Why is my period late?

Reasons for a missed or late period

There are many reasons why you might have a missed or late period, but they all have to do with hormones. Anything that affects your body's production of the hormones that influence your cycle can result in a menstrual cycle that's out of whack. These hormones include progesterone and estrogen (made by the ovaries) and follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (made by the pituitary gland).

Common causes for a missed or late period include:

Pregnancy. A missed period is often the first sign of pregnancy. Your body will start producing human chorionic gonadotropic (hCG), yet another hormone, as soon as the fertilized egg implants itself in the lining of your uterus. A pregnancy test can pick up hCG in your urine when you've missed a period (and sometimes before). If you're unsure of the results (or want confirmation), your healthcare provider can do a blood test.

Stress. Big and little stressors – from a switched schedule (day shift to night shift) or travel that disrupts your circadian rhythms, to an illness or major life event – can result in changes in hormone levels. These changes can cause you to ovulate off schedule or to not ovulate at all.

Breastfeeding. When you breastfeed, your body produces prolactin, which suppresses ovulation. In fact, some moms use exclusive breastfeeding as birth control, though it's risky. Keep in mind that you can get pregnant before you have a period. Your first ovulation comes two weeks before your first period!

Medications. Certain birth control medications can affect your menstrual cycle.  Birth control pills keep your body from ovulating. (You may still have some bleeding, but bleeding when on birth control pills isn't a true menstrual period linked with ovulation. It's a result of the withdrawal from the hormones as you take the placebo pills.) And if you're on extended birth control pills, you'll only get your period every 3 months, because they're designed to cause withdrawal bleeding every 91 days. Other hormonal birth control, like the Depo-Provera shot (which suppresses ovulation) and hormone-containing IUDs, (which thin the uterine lining) can stop or delay your periods, too.

Other medications can also affect your cycle or cause it to stop. These include some types of psychiatric medications and cancer chemotherapy.

Being extremely underweight. Extreme weight loss, low calorie intake, excessive exercise, and/or being very underweight can inhibit the production of estrogen and slow the release of gonadotropin-releasing hormone (GnRH), which affects the timing of your period. A BMI of 18.5 or less can result in irregular menstrual cycles and may stop ovulation completely. Your body stops ovulation and your periods because it thinks that you're starving at this low weight and understands that this isn't a great time to get pregnant. Keep in mind, though, that your cycle can restart at any time, so you still need to protect against pregnancy if you don't want to conceive. In this case, changes in your menstrual cycle are a warning sign for your overall health. Seeing a physician to prevent long-term damage to your body is crucial.

Obesity. Being very overweight can cause your body to produce too much estrogen, leading to a loss of ovulation and menstrual periods. Like being extremely underweight, this is a warning sign for your overall health and should be addressed by your healthcare provider.

Menopause. The time when your body is transitioning to menopause is called perimenopause, and your period may come and go – or lengthen and shorten. That's because the levels of estrogen in your body are rising and falling during this transition. Perimenopause can happen as early as mid-30s, though it's more common in your 40s. The average age of menopause (defined as one year without a period) is 51, plus or minus 5 years. Irregular cycles can start a few years before your last period. If you enter menopause significantly early, it's called primary ovarian insufficiency (early menopause). There are long-term health implications for early menopause, so talk with your healthcare prover about interventions.

Medical conditions. Health issues that cause hormonal imbalances can affect your menstrual cycle. These include:

• Polycystic ovarian syndrome (PCOS) is a common cause of irregular periods. We don't fully understand the condition yet, but it's a communication problem between the brain, ovaries, and uterus that is probably caused by both genetic and environmental factors. PCOS is associated with diabetes and insulin resistance, excessive body hair, and/or acne.
• Thyroid problems can cause irregular periods, or they can cause your period to stop. Hypothyroidism happens when the thyroid gland doesn't produce enough thyroid hormone. And hyperthyroidism is when your thyroid is overactive and produces too much thyroid hormone.
• Pituitary tumors can secrete excessive prolactin and cause missed periods.
• Chronic diseases, such as uncontrolled celiac disease and diabetes, can affect menstruation (though this is rare).

Medical terms for a missed period

Amenorrhea is the medical name for not having a period. Here are the terms that a healthcare provider might use to define your missed periods:

• Primary amenorrhea means a girl or woman has not had any period by age 15.
• Secondary amenorrhea means that menstrual periods are absent for more than 3 to 6 months (3 months in someone who was previously having regular periods and 6 months in someone who was having irregular periods).  
• Oligomenorrhea means having infrequent periods (fewer than 6 to 8 periods per year).

What to do if your period is late

If you just missed it – say your period is 3 days late or 5 days late – take a pregnancy test.

If you're not pregnant and your period is late, keep in mind that missing a period or having a late period occasionally isn't usually something to worry about. But not having a period for an extended time or infrequently can be a sign of a health problem. And these can lead to other problems, such as cardiovascular disease, diabetes, infertility, and osteoporosis.

See your healthcare provider if you:

• Don't get a period for three months or more in a row
• Have fewer than 9 menstrual cycles each year
• Had regular periods but now have irregular periods
• Have a period that lasts more than 10 days
• Have periods that are more than 35 days apart. A typical cycle is 28 days, but some women have a normal range of 21 to 35 days.

If you missed your period because you're pregnant, call your healthcare provider to make a prenatal appointment. In the meantime, use our due date calculator to find out when your baby may arrive.

Learn more:

• Ovulation calculator & calendar
• Am I pregnant?
• Your first prenatal visit