Here's something most of us were never taught: you were born with every egg you will ever have. In fact, your egg supply peaked before you were born, at around five months of pregnancy.
Menopause is what happens when that supply runs out. And it's about far more than periods stopping.
The official definition (and why it's a bit odd)#
Doctors confirm natural menopause once you've gone 12 months in a row without a period, with no other cause such as pregnancy, medication or illness.
That makes menopause, technically, a single day: the one-year anniversary of your last period. Everything after it is called postmenopause.
Some doctors argue the definition doesn't fit everyone. What if you've had a hysterectomy, use a hormonal coil (IUD), had an endometrial ablation, or have PCOS? You may not have periods to count at all. In that case, doctors usually go by your age and symptoms: NICE (opens in a new tab) recommends this for women over 45, including those without a uterus. A blood test is sometimes used, mainly for women under 45, but the results aren't always reliable.
What's happening inside your body#
Think of your eggs as a savings account you can only withdraw from. Girls are born with roughly 1 to 2 million (BioNumbers, Harvard Medical School (opens in a new tab)), and no new ones are made. One modelling study estimated that by 30, most women have about 12% of the eggs they had before birth, and by 40 about 3% (Wallace and Kelsey, 2010 (opens in a new tab)).
When the account empties, your ovaries make very little of the sex hormones:
- Estradiol (the main estrogen) drops to a tiny fraction of its former level
- Progesterone falls too
- Testosterone drops to roughly half or less of its peak, though your body has other ways to make some
Here's why that matters so much. Estrogen works like a key that fits locks in almost every organ, from your brain and heart to your bones, skin and gut. When there's less of it, all those systems notice.
Neuroscientist Dr Lisa Mosconi adds a twist. Many of the best-known symptoms, including hot flashes, night sweats, insomnia, memory lapses, low mood and anxiety, actually start in the brain, not the ovaries. The two are in constant two-way conversation, so the health of one affects the other.
Why does menopause even exist?#
Hardly any animals go through it. Humans do, along with a couple of whale species and possibly one type of giraffe. Most mammals keep ovulating until they die.
One much-debated theory is the "grandmother hypothesis": there may have been a survival advantage in women stopping reproduction and helping raise the next generation instead. Whatever the reason, many women today live for decades after their final period.
When does it happen?#
| Term | Age at final period |
|---|---|
| Premature menopause | Before 40 |
| Early menopause | 40 to 45 |
| Typical range | About 45 to 55 |
| Average | Around 50 to 52 |
A huge pooled analysis of over 234,000 women in 7 countries put the median age at 50. In the US, the long-running SWAN study found a median of about 52 (Gold et al., 2013 (opens in a new tab)), and the 2024 UK study cites about 51. About 10% of women reach menopause before 45, and roughly 2% before 40 (Mishra et al., 2017 (opens in a new tab)).
