On the first day of your period, the ache can start before you're properly awake: a low, heavy pain that tightens, lets go for a moment, and tightens again. Sometimes it spreads into your lower back, or your stomach gets unsettled too.
Period cramps come from the muscle wall of your uterus tightening as it sheds its lining. The tightening is set off by prostaglandins, chemicals the lining releases as your period begins. When a squeeze is strong, it can briefly reduce the blood flow to the muscle, and that's a big part of what you feel.
What sets cramps off
Each cycle, the lining of your uterus thickens to prepare for a possible pregnancy. After ovulation, the hormone progesterone supports that lining. If you don't get pregnant, progesterone drops near the end of your cycle, and the lining starts to break down and shed. That shedding is your period.
As the lining breaks down, it releases prostaglandins, hormone-like chemicals that make the muscle wall of your uterus and its blood vessels tighten. The tightening helps push the lining out, and it's what you feel as cramps, as the American College of Obstetricians and Gynecologists (ACOG) explains.
A strong contraction squeezes the blood vessels in the wall of your uterus, so for a moment the muscle gets less blood. A 2006 review puts the pain down to three things working together: strong contractions, the drop in blood flow they cause, and pain nerves that have become more sensitive. Researchers are still working out the full picture, but prostaglandins are the best-understood part of it.
Why the first day usually hurts most
Prostaglandin levels are highest as your period starts, then fall as the lining sheds, which is why cramps usually ease after the first few days, ACOG explains. A 2017 review adds that people with painful periods tend to have higher levels, with the peak in the first two days of bleeding.
The Society of Obstetricians and Gynaecologists of Canada (SOGC) describes typical period cramps as starting around the beginning of your period, peaking when your flow is heaviest, and lasting two to three days. They can also start a day or two before any bleeding, according to MedlinePlus, the US National Library of Medicine's health site. So if the first day or two feel like the hardest part, that timing matches what's happening in your uterus.
Why cramps come in waves
Your uterus doesn't hold one long squeeze. It tightens and relaxes, again and again, and each strong contraction can bring its own wave of pain. In a small 1988 study of 10 women, blood flow in the uterine lining changed in time with each contraction, and the contractions were strongest from the day before bleeding through the second day.
In people with painful periods, contractions also tend to be more frequent and less coordinated, and the pressure inside the uterus runs higher, which adds up to a poorer blood supply, according to a 2020 review. Ultrasound studies have found more resistance to blood flow in the arteries of the uterus on the first day of the cycle in women with painful periods, as in one study comparing 42 women with painful periods and 50 without. Not every study agrees, though. A 2024 study found no difference in blood flow in women with mild cramps, and its authors suggest milder cramps may have a different cause.
Why you can feel it in your back and legs
Cramps often don't stay in your lower belly. An aching back is one of the usual period symptoms SOGC describes, and some people feel the pain in their thighs as well.
This is referred pain, meaning pain that's felt somewhere other than where it starts. Nerves from your uterus enter the spinal cord at the same levels as nerves from your lower back and belly, and the signals meet there, as a review of reproductive tract pain explains. Pain from internal organs is also hard to pinpoint, so your brain can register it as coming from those nearby areas. A 2017 systematic review describes period pain spreading to the low back, the abdomen and possibly the thighs, areas that share some of their nerve supply with the uterus.
Why your stomach gets involved
Many people notice their bowels change around their period. SOGC explains that the prostaglandins behind uterine contractions can make your bowel contract too, while progesterone can slow it down in the days before your period. That's why constipation just before your period and looser stools in the first day or two are both common.
In the gut, prostaglandins can also draw fluid into the bowel, which may add to loose stools and stomach pain. In a survey of 156 healthy women in Winnipeg, 73% had at least one gut symptom before or during their period, and about a quarter had diarrhea. Nausea during a period was much more common in women who also had painful periods. The researchers point out that it isn't yet known whether prostaglandins from the uterus travel to the gut, or whether levels rise in both places at the same time.
Why cramps hurt more for some people
How much cramps hurt varies a lot from one person to another, and researchers have found a few reasons.
The amount of prostaglandin seems to matter. In a 1978 study, the uterine lining of women with painful periods contained about four times as much of the main prostaglandin linked to cramps on the first day of bleeding as the lining of women without pain. That study was small and is decades old, and a later small study didn't find a difference in a blood marker of prostaglandin, so prostaglandins are likely an important part of the story rather than all of it.
Your flow can make a difference too. Cramps tend to be more severe when periods are longer and heavier, and in people whose periods started at a younger age, according to the 2020 review.
Your nervous system plays a part. Some lab studies have found that people with painful periods react more strongly to certain kinds of pain, even at times of the month when they aren't in pain, although the findings don't all agree. A family history of painful periods also raises the chances of having them, according to a review of 15 large studies.
Cramps can change over time, too. They often ease with age and may get better after giving birth, according to ACOG and MedlinePlus, but that's an average rather than a rule. When 404 nurses were surveyed six years apart, cramps were as likely to get worse as to get better, although improvement was more likely in those who had a baby in between.
Pain can also cost you sleep. In a small sleep-lab study, 10 women with painful periods slept less well on nights with period pain, so if the first days of your period leave you drained, the cramps themselves may be part of the reason.
When cramps are worth talking to a doctor about
Cramps are very common. More than half of people who get periods have some pain each month, usually mild and lasting a day or two, according to ACOG. In a 2005 Canadian survey of more than 2,700 women, 60% of those who were having periods had period pain. Of those, 60% called it moderate or severe, about half said it had limited their activities, and 17% had missed school or work because of it.
Common doesn't mean you have to put up with it. SOGC's 2025 guideline notes that period pain is often undertreated and too often dismissed. It's worth talking to a doctor if:
- your cramps stop you from doing your usual things, or make you miss school or work
- over-the-counter pain relief doesn't help
- your cramps are getting worse over time, or suddenly feel much worse than usual
- you get severe cramps for the first time after about age 25
- you have pelvic pain at other times of the month, pain during sex, or pain when you use the bathroom
- your periods are very heavy, with clots larger than a quarter
- you have a fever along with the pain
These signs come from SOGC, the US Office on Women's Health and MedlinePlus.
Doctors call everyday cramps primary dysmenorrhea. When period pain is caused by another condition, it's called secondary dysmenorrhea, and the signs above can point to it. The most common cause is endometriosis, where tissue similar to the lining of the uterus grows outside it. Health Canada says it affects about 1 in 10 girls and women of reproductive age, and the US Office on Women's Health says it may affect more than 11% of American women aged 15 to 44. Another cause is adenomyosis, where that tissue grows into the muscle wall of the uterus, as SOGC describes.
Endometriosis can take a long time to recognize. In Canada, diagnosis takes at least five years on average, according to SOGC, and its endometriosis guideline asks health care providers to believe patients when they describe their symptoms. A doctor can help work out what's behind your pain and talk through options to ease it, including prescription medication.
If you'd like to see how your cramps change over time, you can log your period days in GetMyMood and select symptoms such as cramps, back pain or nausea. Over a few cycles, that makes it easier to see which days tend to hurt most and whether the pain is changing, which can be useful to bring up at an appointment.