Endometrium: definition and meaning
The endometrium is the inner lining of the uterus. Each menstrual cycle, estrogen makes it thicken and progesterone makes it ready for a fertilized egg to implant. If pregnancy doesn't happen, hormones fall and the top layer sheds as your period. A deeper basal layer stays behind and regrows the lining next cycle.
How it changes in your cycle
| Phase | What the lining does | Typical thickness |
|---|---|---|
| Menstrual | Top layer sheds | Thin, about 2 to 4 mm |
| Follicular (proliferative) | Estrogen rebuilds it | Grows to about 5 to 7 mm |
| Around ovulation | Layered, "triple line" on ultrasound | About 7 to 11 mm |
| Luteal (secretory) | Progesterone makes it soft and nourishing | About 7 to 16 mm |
Why it matters
- Period flow: a thicker lining often means a heavier period
- Fertility: a healthy lining helps an embryo implant
- Conditions: endometrium-like tissue outside the uterus is endometriosis; inside the muscle wall it's adenomyosis
- After menopause: a thickened lining or any bleeding should be checked
Without regular ovulation, as with anovulation or PCOS, the lining can keep building under estrogen with no progesterone to balance it. That can cause irregular heavy bleeding and, over years, raises the risk of thickening (hyperplasia), which is why doctors aim for regular bleeds in those conditions. An endometrial biopsy checks the tissue when needed.
See a doctor about very heavy or prolonged periods, bleeding between periods, periods that come less than every three months for a long time, or any bleeding after menopause.
Frequently asked questions
What is a normal endometrial thickness?
It depends on the cycle phase: thin just after a period and up to about 16 mm in the luteal phase. After menopause it's usually under 4 to 5 mm.
Does the endometrium affect mood?
Not directly. The hormones that shape it, estrogen and progesterone, are the ones linked to mood changes across the cycle.
Sources
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Menstruation: definition and meaningMenstruation is the monthly shedding of the uterine lining through the vagina, known as a period. Why it happens, how long it lasts, and what's normal.
Endometriosis: definition and meaningEndometriosis is when tissue similar to the uterine lining grows outside the uterus. A clear definition, common symptoms, and when to see a doctor.Adenomyosis: definition and meaningAdenomyosis means the uterine lining grows into the muscle wall of the uterus. A plain-English definition, common symptoms and when to see a doctor.The 4 phases of the menstrual cycle, in colorThe 4 phases of the menstrual cycle explained simply: what your hormones do in each, how long each lasts, and how mood and energy usually change.Uterus: definition and meaningThe uterus, or womb, is where a period comes from and where a pregnancy grows. Its parts, how it changes each cycle, and common conditions that affect it.