Main Reasons Your Period Could Be Late, Other than Pregnancy

late period

Most women immediately associate a late period with a baby on the way.  Although a missed period is a reliable sign of pregnancy, there are quite a few reasons, other than pregnancy, for why your period might not arrive on time.

The first step in determining if your period is actually late is to calculate the duration of your menstrual cycle.  While 28 days is considered “average” for menstrual cycle length, cycles ranging anywhere from 21 to 35 days are normal and totally healthy.   To determine your cycle length, simply count the number of days from the first day of a period until the day before you started the next period.  Some women’s cycles naturally vary a bit from month-to-month, so it’s a good idea to count a couple of cycles to get your average and learn what is ordinary for you.

If days have passed since you expected your period to start and you’ve engaged in sexual activity that could lead to a pregnancy, then it’s a good idea to take an at-home pregnancy test.   Home pregnancy tests are sensitive enough to detect a pregnancy as early as 4-5 days before your missed period, so a pregnancy test should be able to give you a definitive answer if your period is late.  If, however, you’ve ruled out pregnancy as a possible reason for your late period, then consider the following list of the most common reasons for a late period, other than pregnancy:

  • Stress – Imagine you are being chased by a grizzly bear – would you rather have your precious and limited energy go to your muscles or to your reproductive system? Your muscles, of course… run!  Under stress, your amazing body is programmed to divert energy toward critical systems required for survival and away from dispensable systems, like your reproductive system.  Stress makes your body slow or stop the production of estrogen and progesterone, hormones required for both conception and menstruation.  Stress can be perceived as an acute threat, like that bear chasing you, or a chronic concern, such as a looming deadline on a very important project at work.
  • Birth control – If you’ve recently changed your birth control, then you can likely expect some changes in your menstrual cycle for at least a couple of months. Hormonal birth control, such as pills, patches, and rings, introduce synthetic hormones into your system that prevent ovulation.  Starting a new routine will almost certainly cause some irregularities in your menstrual patterns.  Also, some birth control pills are delivered via three months of an active dose, followed by one week of inactive placebo pills.  If you are taking this type of birth control, then you should only get your period four times per year.
  • Certain Medications – In addition to birth control, many medications that seem unrelated to reproductive health can also impact your period. Antipsychotics, commonly used to treat schizophrenia and bipolar disorder, can cause dramatic increases in the level of the hormone prolactin, which in turn can suppress ovulation.  Antidepressants like fluoxetine and some other SSRIs often cause an increase in menstrual cycle length, which would lead to a late period.  Certain antidepressants are more likely to impact your cycle, so it’s important to talk to your doctor about side effects you’re experiencing.  In addition to medications that are primarily used to treat mood disorders, some medications used for cancer or epilepsy have also been shown to cause late periods or complete absence of periods.
  • Extreme diet/exercise – Maintaining a very low body weight by following a restrictive diet or exercising excessively can prevent your body from ovulating. When body fat levels are extremely low, your body enters a sort of starvation mode that can shut down reproductive function. In fact, studies of high intensity sports where leanness is rewarded, such as ballet and running, have reported that over 60% of female athletes cease menstruation during extreme training.
  • Illness – It’s unlikely that a minor cold would significantly affect your period, but more severe illnesses such as the flu could definitely cause a shift in your cycle. When you get sick, your immune system goes into overdrive.  This requires a lot of energy, so regular reproductive function can be compromised in the process of fighting off an infection.
  • Travel – Most of us are familiar with jet lag and the sleepiness that often accompanies travel across time zones. Jet lag occurs because your body is used to your home time zone, and therefore releases hormones that regulate sleep/wake patterns according to that time zone – not your travel destination.  The sleep hormone melatonin can cause changes in reproductive hormone levels, so disruptions in your sleep cycle can impact your menstrual cycle.
  • Irregular work schedules – Certain fields of work, such as healthcare and aviation, often come with extremely variable work hours. Research studies in nurses and flight attendants (the latter working in the same time zone during the study) have shown that large variances in the hours that you work increase the likelihood of irregular periods.  Therefore, if you have a job with rotating shift work, your cycle length might vary between months.
  • Hormonal Disorders – Women with hormonal disorders are at a higher risk of having abnormal and irregular periods. Polycystic ovary syndrome (PCOS) is a common disease that impacts approximately 1 in 10 women, in which the ovaries develop follicles and release eggs at irregular intervals, thus causing irregular periods.  Just as common as PCOS are thyroid disorders, which can also affect your menstrual cycle.  Hyperthyroidism, when your body makes too much thyroid hormone, can lead to shorter periods or complete absence of periods, while hypothyroidism, when your body doesn’t make enough thyroid hormone, can lead to more frequent and longer periods.  It’s critical to talk to your doctor about if you suspect you might have PCOS or another hormonal disorder.
  • Perimenopause – Menopause is the absence of a period for 12 consecutive months. It typically occurs in women between the ages of 45-55, but more rare instances of early menopause have been reported by women in their 30s.  Perimenopause is defined as the time surrounding menopause, often lasting for a few years.  During perimenopause, periods become irregular as ovarian function declines.
Kristen Hollinger
Dr. Kristen Hollinger has a Ph.D. in molecular and cellular biology from Pennsylvania State University. She currently resides in Maryland and works as an Instructor in the Departments of Psychiatry and Neurology at Johns Hopkins University School of Medicine. Her research focuses on neurological diseases including depression and multiple sclerosis.

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