9 reasons why your period may be late, including stress, changes to sleep schedule, and more

• Your period may be running late because of breastfeeding, weight loss, too much exercise, changes to your sleep schedule, perimenopause, and of course, pregnancy.
• Weight gain may also cause a late period because body fat produces estrogen that may interfere with your hormones.
• Illnesses like PCOS, celiac, and diabetes, may also delay your period.
• This article was reviewed by Jason R. McKnight, MD, MS, a family medicine physician and clinical assistant professor at Texas A&M College of Medicine.

The first thought many sexually active people will have when their period is late is that they are pregnant. However, there are many reasons aside from pregnancy that your period may be late, or even missing altogether.

The process of ovulation and menstruation relies on a delicate balance of hormones, and if these hormones are thrown off even a little bit, it can cause you to have a late period.

This article breaks down nine reasons why your period may be late and what's happening biologically behind the scenes.

1. Breastfeeding

If you are exclusively breastfeeding your baby, it's common to experience amenorrhea, or, absence of a period.

This is because breastfeeding is associated with high levels of the hormone prolactin, says G. Thomas Ruiz, MD, OB/GYN Lead at MemorialCare Orange Coast Medical Center in Fountain Valley, California.

Elevated prolactin levels suppress the secretion of two hormones that are important for the menstrual cycle and ovulation: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Some people may not get their period again until they completely stop breastfeeding.

2. Stress

Stress doesn't just affect your mental wellbeing. It can also affect your hormones, throwing off the menstrual cycle.

Ruiz says when you're stressed, you have an increased level of the hormone cortisol — also known as the stress hormone. Elevated cortisol will suppress LH secretion, which will in-turn affect ovulation.

Stress can also lower your levels of estrogen and progesterone hormones, which can make your period irregular.

Ruiz says it's not uncommon to experience a missed period in high-stress situations, such as during college final exams.

3. Changes to sleep schedule

A consistent sleep schedule keeps your biological schedule consistent too. But when your sleep schedule is thrown off, the secretion of hormones is not as consistent and regular as it usually is, which in turn affects the secretion of reproductive hormones, says Evelyn Mitchell, MD, obstetrician and gynecologist with Keck Medicine of USC.

If your body isn't releasing the hormones associated with menstruation, then you could have a late or missed period.

Furthermore, lack of sleep can contribute to stress, which, again, will lead to an increase in cortisol, which can also affect your period's timing.

4. Weight loss

Mitchell says that exercising and eating very little can lead to extreme weight loss that can cause a missed or late period.

When losing weight, the brain may not secrete gonadotropin-releasing hormone (GnRH), Mitchell says. GnRH is a hormone that controls the secretion of other hormones released by the pituitary gland, including reproductive hormones like FSH and prolactin.

Because of this, your pituitary gland won't receive a signal that it needs to secrete reproductive hormones, and this will result in having a late or missed period.

5. Exercising too much

Exercising too much is often associated with weight loss, which can lead to a late or missed period.

But even if you're at a consistent weight, excessive exercise, alone, can cause you to miss your period. Case in point, female athletes are more likely to have missed periods than non-athletes.

6. Weight gain

Just as weight loss can cause you to miss a period, so can weight gain, Mitchell says. Adipose tissue, or body fat, produces estrogen. So when you have excess weight, you have excess estrogen circulating in your body, which can interfere with the hormonal processes in your brain.

"Estrogen can give negative feedback to the brain that tricks the brain into thinking, 'Oh, maybe I don't need to secrete as much of this hormone,' and then that in turn causes a lack of ovulation and lack of periods," says Mitchell.

Additionally, if you are overweight and have chronically high estrogen levels, your brain may not sense increased estrogen production from the ovary when it comes time to ovulate, and then ovulation and menstruation will not occur, says Ruiz.

"Each woman seems to have a critical weight at which they may have trouble ovulating. Not all women with BMI between 30 to 35 will have ovulatory dysfunction. So women will do fine until they get to a BMI of 35, others will start having dysfunction at BMI of 33," says Ruiz.

When an overweight person loses weight and estrogen levels return to a more normal baseline, periods will likely return to normal.

7. Perimenopause

Perimenopause is the time leading up to menopause when your period will completely stop. Usually, perimenopause occurs in your 40s.

When you are perimenopausal, your ovaries are running out of eggs, Ruiz says, and your levels of estrogen may be irregular. The amount of estrogen produced from cycle to cycle might not be enough to trigger ovulation and therefore menstruation.

Additionally, Mitchell says that perimenopause can sometimes prevent the ovaries from responding properly to estrogen, which could also delay menstruation. Though having regular periods is still possible during perimenopause, it may vary month to month because of changing hormone levels.

After a year of no periods, you are considered menopausal.

8. Undiagnosed illnesses

If you're having a late period and still getting enough sleep, are at a consistent, healthy weight, aren't severely stressed, and aren't premenopausal or breastfeeding, it's time to consider other possibilities like illness.

Some examples of underlying conditions that can affect the menstrual cycle are:

• Polycystic ovary syndrome (PCOS): Irregular periods are very common in people with PCOS. This condition may result in someone not having a period for months on end, Ruiz says, and very heavy periods when you do menstruate.
• Celiac Disease: Missed periods can be a symptom of celiac disease in adults, according to the Celiac Disease Foundation. That's because celiac disease is an autoimmune disease that — if left untreated — can impair your body's ability to absorb nutrients, which can affect the menstrual cycle.
• Diabetes: Some people with diabetes, or who are at risk, may experience missed periods. Though the reason isn't entirely clear, researchers think that insulin resistance plays a role.
• Thyroid conditions: Whether you have hyperthyroidism or hypothyroidism, your period may be affected, Mitchell says. The thyroid produces the thyroid-stimulating hormone, which can affect LH and FSH secretion.

9. Pregnancy

The most common cause for a late period in sexually active people is pregnancy.

When you are pregnant, you will not menstruate. Hormonally, pregnancy results in high levels of the hormone progestin, and high progestin suppresses ovulation, Ruiz says.

Your body will be releasing hormones and undergoing changes to accommodate the growing baby rather than releasing the hormones responsible for ovulating and menstruating.

When to see a doctor

The occasional late or missed period is normal and can be caused by a number of factors including stress, lack of sleep, overexercising, or significant changes in weight. But if you've missed three or more periods in a row, you have a condition called amenorrhea and should make an appointment with an OB-GYN.

Your doctor can help determine what the cause of your late or missing periods is and make sure that you don't have any underlying conditions, and get you back on track to having normal, regular periods.

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