Endometrial Thickness: What Is Normal?

✓ Medically reviewed by a board-certified obstetrician and gynecologist
Written & medically reviewed by: Op. Dr. Ayşet Jane Özcan, Obstetrician & Gynecologist
Istanbul Faculty of Medicine · Medical Park Bahçelievler · Member of TJOD and TSRM
Last updated: 9 August 2026 · Last medically reviewed: 9 August 2026

If you have recently had a pelvic ultrasound, you may have noticed a line in the report giving your endometrial thickness in millimetres, and wondered whether that number was reassuring or something to worry about. It is one of the most frequently measured and most frequently misunderstood figures in gynaecology.

This article explains what the endometrium is, how its thickness is measured, how it changes naturally through the menstrual cycle, what is considered normal before and after the menopause, and what a thin or thick lining can mean for your bleeding pattern, your fertility and any further tests you may be offered.

What Is the Endometrium and Why Does Its Thickness Matter?

The endometrium is the inner lining of the uterus. It is the tissue that builds up each month in preparation for a possible pregnancy and sheds as your period when pregnancy does not occur. Unlike most tissues in the body, it is designed to grow, transform and break down again on a monthly schedule, and it does this under the direction of two hormones: oestrogen, which makes it grow and thicken, and progesterone, which makes it mature, soften and stabilise.

Because the lining responds so directly to hormones, its thickness acts as a visible record of what those hormones have been doing. A lining that is thicker than expected suggests prolonged or unopposed oestrogen stimulation. A lining that stays stubbornly thin may point to low oestrogen levels, scarring, inflammation or a reduced blood supply.

Thickness matters most to two very different groups of women. For those trying to conceive, the endometrium is the surface on which an embryo has to implant, so its quality and depth are part of any fertility assessment. For women past the menopause, particularly those with any bleeding, thickness is one of the earliest signals that the lining deserves a closer look. In both situations the millimetre reading is a starting point for a conversation, not a diagnosis in itself.

How Endometrial Thickness Is Measured

Endometrial thickness is measured with ultrasound, and in almost all cases this means a transvaginal scan. A slim probe is placed in the vagina, which brings it close to the uterus and gives a far clearer picture than scanning through the abdominal wall. The examination usually takes a few minutes, does not require a full bladder, and most women describe it as similar in comfort to a routine internal examination.

On the screen the lining appears as a bright stripe between the two muscular walls of the uterus. The measurement is taken in the long axis of the uterus, at the thickest point, and it includes both layers of the lining, front and back. This is why the figure is sometimes called the double layer thickness. If there is fluid or blood inside the cavity, the two layers are measured separately and the fluid is excluded, otherwise the reading is falsely high.

Several practical factors change the number. Fibroids, adenomyosis, previous surgery, a tilted uterus and body habitus can all make the border of the lining harder to define. A measurement described in the report as unclear or not adequately visualised is not a hidden warning; it usually means the scan needs repeating on a different cycle day or with a different technique.

How the Lining Changes Across the Menstrual Cycle

There is no single normal endometrial thickness, because the lining is meant to change. In a woman who is still having periods, the measurement is only interpretable if you know which day of the cycle the scan was performed on.

During the menstrual phase, while you are bleeding and immediately afterwards, the lining has just shed and is at its thinnest, often measuring roughly 2 to 4 mm. Through the proliferative phase, which runs from the end of your period to ovulation, rising oestrogen from the developing follicle rebuilds the lining, and it typically climbs to somewhere around 5 to 11 mm. Close to ovulation it often takes on a layered appearance sometimes described as a triple line, which simply reflects the structure of the growing tissue.

After ovulation, in the secretory or luteal phase, progesterone takes over. The lining stops growing rapidly and instead becomes richer in glands and blood vessels, appearing brighter and more uniform on the scan, commonly in the range of about 7 to 16 mm. All of these ranges vary between reference sources, between machines and between individual women, so they should be read as orientation rather than as fixed cut-offs.

What Counts as a Normal Endometrial Thickness Before the Menopause?

Before the menopause, the honest answer is that almost any measurement between roughly 2 mm and 16 mm can be entirely normal, depending on when in the cycle the scan was done and what your symptoms are. A reading of 4 mm on day three of your cycle is expected. The same 4 mm reading a week before an expected period would be unusual and would prompt questions about your hormones.

This is the reason your doctor will always ask for the first day of your last period before interpreting the scan. Where possible, and particularly when the reason for the scan is abnormal bleeding, the examination is scheduled in the first half of the cycle, just after bleeding stops. At that point the lining is thin and any polyp, fibroid or focal thickening stands out clearly instead of being hidden inside a naturally thick lining.

If your cycles are irregular or unpredictable, timing the scan is harder, and the measurement carries less weight on its own. In that situation your doctor is more interested in the pattern: how often you bleed, how heavy it is, whether you ovulate, and what your hormone profile suggests. A single number taken in isolation from an unknown cycle day rarely settles anything.

Endometrial Thickness After the Menopause

After the menopause the situation changes completely, and this is where the measurement becomes genuinely important. Once ovarian oestrogen production has stopped, the lining is no longer being stimulated to grow, so it should be thin, inactive and stable. In most postmenopausal women it measures around 5 mm or less, and often considerably less.

The measurement carries the most weight when there is bleeding. Any vaginal bleeding after the menopause, even a single episode of light spotting, needs to be assessed; it is never dismissed as normal. In a woman with postmenopausal bleeding, a thin lining, usually defined at a threshold of about 4 mm or 5 mm depending on local guidance, makes a serious cause such as endometrial cancer very unlikely, although it does not remove the need to explain the bleeding. A lining above that threshold does not mean cancer either, but it does mean the tissue itself needs to be sampled rather than simply watched.

In a postmenopausal woman with no bleeding at all, a slightly thicker lining picked up incidentally on a scan done for another reason is a much weaker signal, and the thresholds used are higher. Hormone therapy also changes what is expected, so your doctor will always factor in whether you are taking it and in what form.

Thin Endometrium: What Can Cause It

A thin lining outside the expected phase of the cycle usually reflects one of three problems: not enough oestrogen reaching the tissue, damage to the tissue itself, or a poor blood supply to it. Recognising which of these applies matters, because the approach to each is different.

• Low oestrogen levels: the perimenopause and menopause, reduced ovarian reserve, prolonged breastfeeding, very low body weight, intense endurance exercise and hypothalamic suppression from stress or restrictive eating all lower oestrogen and thin the lining.
• Intrauterine adhesions or scarring: scar tissue inside the cavity, sometimes called Asherman syndrome, can follow a difficult dilatation and curettage, surgery for fibroids, retained placental tissue or a severe uterine infection.
• Chronic inflammation of the lining: low grade, often silent inflammation of the endometrium can leave the tissue unresponsive to normal hormone signals.
• Reduced blood flow: longstanding damage to the small vessels supplying the uterus, including the effects of previous pelvic radiotherapy or uterine artery embolisation.
• Medication effects: some ovulation induction tablets and long term hormonal contraception can keep the lining thin while they are being used.

A thin lining does not automatically mean anything is wrong. If your periods are regular and you are not trying to conceive, a thin measurement on a scan taken early in the cycle is simply the expected finding.

Thick Endometrium: What It Can Indicate

A lining that is thicker than expected for your stage of life has a long list of possible explanations, and most of them are not sinister. Early pregnancy is always considered first in a woman of reproductive age. Beyond that, the common findings are structural: an endometrial polyp or a fibroid pushing into the cavity can both be reported as a thickened lining until they are looked at more closely, which is why our guide to uterine polyps is often the next thing women read after an ultrasound report.

The finding that most concerns doctors is endometrial hyperplasia, an overgrowth of the lining caused by oestrogen acting on it for long periods without the balancing effect of progesterone. This happens when ovulation is infrequent, as in polycystic ovary syndrome, in women with a higher body weight because fat tissue produces oestrogen, in the perimenopause when cycles become erratic, and with oestrogen only hormone therapy in a woman who still has a uterus.

Hyperplasia is described in two broad categories: without atypia, where the cells look normal but there are too many of them, and with atypia, where the cells themselves are abnormal. The first is usually managed with progestogen treatment and follow up sampling. The second carries a meaningful risk of harbouring or progressing to cancer and is managed more definitively. Only a tissue sample can tell these apart; ultrasound cannot.

How Hormones and Medicines Change the Lining

Many women are given a thickness measurement without being asked what they are taking, and then worry unnecessarily. Several very common medications reshape the endometrium in predictable ways.

Combined hormonal contraception and progestogen only methods, including the hormonal intrauterine system, all thin the lining over time. This is expected and is part of how they reduce or stop bleeding; a lining of 3 mm in a woman using a hormonal coil is a normal finding, not a problem. Ovulation induction tablets that work by blocking oestrogen receptors can also have a temporary thinning effect while they are being taken.

In the opposite direction, oestrogen therapy thickens the lining, which is precisely why women who still have a uterus are given a progestogen alongside it. Certain medications used after breast cancer act on the uterus in a way that can thicken the lining, produce cysts within it and encourage polyps, so any bleeding while taking them is always investigated rather than watched.

Sequential hormone therapy makes the lining fluctuate month by month, so the timing of a scan matters just as it does in a natural cycle. Whenever you have a pelvic ultrasound, tell the person scanning you about every hormonal medication, coil or implant you use. It changes the interpretation more than almost anything else.

Endometrial Thickness, Fertility and Implantation

An embryo cannot implant on a surface that is not ready for it, so the endometrium is assessed as part of most fertility investigations. The lining needs to have grown adequately under oestrogen, then to have been transformed by progesterone into a receptive tissue during the short window when implantation is possible. Thickness is the easiest part of that to measure, which is why it receives so much attention, but receptivity depends on far more than millimetres.

In practice, most clinicians look for a lining of at least about 7 mm around the time of ovulation or transfer, and become more cautious below that. It is important to be clear that this is a threshold for concern rather than a barrier: pregnancies do occur with linings measured below it, and a thicker lining does not guarantee implantation. If a persistently thin lining is found, the useful next step is to look for a cause, such as adhesions or chronic inflammation, rather than to focus on the number alone.

Endometrial Thickness in IVF Cycles

During an IVF cycle, the lining is monitored by ultrasound alongside follicle growth, and again before an embryo transfer. In a frozen transfer cycle the lining is usually prepared with oestrogen, with progesterone added at a planned point to open the implantation window. If the lining has not responded adequately, the transfer can be postponed and the embryos kept frozen. Deferring a transfer is a normal, low risk decision, and it is often the most sensible one.

What Happens If Your Measurement Is Abnormal

An unexpected measurement leads to more information, not straight to treatment. The first step is usually to repeat the scan at a better point in the cycle, since a great many apparently thick linings are simply normal secretory phase tissue seen at the wrong moment. Where the picture is unclear, a saline infusion scan, in which a small amount of sterile fluid is placed in the cavity to separate its walls, can distinguish a diffusely thick lining from a discrete polyp or fibroid.

If the tissue itself needs to be assessed, an endometrial biopsy is taken. This is done in the clinic with a fine flexible tube passed through the cervix, takes a minute or two, and typically causes a strong period like cramp during the sampling. When a focal lesion is suspected, or when a biopsy has not given a clear answer, hysteroscopy allows the cavity to be inspected directly with a fine camera so that a polyp can be removed or a targeted sample taken.

This is also the point to say plainly that a single measurement is never interpreted alone. Your doctor weighs it against your age, your menopausal status, the day of your cycle, your bleeding pattern, your body weight, your medications and your family history. Two women with an identical reading can quite reasonably be advised differently.

When to See a Doctor

Numbers on a report are less important than symptoms. Arrange an appointment, without waiting for a scheduled scan, if you notice any of the following.

• Any vaginal bleeding or spotting after the menopause, however light, and even if it happens only once
• Bleeding between periods, after sex, or periods that have become noticeably heavier or longer than they used to be
• Bleeding that starts while you are taking hormone therapy or a medication used after breast cancer
• Persistent pelvic pain or pressure, or a sense of fullness that does not settle
• Difficulty conceiving after twelve months of trying, or six months if you are over 35
• A previously diagnosed thickened lining that has not been followed up as planned

If you have been told your lining is thick or thin, ask two questions: what day of my cycle was this measured on, and what is the plan if it is the same next time. Those two answers turn an alarming millimetre figure into a manageable follow up plan, and they are usually enough to put the finding into proportion. The assessment can then be tailored to your own history rather than to a single number, and repeated at a sensible interval instead of an anxious one.