Okay, so we have periods. Whether you refer to it as a “visit from Aunt Flow,” “that time of the month,” or one of the less polite euphemisms…it happens. The medical term for a period is menses, which is the Latin word for “months.” Many of us assume that a period happens monthly, but through our lifetime there is more variation than many would think.

The menstrual cycle is the process of making a thick, spongy, blood-filled lining within the uterus that is ripe for the implantation of a fertilized egg. When a woman becomes pregnant, the placenta attaches to this spongy lining and the baby grows inside the uterus. If an egg is not fertilized, this lining is shed as a period. This process starts in puberty and repeats itself until most of a woman’s healthy eggs have been given a chance to be fertilized.

Your period as a teenager

The first period usually occurs around age 13 in the United States, but can occur anywhere between 10 and 16 years of age. It may take several years before the menstrual cycle becomes a regular monthly event. During this time periods seem to happen somewhat randomly and the amount of bleeding or number of days of bleeding often vary as well.

Peak reproductive years: 20 – 40

Women tend to have their most regular menstrual cycles during their peak reproductive years, between the ages 20 to 40. A menstrual cycle is counted from the first day of one bleeding episode to the first day of the next. During the reproductive years, an average menses occurs 28 to 35 days from start to start and lasts four to six days. The range of normal can be anywhere between 21 to 35 days from start to start with two to seven days of bleeding. A small amount of bleeding at the time an egg is released from the ovary, usually 14 days before the onset of the next menses, is also considered normal.

Subtle changes likely: late 30s and 40s

Women may notice subtle changes in their menstrual cycles starting in their late 30s and 40s. At first, these women may notice shorter intervals between their menses and later, a woman may skip several cycles and or have longer times without a period. These are common cycle changes of perimenopause, or the two to eight years prior to menopause, and may be accompanied by hot flashes or other symptoms of hormonal change.

The big change: menopause

Menopause officially is diagnosed when a woman has been without a period for 12 months in the absence of other reasons. The median age of menopause is 51 years of age. One percent of women will undergo menopause before age 40 and two percent of women are still not menopausal at age 55. Women who smoke, have Type 1 diabetes, live at high altitudes, are undernourished, or vegetarian tend to undergo menopause at younger ages. A family history of menopause before age 45 may also predict earlier menopause. There are no accurate blood tests to diagnose menopause, but other conditions that may also cause irregular menstrual cycles can be excluded.

Other causes of change

Besides age, there are many other reasons for irregular or missed cycles. Pregnancy and breastfeeding naturally prevent the body from going through the menstrual cycle. Contraceptive methods, such as extended-cycle birth control pills, Nexplanon, DepoProvera, or the Mirena IUD, may safely change how frequently a woman has her period. These can also be used as treatment for painful or heavy periods. Stress, such as a death in the family or midterm exams, can delay a menses on rare occasions for some women. More concerning causes of irregular menses are excessive exercise, polycystic ovarian syndrome (PCOS), extreme weight loss or gain, uterine fibroids, uterine cancer and thyroid disease.

Know what’s normal

The best way to know what is normal for you is to keep a record of when your period starts, how many days you bleed, whether the bleeding seems heavy or light and whether you notice cramping or other symptoms. (Many people use a menstrual tracking app for this – check your app store for one of the many free options.) Three months of keeping track should be enough to get an idea of what normal is for your body. Seek medical advice if your periods suddenly stop for more than 90 days, your periods become irregular after having been regular, you bleed for more than seven days, you bleed more heavily than usual (soaking through more than one pad or tampon every hour), or you bleed at random times between periods.