Our period is the visible part of something bigger: a full monthly cycle our body runs to prepare for a possible pregnancy. That cycle is the foundation of our fertility, and it’s important we understand how it works.

What our body is doing each month

Every month, our body runs the same plan. 

It matures an egg and releases it, and it thickens the lining of the uterus into a soft, blood-rich cushion that is meant to support a fertilized egg. If no pregnancy happens that month, our body doesn't need the lining, so it sheds it, and that shedding is our period.

Then the whole thing starts over. One "cycle" is a single round of this, counted from the first day of one period to the first day of the next.

The Four Phases

The period (menstruation) is one phase of the cycle (the part we just covered). 

Beyond that first phase, the cycle moves through three more, each run by its own hormones:

• The build-up (follicular phase): This is the first stretch, starting on day one of our period. A hormone called FSH (follicle-stimulating hormone) tells our ovaries to grow a batch of follicles, the tiny fluid-filled sacs that each hold an egg. As the follicles grow they release estrogen, and estrogen does two jobs at once: it ripens one lead egg while the rest bow out, and it rebuilds the uterine lining that just shed, thickening it back up for a possible pregnancy.

• The release (ovulation): When estrogen reaches its peak, it sets off a sharp surge of LH (luteinizing hormone), and that surge is what releases the ripened egg from the ovary. The egg is swept into the fallopian tube, where sperm can reach it. Ovulation is quick, essentially a single day, and it's the hinge the whole cycle turns on: it splits the month into before and after, and it's the only time a pregnancy can start.

• The wait (luteal phase): After the egg is released, the ovary shifts to making progesterone, the hormone that keeps the freshly built lining thick and steady in case the egg is fertilized. This phase holds fairly steady at around two weeks. If a pregnancy takes hold, progesterone stays high to support it. If it doesn't, progesterone falls, the lining lets go, and it leaves as our next period, putting us back at day one.

The short version: estrogen builds the runway, LH launches the egg, progesterone holds the pattern.

The days that matter for pregnancy

Pregnancy can only happen around ovulation, when there's an egg to fertilize. That stretch is called the fertile window, and it's about six days: the day of ovulation and the five days before it. 

The reason it opens before ovulation is timing: sperm can survive up to about five days inside the body, while the egg lasts only 12 to 24 hours after it's released. So the best odds come from sperm already waiting when the egg arrives.

One thing that surprises many of us is that ovulation usually lands about 14 days before our next period, so on anything other than a neat 28-day cycle, it happens earlier or later than the classic "day 14."

How long a cycle usually lasts

If your cycle isn't 28 days, that's completely fine. 

A typical adult cycle runs anywhere from about 24 to 38 days, and 28 is just the average. What tells you more than the exact number is consistency: a cycle that shows up around the same length each time is your body running its loop on schedule, whatever that length is for you.

Once you can read our own pattern, the cycle stops being something you wait on and becomes information you can use, the first real step in understanding your own fertility. It's more than the week you bleed, a monthly report on how your body is working, and now you can read it.