Small, muscular, and busier than anything else you own
Non-pregnant, the uterus is roughly the size of a small pear and weighs about 60g. In pregnancy it expands to hold several litres, then returns. No other organ in the human body does this.
It sits low in the pelvis, tilted forward over the bladder in most people and tilted back toward the spine in around a quarter — a normal variant called retroversion that can make cramps register as low back pain.
Three layers
- Endometrium
- The inner lining. Thickens from ~1mm to 7–16mm across a cycle, then sheds. This is your period.
- Myometrium
- Thick smooth muscle. It contracts to shed the lining and, later, to deliver a baby.
- Perimetrium
- The thin outer covering, continuous with the pelvic lining.
Why cramps happen
As the lining breaks down it releases prostaglandins — signalling molecules that make the myometrium contract and briefly squeeze its own blood supply. That's the cramping sensation, and it is genuinely a small oxygen-deprivation event in muscle.
Prostaglandins also act on nearby smooth muscle, which is why nausea, loose stools and headache cluster around day 1. Higher prostaglandin levels mean stronger contractions — the biological reason two people can have wildly different period experiences and both be 'normal'.
Self-check
Check yourself
1. What causes period cramps?