Listed below is a rough outline for a regular cycle. Many women experience irregular menstrual cycles. You can tell if you have an irregular cycle if your period comes sporadically. Although this is natural, especially for younger women, you might want to talk to your provider to make sure your cycle isn’t affected by another condition. Some women use birth control pills to even out their cycles, which can also control symptoms of PMS.

The first day of the cycle is the day your period starts. Of course, this won’t happen if the egg was fertilized, because that means you’re pregnant. Otherwise, you usually know when to expect day one (about every 28 days). Some women experience the most severe cramps on the first day, that’s because the uterus is contracting to push the blood and tissue and the unfertilized egg out and through the cervix. The bleeding will last from three to five days usually, but anywhere between two and seven days can be expected.

The first half of your cycle, from the start of your period until about day 14, is the “estrogen phase” because the level of estrogen in your body is rising throughout these two weeks. FSH is the hormone that causes estrogen to be produced by your ovaries as they prepare an egg for ovulation.

Toward the end of this time, from about day 7 to day 14, the egg is growing and the estrogen is stimulating the uterus to thicken with blood and extra tissue.

Around day 13 to 15, you experience ovulation. Ovulation happens when the ovaries release an egg in response to a luteinizing hormone surge from the pituitary gland in your brain. Some women are in tune with their bodies enough to sense ovulation, perhaps through mittleshmerz (cramps) or a very slight rise in body temperature. The egg travels through the fallopian tubes and toward the uterus. When the egg is in the fallopian tubes, fertilization is most likely. After ovulation, progesterone is produced.

The second phase of your period is called the “progesterone phase.” That means the cycle is now controlled by the hormone progesterone. The uterus continues to prepare for pregnancy. This is the time when you are most prone to PMS symptoms, the hormones are increasing up until around the time the unfertilized egg is expelled from your body. The egg eventually gets to the uterus, and if it isn’t fertilized, then day one arrives once again.