The menstrual cycle is the natural series of changes that occur every month in women. There are hormones, ovulation, premenstrual symptoms (PMS), and periods (bleeding). 

In terms of hormones, the only ones you need to know about are estrogen and progesterone. In the first half of the cycle, estrogen goes up. In the second half of the cycle, progesterone goes up. The lining of the uterus changes monthly in response to these fluctuating hormones. When they both taper off, a period happens. 

DAY 1-13

Your cycle starts on the first day of your period. Your brain tells your ovaries to start making eggs. Not just one egg. Lots of eggs. However, although many eggs start developing, just one gets released. The egg with the best development becomes dominant while all the others stop growing at various stages and soon get broken down. 

Each developing egg and the cells surrounding it are collectively called a follicle. Developing follicles produce estrogen and release it into the bloodstream. Estrogen triggers the lining of the uterus to get thicker. So, at the same time as the egg is developing, the lining of the uterus is getting ready for its release.

Follow me so far? An egg develops in the ovaries while the uterine lining gets thicker. Estrogen goes up. That’s the first part of the cycle.

DAY 14

Next comes ovulation. In a 28-day cycle, ovulation occurs on Day 14. As the dominant follicle matures, the brain releases a surge of hormones that causes the follicle to burst open and the egg to be released from the ovary.

Some women feel mild pain during ovulation, typically on one side of their abdomen. Sometimes there’s spotting. Many women don’t notice ovulation at all. Either way, ovulation is a big deal: it’s essential to fertility, and it signals the transition into the second part of the menstrual cycle.

DAY 15-28

The fimbriae of the fallopian tubes are waiting for the newly released egg. They gently sweep it down toward the uterus. The whole journey down the tube takes the better part of a week. It’s during this time the egg may or may not meet one lucky sperm and become fertilized. Let’s just assume there are no sperm in this cycle and the egg continues on unfertilized. We’ll talk more about fertilization and pregnancy later. 

Although the big follicle in the ovary has released its egg, its job isn’t done. The follicle now starts producing progesterone, the second important hormone in the menstrual cycle. Progesterone stabilizes the uterine lining, so it stays thick, but not so thick that it starts falling apart. The uterine lining – after two weeks of estrogen and about a week of progesterone – is just right for a fertilized egg to implant and cause a pregnancy. 

However, in most menstrual cycles, nothing implants in the uterine lining. Without implantation, the ovary runs out of steam – the empty follicle stops producing progesterone and the uterine lining starts falling apart. This is the time women get premenstrual syndrome (PMS), which encompasses various unpleasant symptoms that signal your period is coming soon. Cramping, irritability, increased appetite, sadness, fatigue, and breast tenderness are some of the most common PMS symptoms.

DAY 1-7

Without a pregnancy, the uterine lining sheds itself. This causes bleeding and cramping. The thickness that was built up during the previous cycle gets sloughed off and empties out the cervix into the vaginal canal. 

Then the whole cycle starts again. The ovaries heal up and start producing a fresh batch of follicles. After menstruation (a period), the uterine lining starts building up again from the estrogen released by the follicles.

NORMAL'S NOT ALWAYS NORMAL

All that describes a normal menstrual cycle. However, as most women will tell you, abnormal cycles are common. All sorts of things can go awry with the brain’s hormones, follicle development, estrogen and progesterone, the uterine lining, bleeding, and any other aspect of the menstrual cycle. We won’t get into any of them here. If you have abnormalities with your cycle, talk to your doctor; often there are treatments available for various problematic conditions.