10 Causes of Irregular Periods in Your Late 30s

Last updated 01.29.2024 | by Dr. Karen Pike | 6 Minutes Read

This article has been reviewed and fact checked by Dr. Karen Pike, a senior physician administrator and board-certified emergency room doctor actively working in northern California. Read more at our medical disclaimer page.

Irregular periods can happen at any time in life, but in your late 30s, a change in your cycle could be an indication that perimenopause is on the way. Most women enter perimenopause in their 40s, but some women can begin to notice the signs earlier.

However, there are several potential causes of irregular periods, and your menstruation patterns may have changed for a variety of different reasons. So, it’s important to discover why your periods are irregular so that you can address the underlying cause.

When I was 38, I began to notice irregularity in my own menstrual cycle. I experienced bleeding between periods and painful periods, and I could no longer predict when my period would arrive. This was unusual for me, as I had been regular for many years.

This sudden change took me by surprise, and I immediately assumed that perimenopause was to blame. Being 38, I was still relatively young. Still, I was convinced that I was on a fast track to menopause, and I was concerned. At this stage in my life, I was keen to try for another pregnancy, and I already knew the clock was ticking.

And so, I visited my doctor and underwent some testing to find out the cause of my irregular cycle. Thankfully, investigations showed that it was more likely stress that had been affecting my menstrual period, not perimenopause. And later that year, I’m happy to say I was able to get pregnant.

Are you in your late 30s, and have you noticed abnormal periods or irregular bleeding? Before you jump to conclusions, let’s look at all the possible reasons why these changes could be happening.

In this post, I’ll explain ten causes of irregular menstruation in your late 30s so that you can better understand the potential factors at play.

The Most Common Causes of Irregular Periods in Your Late 30s

The Most Common Causes of Irregular Periods in Your Late 30s

Irregular periods can happen for a variety of reasons, including:

1. Perimenopause

Perimenopause is the build-up to menopause. This phase of a woman’s life usually begins in her mid to late 40s; however, it can occur earlier.

One of the first signs of perimenopause is irregular menstrual bleeding. Your menstrual cycle may be lighter or heavier, your cycle length may increase or decrease, or you may experience an increase in period cramps. These symptoms are due to hormonal changes, specifically fluctuations in the levels of estrogen and progesterone in the body.

2. Pregnancy

If you are sexually active, a missed period may indicate that you’re pregnant. During pregnancy, ovulation stops, and as a result, your periods stop, too.

3. Breastfeeding

If you breastfeed, period changes are normal. This is due to hormonal shifts similar to those found during pregnancy.

Most breastfeeding women will not experience a period of around six months after giving birth. After this time, your periods will likely return; however, they may take a while to settle back into a regular and predictable cycle.

4. Stress

During times of intense stress, your body has a harder time regulating reproductive hormones. The delicate balance of estrogen and progesterone may become disrupted, which can cause your period to be irregular.

5. Weight loss or Weight Gain

Significant changes in body weight can directly impact your periods. Carrying excess fat increases your body’s production of estrogen, which can cause your periods to become lighter or heavier.

Conversely, if your weight dips below healthy levels, your body may struggle to produce enough estrogen to regulate your periods. In this case, you may find they stop altogether.

6. Overexertion

Engaging in extreme exercise, such as the levels at which professional athletes engage, has been shown to alter the menstrual cycle. It can lead to spotting between periods and lighter periods, and in some cases, your period may disappear completely.

7. Polycystic Ovary Syndrome

Polycystic ovary syndrome(PCOS) affects as many as 10% of women, and you can develop the condition at any time, even during your late 30s.

PCOS is characterized by multiple benign cysts which form on the ovaries. These cysts can change the body’s hormonal balance and lead to missed or irregular periods.

8. Uterine Fibroids

Uterine fibroids are growths which form on and in the uterus. While they are noncancerous, they can cause significant pain and discomfort. They can also cause heavier-than-usual bleeding, increased pain during periods, and unpredictable cycle changes.

9. Hormonal Birth Control

Hormonal birth control pills deliver synthetic hormones, which can change the balance of naturally occurring hormones in the body. As a result, many women find that when they stop taking this type of medication, their periods are erratic and unpredictable for a while. Usually, this unpredictability settles down over time, and your normal cycle will return.

10. Insulin Resistance and Diabetes

Spikes in blood sugar can have a direct impact on the production of reproductive hormones. These hormones also regulate the menstrual cycle and may cause periods to become irregular.

Usually, treatment for these conditions helps to restore hormonal balance, and the menstrual cycle eventually returns to normal.


Are there any underlying health conditions that can cause irregular periods?

Aside from the common health disorders listed above, irregular periods may be a sign of a less common condition, including thyroid issues and certain types of cancer.
If you’re concerned about unexplained changes to your menstrual cycle, it’s important to visit your doctor as soon as possible.

Are there any medications that can cause changes to a person’s menstrual cycle?

Yes. Certain medications have been shown to alter menstrual bleeding. Some of the most commonly prescribed medications with this side effect include steroids, antidepressants, weight loss medications, antipsychotics, and hormone-based medications such as birth control or hormone replacement therapy (HRT).

How can I tell the difference between perimenopause symptoms and other causes of irregular periods?

There are many possible causes of irregular periods. Your symptoms could be a sign of perimenopause, but the only way to know for sure is to visit a healthcare professional for testing and evaluation. I was convinced that I was entering perimenopause in my late 30s. However, further investigation showed that I simply had irregular bleeding due to stress.
Once I addressed the underlying cause, my periods returned to normal, and I went on to become pregnant with my daughter.


Irregular periods can be caused by many different factors. If you’re in your late 30s, it may be a sign of perimenopause. However, there are plenty of other potential factors that can affect your menstrual cycle at this stage of life.

So, if you’re concerned, book an appointment with your doctor as soon as possible. They will be able to evaluate your symptoms, carry out necessary testing, and provide an accurate diagnosis. Then, if necessary, they can help you find a treatment plan that works for you.


  • Dr. Karen Pike

    Dr. Pike is a senior physician administrator and board-certified emergency room doctor actively working in northern California. She received her undergraduate degree at Dartmouth College, where she graduated Phi Beta Kappa and played collegiate soccer. She attended Georgetown University for medical school and performed her residency in emergency medicine at Stanford University. She was part of the first-ever, women-majority emergency medicine program in United States. Dr. Pike is also the primary medical consultant for “Grey’s Anatomy,” a role she has held since the pilot episode when she partnered with Shonda Rhimes as the show’s original medical consultant. At her hospital, she was the second woman Chief of Staff. Today serves as the Director of the Emergency Department. Whether in leadership or direct patient care, her dedication to excellence in communication, quality, and collaboration is unwavering.