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Endometrial Thickness: Things You Need to Know About Endometrial Size
Nov 27, 2025
What endometrial thickness actually measures
Endometrial thickness is the depth of the uterine lining measured in millimeters during an internal ultrasound (a small probe placed gently in the vagina). The sonographer measures the lining across the middle of the uterus and adds the front layer + back layer to get one number. Fluid in the cavity isn’t counted. If the uterus is tilted or fibroids are present, the view can be tricky—so a skilled scan matters and, sometimes, a repeat scan is smarter than arguing with a bad picture.It changes across the month—on purpose
Right after a period, the lining is thin—about 3–5 mm.As ovulation approaches, it builds—about 6–11 mm—and often shows a “three-line” look (a clear middle line with lighter bands on each side).
After ovulation, it thickens a bit more—about 7–16 mm—and looks even and bright, because glands have filled in.
That rise is normal. The lining is getting ready for a possible implantation. A number without a cycle day is like a speed reading without knowing the speed limit.
Fertility: which numbers actually line up with results
In natural cycles or when you’re tracking ovulation, pregnancies often happen when the lining near ovulation is about 7–14 mm and has that three-line look. Pregnancies can happen below 7 mm, but the odds tend to dip. In IVF (embryo transfer), many clinics prefer ≥7–8 mm before transfer. If the number is very high (≈14–16 mm or more) and the pattern looks off, clinicians look for polyps (small growths), trapped fluid, or irritation. Think of these as useful ranges, not hard gates.Too thin”: what it means and what to check
A lining under ~7 mm right before ovulation or transfer can make implantation harder. Common reasons: not enough estrogen effect, scar tissue in the uterus (from past surgery or infection), smoking, poor blood flow to the uterus, medicines that overly suppress hormones, or simply being scanned too early. What helps (chosen to fit the cause):- More or longer estrogen in medicated cycles (pills, patches, or gel—route can matter).
- Treat scar tissue with a hysteroscopy (a tiny camera through the cervix to see and remove it).
- Remove polyps or small fibroids bulging into the cavity so the lining can grow evenly.
- Treat lining inflammation/infection when tests suggest it.
- Lifestyle fixes: stop smoking; steady weight; correct thyroid or prolactin
Many “thin lining” labels vanish when the follow-up scan is timed correctly.
Too thick: context is everything
Before menopause with irregular cycles (often PCOS): if you don’t ovulate on time, the lining keeps building past 12–16 mm because progesterone (the hormone that tells it to shed) didn’t arrive. That can cause heavy, random bleeding, polyps, and, over years, a risk of overgrowth (hyperplasia). The fix is to control the cycle: scheduled progesterone, combined pills/patches/ring, or inducing ovulation if you’re trying to conceive.After menopause: if there is any bleeding, a lining over 4 mm usually needs a biopsy or a camera look to rule out overgrowth or cancer. If there’s no bleeding, many doctors wait until >10–11 mm or worrisome features before sampling. Tamoxifen (a breast cancer drug) is a special case: it can make the lining look thick and bumpy, so plans are personalized.
Symptoms decide how fast to move
- Very heavy periods (soaking a pad an hour, big clots),
- Bleeding between periods,
- Bleeding after sex,
- Any bleeding after menopause.
These push the work-up forward whether the number is 6 mm or 12 mm. Fever with pelvic pain or foul discharge points to infection—that needs prompt care, not a debate about millimeters.
Medicines and devices that change the number (and the look)
- Estrogen therapy thickens the lining.
- Progesterone and the hormonal IUD (levonorgestrel IUD) thin and quiet the lining; with an IUD the lining can look patchy but still be normal.
- Tamoxifen can stimulate the lining and create polyps—extra caution.
- Fertility meds: letrozole and clomiphene can nudge thickness and pattern; clomiphene sometimes leaves the lining thinner in sensitive users.
When thickisn’t just thick—what else the scan can show
- Polyp: a sma ll, soft bump inside the cavity; can cause spotting. A saline ultrasound (saltwater in the uterus for contrast) or hysteroscopy finds and removes it.
- Submucosal fibroid: a firm knot of muscle pushing into the cavity; heavy bleeding, miscarriages, and a distorted number; surgical shaving from inside the uterus can fix it.
- Adenomyosis: lining tissue grows into the uterine muscle; the border looks blurred, periods are painful and heavy; thickness alone won’t explain the symptoms, so the whole picture
Time your scan so the number means something
- Heavy or irregular bleeding: scan early (cycle days 5–9) to avoid mistaking a normal late-month bulge for a problem.
- Fertility tracking: scan right before ovulation (or right before trigger/transfer) to judge readiness.
- Postmenopausal bleeding: scan as soon as you can; if >4 mm or the shape looks suspicious, plan a biopsy or camera look.
Thin-lining playbook (trying to conceive)
- Confirm timing and repeat the scan near ovulation/transfer.
- Check the cavity: saline ultrasound or hysteroscopy for scar tissue, polyps, small fibroids.
- Tune hormones: extend estradiol days or change how you take it (patch/gel vs pills) in programmed cycles.
- Test and treat lining inflammation when indicated.
- Be selective with “add-ons”: low-dose aspirin or vaginal sildenafil have mixed evidence—use only if there’s a clear reason.
Thick-lining playbook (not postmenopausal)
- Map bleeding and do a pregnancy test if the period is late.
- Check for missed ovulation (PCOS, thyroid, prolactin).
- Protect the lining with scheduled progesterone or a hormonal IUD if pregnancy isn’t the goal now.
- Remove focal problems (polyp/fibroid) that keep the lining overbuilt.
- Biopsy if bleeding keeps going, risks are high, or the scan looks unusual.
Postmenopause: fast rules that prevent misses
- Any bleeding + lining >4 mm → biopsy or hysteroscopy.
- On tamoxifen with bleeding → evaluate even if the number is small.
- No bleeding but very thick or irregular → individual call based on risks and a second look.
How to read your own report—line by line
Find these items and you’ll know your next step:- Cycle day or menopause status (so the number has context).
- Exact thickness in mm.
- Pattern: “three-line” (pre-ovulation) or “even and bright” (after ovulation).
- Any focal lesion: polyp, fibroid, or fluid.
- Comment on image quality: clear vs limited.
If key details are missing, ask for an addendum or a repeat scan at the right time.
Quick answers
- “What’s a good size?” Depends on timing. Near ovulation/transfer, many pregnancies happen around 7–14 mm. After menopause, >4 mm with bleeding needs a closer look.
- “Can I get pregnant with 6 mm?” Yes—just less likely than at ≥7–8 mm; fix timing and fix fixable causes.
- “Is thicker always better?” Very thick without regular progesterone or with odd features needs evaluation.
- “Do supplements thicken the lining?” No solid proof. Start with timing, hormones, and fixing the cavity.
Take this with you
Endometrial thickness is a moving target that should match where you are in the cycle and what you feel. Put the number next to timing, symptoms, and the scan pattern. Then act: adjust hormones, repair the cavity when needed, sample the lining when risk says so, and stop chasing a single “perfect” millimeter. That is how a worrying number turns into a plan you can use—and if you want coordinated gynecology and fertility support, centers like Birthright by rainbow Hospitals can help align the steps.FAQs
1) Does one “endometrial thickness” number mean the same thing for everyone?No, the same number means different things depending on cycle day or menopause status. Thickness rises after a period because estrogen builds the lining; therefore 7–14 mm near ovulation/transfer is common. After menopause, any bleeding with >4 mm needs evaluation because progesterone cycling is absent; therefore the threshold for action is lower. Always read the number with timing so you don’t mistake normal growth for a problem.
2) My report says 6 mm before ovulation—can I still get pregnant?
Yes, pregnancies can occur at 6 mm because implantation depends on more than thickness (timing, embryo quality, cavity health); therefore chances are lower than at ≥7–8 mm but not zero. Improve what you can: confirm precise timing, treat cavity issues (polyps/scar tissue), and optimize estrogen support in medicated cycles in order to raise the odds without chasing a single “perfect” millimeter.
3) What does “three-line pattern” mean, and why do clinics like it?
It’s the classic pre-ovulation appearance (a bright central line with two lighter side bands). This pattern correlates with estrogen effect because glands and fluid align before ovulation; therefore many pregnancies occur when this look is present near trigger/ovulation. But it’s not mandatory—some cycles implant with a uniform (“bright”) pattern, so use it as a helpful sign, not a gate.
4) When is a “thick” lining a problem rather than just well-prepared?
Context decides. In reproductive-age anovulatory cycles (often PCOS), thickness >12–16 mm can build because progesterone never arrived to shed the lining; therefore bleeding becomes heavy and irregular, and long-term risk of hyperplasia rises. After menopause, any bleeding with >4 mm triggers sampling so overgrowth or cancer isn’t missed. If thickness is high and the pattern looks uneven or lumpy, look for polyps or submucosal fibroids in order to fix the focal cause.
5) What actually helps a “thin lining” before ovulation or transfer?
Start with timing (repeat the scan at the right day) because early measurements under-call thickness; therefore many “thin” labels vanish on a correctly timed repeat. If still thin, adjust estrogen dose/route or duration in medicated cycles, treat intrauterine adhesions or polyps via hysteroscopy, correct thyroid/prolactin issues, and stop smoking in order to improve blood flow. Add-ons (e.g., low-dose aspirin, vaginal sildenafil) have mixed evidence; use only with a clear indication.
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