“Why Did My Period Come Early?” Here Are 7 Reasons for Menstrual Irregularity, According to a Gynecologist

There’s nothing more annoying than being caught unawares by Aunt Flo—so when your body and your period tracker aren’t in agreement and bleeding happens sooner than expected, it’s only natural to ask (or scream) “why did my period come early?” Fortunately, we tapped gynecologist Dr. Alyssa Dweck for some answers that will help you identify the root cause of the surprise attack ASAP.

Dr. Alyssa Dweck, MS, MD, FACOG, is a practicing gynecologist in New York. A graduate of Barnard College, she has a master's degree in human nutrition from Columbia University and a medical degree from Hahnemann University School of Medicine in Philadelphia (now named Drexel University). She is also the author of The Complete A to Z for Your V.

Are you sure that was your period? If bleeding starts ahead of schedule and there’s even the slimmest chance you’re pregnant, it might be a good idea to investigate the possibility.

Bleeding in early pregnancy is quite common, and can be caused by the implantation of the embryo into the uterine lining (aka implantation bleeding) or a result of the increased blood flow and hormonal fluctuations that take place after conception. (It’s also sometimes, but not always, a harbinger of miscarriage.) 

Whatever the case may be, Dr. Dweck tells us that pregnancy-related bleeding is commonly mistaken for early menses—so you’d be wise to rule out the possibility with an at-home test or a trip to the doctor’s office where a physical exam, blood test and/or ultrasound can be used to tell you one way or another.

Polycystic Ovarian Syndrome (PCOS) is a condition in which the ovaries produce more androgens (i.e., male sex hormones) than they’re supposed to. This, in turn, causes irregular ovulation or none at all, and menstrual cycles that come early or late, says Dr. Dweck. In addition to irregular menstruation, symptoms of PCOS can include abnormally heavy or light bleeding, excess body hair, weight gain (particularly in the lower abdominal region), acne and fertility issues (among other things). 

What’s more, it’s not uncommon for women of childbearing age to have PCOS and not know it. If you suspect you might have PCOS because of early periods and/or any combination of the other aforementioned symptoms, your physician can use ultrasounds and blood tests to diagnose the condition and help you manage it.

Per Dr. Dweck, the thyroid gland in the neck regulates metabolism and is controlled by hormones that come from the same area of the brain that regulates menstruation. As a result, thyroid disorders like hypo and hyperthyroidism, can result in irregular menstruation and early flow.

Research is ongoing and the explanation not yet well-understood, but Dr. Dweck confirms that the existing literature suggests that both the Covid vaccine and Covid infection might transiently alter menstrual flow. If you’ve recently been vaccinated, boosted or infected with Covid and your period is making an early appearance, that may very well be the reason. The good news is that this unfortunate side effect is not likely to persist over the long term, so you can probably just ride it out.  

According to Dr. Dweck, a whole host of factors influence the timing of menstruation—including diet, lifestyle and even stress. While it’s unlikely that your early period is the result of that time you spilled coffee on your blouse before a meeting a couple weeks ago, more significant life changes (like moving to college, going through a divorce or losing a job) can certainly screw up your cycle for a sec.

Early menses and breakthrough bleeding are commonly experienced by women who have just started taking birth control pills (including Plan B) or recently welcomed a hormonal IUD into their womb. That said, the doctor notes that this symptom typically resolves once a woman’s body has become acclimated to the hormonal contraceptive in question, and that “prolonged or worsening menstrual irregularity may warrant a workup with a health care provider.”

Menopause begins when a woman has gone 12 consecutive months without a period. Before you reach that milestone, however, you’ll have to go through perimenopause—a preceding phase that’s characterized by “volatile hormonal levels, as well as irregular ovulation and menstruation, including early flow.” It’s also worth noting that perimenopause can last for four to eight years, or longer—so if that’s the culprit, you might have to grin and bear it for quite some time.