Regular periods are one of your body's vital signs. In other words, having regular periods, starting at puberty and ending when you reach menopause, lets you know that your body is working as it should.
However, how many days pass between your periods, how much blood you lose, and how long each period lasts may differ from other people. In fact, your periods may vary from cycle to cycle. For the most part, slight variations are nothing to worry about.
Here's what you should know about a regular period, how many days should pass between periods, and when to see a healthcare provider.
What Is the Average Menstrual Cycle?
The menstrual cycle prepares the female body for pregnancy. During the menstrual cycle, estrogen and progesterone rise and fall, causing menstruation and ovulation.
Menstruation is also known as the menstrual period, menses, or a periods. The first day of bleeding marks the start of each menstrual cycle. At the start of a period, the tissue lining the inside of the uterus is thick and filled with blood. Estrogen and progesterone levels fall, which signals the uterus to shed the thick tissue through the vagina.
Typically, periods happen every 21–34 days. In adolescents, cycle length can be less predictable, ranging from 21-45 days. Two to seven days is considered normal length for menstrual bleeding. On average, people lose two to three tablespoons of menstrual blood throughout each period. However, some people have heavy periods, while others have light periods.
Then, in a 28-day cycle, ovulation happens between days 11–14. Ovulation is when one of the ovaries matures and releases an egg. The egg leaves the ovary and enters the fallopian tube. In the fallopian tube, if sexual intercourse occurs, sperm may fertilize the egg.
After ovulation, estrogen and progesterone levels rise, to further prepare the uterine lining. A fertilized egg may attach to the thick uterine tissue, and pregnancy starts. If pregnancy does not happen, estrogen and progesterone levels fall and a new menstrual cycle begins with menstruation.
In the U.S., the age of onset for the first period, or menarche, is between ages 12–13. People continue to get regular periods for nearly 40 years. On average, menopause occurs at age 51. Menopause is the natural cessation of periods after 12 months.
How To Track Days Between Periods
Keeping track of how many days pass between your periods can tell you if your periods are regular. To track your periods, note the first day of each one. You will need to mark the first day of a few periods before seeing a pattern.
You can mark the first day of each period on a calendar and journal your symptoms. For example, you may note any premenstrual syndrome (PMS) symptoms and when they happen.
In addition to PMS symptoms, other factors to keep track of include:
- How much menstrual blood you lose
- When your period is heaviest and lightest
- How many pads and tampons you use, and how often you change them
- If you have menstrual cramps that keep you from engaging in your daily activities
- How long your period lasts
There are several apps that can help you track your periods and symptoms.
If you know when to expect your next period, you can approximate when you ovulate. Knowing when you ovulate helps if you're trying to get pregnant.
In contrast, if your periods are irregular, keeping track of how many days pass between each one can still help. For example, suppose you have any symptoms like severe menstrual cramps. Keeping track of symptoms and when they occur can help a healthcare provider diagnose underlying health conditions.
What Are Irregular Menstrual Cycles?
Periods are different for every person. In actuality, having one period every four weeks is an approximation. If your period comes once every 21–34 days, it's regular. Also, if your cycle varies by a few days (e.g., they average 31 days apart, but in some months, there's a 21- or 34-day gap), that's also normal.
However, as many as 25% of people with periods have irregular menstrual cycles. Some people have fewer or more days pass between periods than average.
For example, oligomenorrhea is when 35 days or more pass between each period. In contrast, polymenorrhea is when 21 days or less pass between each period. Amenorrhea is when periods don't happen for three months or longer, in the absence of a pregnancy, or if someone doesn't get their period by age 16.
Factors Affecting Days Between Periods
Several factors increase your risk of irregular menstrual cycles, such as:
- Perimenopause: This is the transition to menopause during your 40s and 50s. Your period becomes irregular until it eventually stops.
- Primary ovarian insufficiency (POI): This is when the ovaries stop functioning normally and menstruation ceases before age 40.
- Eating disorders: For example, anorexia may cause irregular periods.
- Thyroid problems: An under-active or overactive thyroid can interrupt the menstrual cycle and cause irregular periods.
- High prolactin levels: Prolactin is a hormone that the pituitary gland in the brain makes to make breast milk. This is why postpartum breastfeeding parents experience lactational amenorrhea. Too much prolactin, in the setting of other disorders, can cause irregular cycles.
- Diabetes: If untreated, diabetes can cause irregular periods.
- Cushing's syndrome: This condition causes high cortisol levels. Cortisol is a hormone that the body produces during stressful times. Due to this, you may experience in frequent or absent periods.
- Congenital adrenal hyperplasia: This condition causes problems with the adrenal glands, which are found on top of the kidneys, and can cause irregular cycles.
- Hormonal birth control: Pills, patches, injections, implants, and intrauterine devices (IUDs) control estrogen and progesterone to stop ovulation, and can impact your cycle. Also, you can use some types of hormonal birth control, like the pill, to stop periods.
- Asherman's syndrome: This condition causes scarring inside the uterus, which can lead to irregular periods.
- Certain medications: Drugs that treat epilepsy or mental health conditions can cause irregular or absent periods.
In addition, your age affects your cycle regularity. In adolescents, menstrual cycles can take up to three years to become regular. By the time you reach your 20s and 30s, your period will likely have an established pattern. However, your cycle may be less regular if you have a health condition that affect your period. In the perimenopausal years, your cycle length can once again change.
Do Long Menstrual Cycles Affect Fertility?
Some people with long menstrual cycles may worry about their fertility. Ovulation, when an ovary releases an egg, plays a key role in fertility.
The timing of ovulation can vary your cycles. Each menstrual cycle has three phases, the follicular phase, ovulatory phase, and the luteal phase. The follicular phase occurs before ovulation and the luteal phase after. The luteal phase is relatively constant in every person, with a duration of 14 days, no matter the length of your cycle. The follicular phase, however, can vary in duration, from 10 to 16 days. A longer follicular phase may mean you ovulate later in your cycle, and can lead to a longer menstrual cycle overall.
Still, many people with long or irregular menstrual cycles get pregnant without problems. In contrast, infertility occurs when you cannot become pregnant after one year of trying to conceive. If you're experiencing infertility, check in with your healthcare provider for evaluation. They may recommend that you use medications like clomiphene citrate, which stimulate ovulation. Artificial reproductive technology (ART), such as in vitro fertilization (IVF), can also be an option to get pregnant.
When To See a Healthcare Provider
If your periods are irregular, consulting a healthcare provider may help. Periods more than 38 days apart may indicate an underlying health condition, like POI, eating disorders, or thyroid problems. A healthcare provider can run blood tests to check your hormone levels and prescribe treatments.
Further, bleeding after your period within three weeks may be a sign of an infection, fibroids, or, in rare cases, a cancerous tumor. A healthcare provider will likely send you for an ultrasound to figure out any issues.
A Quick Review
Typically, periods happen every 21–35 days and last two to seven days. If fewer or more days than that pass between periods, your periods may be irregular. To determine the regularity of your periods, keep track of the first day of a few periods until you see a pattern.
If your periods are irregular, or you have other symptoms like severe menstrual cramps, consult a healthcare provider. They can run tests and prescribe treatments to help restore regular periods.
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