Each menstrual cycle is a coordinated sequence of hormonal signals that recruit an egg, release it, and prepare the uterus for a possible pregnancy. The average cycle lasts about 28 to 29 days, though healthy cycles can be shorter or longer. A large Harvard study analyzing over 165,000 cycles found the average length was 28.7 days, with natural variation even among regular cycles. The process breaks down into three main phases: the follicular phase, ovulation itself, and the luteal phase.
The Follicular Phase: Growing an Egg
The cycle begins on the first day of your period. While the uterus sheds its lining, your brain’s pituitary gland ramps up production of follicle-stimulating hormone (FSH). This hormone signals your ovaries to start developing a batch of egg-containing sacs called follicles. Anywhere from 11 to 20 follicles begin maturing in a given cycle, but only one will make it to the finish line.
Within the first week or so, one follicle pulls ahead of the others. This dominant follicle grows faster and pumps out increasing amounts of estrogen. That rising estrogen does two important things simultaneously. First, it tells the uterine lining to thicken, building up to about 12 to 13 millimeters (roughly half an inch) in preparation for a fertilized egg. Second, it signals the pituitary gland to dial back FSH production. Without enough FSH to sustain them, the remaining follicles wither away and get reabsorbed by the body. This is how your ovaries typically release just one egg per cycle.
The follicular phase is the most variable part of the cycle. It can last anywhere from 10 to 21 days depending on how quickly the dominant follicle matures. This variability is why cycles differ in total length from person to person and even month to month.
The LH Surge and Ovulation
When estrogen from the dominant follicle reaches a critical threshold, the pituitary gland responds with a sudden burst of luteinizing hormone (LH). This spike, called the LH surge, is what ovulation predictor kits detect in your urine. Once LH levels rise in the bloodstream, ovulation follows about 36 to 40 hours later.
During those hours, enzymes break down the wall of the mature follicle while prostaglandins (the same type of inflammatory compounds involved in period cramps) help trigger the rupture. The follicle opens and releases the egg into the fallopian tube. Some people feel a twinge or mild cramping on one side of the lower abdomen during this process, sometimes called mittelschmerz, though many feel nothing at all.
The released egg is viable for about 12 to 24 hours. Sperm, by contrast, can survive inside the reproductive tract for 3 to 5 days. This mismatch is why the fertile window extends to roughly five or six days: the few days before ovulation plus the day of ovulation itself. Having sperm already waiting in the fallopian tubes when the egg arrives gives the best chance of fertilization.
The Luteal Phase: Waiting for a Signal
After the egg is released, the empty follicle transforms into a temporary hormone-producing structure called the corpus luteum. Its primary job is making progesterone, the hormone that shifts the uterine lining from “building” mode to “receiving” mode. The lining becomes spongy and nutrient-rich, swelling to about 16 to 18 millimeters by the end of this phase.
Progesterone levels peak about 6 to 8 days after ovulation. Unlike the follicular phase, the luteal phase is relatively fixed in length, typically lasting 12 to 14 days (though it can range from 11 to 17 days). If a fertilized egg implants in the uterine lining, the embryo sends a hormonal signal that keeps the corpus luteum alive and producing progesterone. If no implantation occurs, the corpus luteum breaks down, progesterone and estrogen drop sharply, and the thickened uterine lining sheds. That shedding is your period, and the cycle starts over.
How Your Body Signals Each Phase
Your body produces visible and measurable clues about where you are in the cycle, which is useful whether you’re trying to conceive or simply want to understand your own patterns.
Cervical Mucus
In the days after your period, cervical mucus is typically dry or sticky, with a thick, paste-like texture. As estrogen rises during the follicular phase, mucus gradually becomes creamier, similar to yogurt in consistency. Right around ovulation, it shifts dramatically: clear, slippery, stretchy, and often compared to raw egg whites. This type of mucus is easiest for sperm to travel through. After ovulation, progesterone makes mucus thick and dry again, where it stays until your next period.
Basal Body Temperature
Your resting body temperature rises slightly after ovulation, typically by 0.4 to 1.0°F (0.22 to 0.56°C). The increase is caused by progesterone from the corpus luteum. Because the temperature shift happens after the egg is already released, tracking it over several months helps you identify a pattern and predict when ovulation is likely to happen in future cycles, rather than catching it in real time.
When Ovulation Doesn’t Happen
A cycle without ovulation, called an anovulatory cycle, is more common than most people realize. You can still have a period (or what looks like one) without having released an egg. Anovulation is most often caused by hormonal imbalances and is especially likely in certain situations: during the first few years of menstruation, during perimenopause, or if you have polycystic ovary syndrome (PCOS). A very low body mass index, whether from restrictive eating or intense exercise, can also suppress ovulation.
Signs that you may not be ovulating include very irregular cycle lengths, cycles shorter than 21 days or longer than 40 days, and the absence of typical mid-cycle changes like egg-white cervical mucus or a temperature shift. Cycles consistently longer than 40 days have also been linked to higher risks of metabolic conditions like type 2 diabetes and heart disease, so persistent irregularity is worth investigating beyond fertility concerns alone.
Putting the Timeline Together
Here’s what a typical 28-day cycle looks like in sequence:
- Days 1 to 5: Menstruation. The uterine lining sheds while FSH begins stimulating new follicle growth.
- Days 6 to 13: The dominant follicle matures. Estrogen rises, thickening the uterine lining. Cervical mucus shifts from dry to creamy to slippery.
- Day 14 (approximately): The LH surge triggers ovulation 36 to 40 hours later. The egg enters the fallopian tube.
- Days 15 to 28: The corpus luteum produces progesterone. The uterine lining reaches maximum thickness. If no pregnancy occurs, hormone levels drop and menstruation begins again.
Keep in mind that day 14 is an average, not a rule. If your follicular phase runs longer or shorter, ovulation shifts accordingly. Tracking your own cervical mucus changes or using ovulation predictor kits gives you a much more accurate picture than counting calendar days alone.