decreases, meaning your cells don’t respond to insulin as efficiently. Your body compensates by producing more insulin, while circulating blood sugar and triglycerides actually drop. This combination may explain the increased carbohydrate cravings and hunger many people notice before their period.
Research from NHANES data confirms that insulin sensitivity undergoes small but clinically meaningful cycling across the menstrual cycle, and the effect is more pronounced in people with higher body weight or lower fitness levels. If you have diabetes or prediabetes, this is worth being aware of, since your blood sugar readings may run slightly higher in the days before your period without any change in diet or medication.
Why the Speed of Change Matters
One of the most important findings in menstrual cycle research is that premenstrual symptoms are driven less by where your hormone levels end up and more by how fast they get there. People with severe PMS or PMDD (premenstrual dysphoric disorder) often have hormone levels that look identical to those without symptoms. The difference appears to be in how their brains respond to the rapid withdrawal of progesterone and its calming byproducts.
This is why premenstrual symptoms vary so much from person to person and even cycle to cycle. A month where your corpus luteum breaks down more abruptly can produce worse symptoms than a month with a more gradual transition, even though the starting and ending hormone levels are the same.