Understanding the Normal Menstrual Cycle

Irregular cycles

If you have irregular cycles, you can become pregnant during bleeding. This may not be a true menstrual period.

An egg is released

Eggs are female reproductive cells stored in the ovaries. During each cycle, the hormones trigger an egg, (usually one), to mature and be released from an ovary. This is called ovulation. The egg then travels from the ovary to a fallopian tube.

Front view of uterus, fallopian tubes, and ovaries. Half has been cross sectioned to show inside. Vagina is opening to outside. Cervix is lower part of uterus connected to vagina. Two fallopian tubes lead from uterus to ovaries, one on either side of uterus. Ovaries have developing eggs inside. Uterus is lined on inside with endometrium.

The egg travels through a tube

The egg moves through the fallopian tube toward the uterus. If sperm are present in the tube, the egg may be fertilized, and pregnancy could result.

The uterine lining grows thicker

The lining of the uterus is made up of blood, tissue, and fluid. During each cycle, hormones cause the lining to thicken. This helps prepare the uterus to receive and nourish a fertilized egg.

The egg and lining are shed

If pregnancy doesn’t happen, the egg and thickened lining of the uterus are no longer needed. They are then shed through the vagina. This is called a menstrual period.

How long is each cycle?

It's normal for a cycle to take 21 to 34 days. For teens, the time between periods might be as much as 45 days. For adults, it will be around a month from the first day of one period to the first day of the next. That’s why you may hear about a “monthly cycle,” even though cycle length can vary from one month to another, and anywhere from 3 to 5 weeks is normal. Not everyone has a 28-day cycle.