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Why is my period late?

Written by:, PhD, Founder and Inventor of the Proov test — the first and only FDA-cleared test to confirm successful ovulation at home.

why is my period late?

Being late triggers different thoughts and feelings depending on your current family planning intention. If you are intentional on a journey towards parenthood - and even performing some fertility testing - being late can mean feelings of excitement and anticipation as you think about possible pregnancy and whether you should do a home pregnancy test.  

If you’re not trying to conceive, being late can bring on a bit of anxiety as you think back to when you last had sex, etc.

Let’s remember: Pregnancy is only one of the many reasons why “Aunt Flo” may be late. Whether you are trying to conceive or not, it’s important to be aware of the other reasons because it may mean something's up with your hormonal health. 

When is a period considered late?

This is a really important question to discuss before diving into reasons why menstruation is delayed. The answer relates to what is considered a normal cycle length. 

Less than 10% of women have a cycle that is always 28 days long. This is key because it means calculations based on a 28-day cycle are inaccurate the vast majority of the time.

This may include calculations for when you ovulate, and when you are late — late enough to warrant a pregnancy test. Beware of anything that assumes a 28-day cycle!

By cycle length we are referring to the first day of flow that you know is your menstrual flow all the way until the last day before your next flow. (Cycle length is not the length of your bleeding.)  

A normal cycle length is 21 to 35 days with most women averaging somewhere in and around 29 days. Cycle length also is expected to vary a little bit. One month it’s 27 days, the next it’s 30, then maybe it goes down to 26 days.

why is my period late?

It’s all good, all within normal range. Having a cycle that varies from 2 to 5 days in length with no other irregularities (i.e. pain, unusual bleeding) is no cause for concern. 

So, if a normal cycle is 21 to 35 days long give or take 2 to 7 days, what does this tell us about when you are officially late? It’s highly personal. 

If you're the type of woman who is always on time with a bleed every 24-26 days, you’re probably right to say you’re late if it’s been 28 days and counting since your last menstrual period. 

If on the other hand, you have a wider range of cycle length variability, i.e. 24 to 40 days, it’s a bit more guesswork to figure out when you can be considered late. (If your cycle length is that level of variability on a regular basis, please consider consulting a doctor who knows how to help you figure out why.)  

The takeaway: Track when you bleed to know how long your typical cycle is and what your range of cycle length variability is. This is going to be the most accurate, most personalized way to know when you are late. 

Why is my period late?

why is my period late?


This is the most common thought among women who are trying to conceive. Late period means possible pregnancy. Yes, that’s 100% true and here’s why. 

If conception and implantation have occurred, the new little life that has embedded itself in the lining of your uterus will cause hormones like hCG to rise. (Yes, hCG is the hormone you test for in a pregnancy test.)

HCG does many things to help the pregnancy progress, one of which is rescuing of the corpus luteum from reabsorption after ovulation so that it can continue to churn out progesterone. Continued production of progesterone keeps the lining of the uterus stable. Stable lining, no shedding, no menstruation. Read more about the physiology of your cycle here

Ovulation occurs 11 to 17 days before menstruation if there’s no pregnancy. If you’re trying to figure out if you should do a pregnancy test and you know when you ovulated, go ahead and do a test if you are more than 17 days post-ovulation.  

If you don’t know when you ovulated, you can still do a pregnancy test instead of continuing to wonder. Think first about the information in the previous section and consider if you are truly late based on what is known about cycle length variability.  


One of the common reasons for cycle length variability is stress. It is thought that this goes back to caveman days when our stress came in the form of no food, no shelter, or a bear running after us. When there is perceived danger and scarcity, our body says, “nope, not a good time to get pregnant” and it delays ovulation. Delayed ovulation caused the entire cycle to lengthen. Late menstruation. 

Stress for the modern gal comes in many different forms, none of which involve voracious animals. Job and school deadlines, rocky relationships, restrictive dieting, exercising to the point of exhaustion without sufficient time to recover, chronic or sudden sleep deprivation, mental health issues like depression or anxiety, civil unrest due to politics — all of these are forms of stress that can throw off your cycle. 

Is stress the reason you’re late? Tracking your cycle gives you the data to know how resilient your cycle is in times of stress. Track when you bleed and rank your stress level throughout your cycle.

Weight Changes

Gaining or losing weight can offset your hypothalamic-pituitary-ovarian (HPO) axis, a fancy term for the hormonal pathway that connects your brain to your ovaries. Weight changes are perceived as a form of stress and the more sudden the change is the more likely your ovulation will be delayed or non-existent. Late or absent ovulation, late or absent menstruation. 

There’s nothing wrong with wanting to manage your weight, just be smart about how you do it. Adopt small, sustainable changes instead of crash dieting or going from 0 to 100 with your level of physical activity. Consistency is the key and what your HPO axis tends to like.  

Premature Ovarian Failure

Not to be confused with premature ovarian insufficiency, premature ovarian failure (POF) means your ovaries have ceased to ovulated prior to natural menopause, but it’s not early menopause. Only about 1% of women have POF and they have a 5-10% chance of spontaneous ovulation and pregnancy. Symptoms of POF include amenorrhea, or an absence of menstruation, which is why it could explain being late.   

Hormone Imbalances

A cycle that is running long (i.e. longer than 35 days) often means an imbalance in estrogen and progesterone, two hormones that help regulate your cycle. Hormone imbalances can also mean suboptimal insulin, thyroid, prolactin or testosterone levels. Consult a physician who knows about the hormonal complexities of the female cycle and ask about tests that can shed light on why your period is unpredictable and often late. 

why is my period late

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