Table of content

• What is Endometrium?

• Functions of the Endometrium

• Endometrial Thickness Chart - What is Normal Size of Endometrium in mm?

• How to Measure Normal Endometrial Thickness?

• 4 Major Conditions That Can Cause A Change In The Endometrial Thickness Range

• Take-Home Points

• FAQ on Normal Endometrial thickness, measurement, changes and treatment

What is Endometrium?

The endometrium is the inner lining of the mammalian uterus. The endometrium is an inner epithelial layer of the uterus along with its mucous membrane that has a basal and functional layer.  

The layer that thickens and sheds during menstruation is a functional layer of the endometrium. Most mammals including humans shed endometrium during the menstrual cycles. In other mammals, the endometrium is reabsorbed in the estrous cycle.

The normal thickness of endometrium varies throughout the menstrual cycle. Regular monitoring of the endometrial thickness is important for women undergoing fertility treatment or those experiencing abnormal bleeding.

Functions of the Endometrium

• The endometrium is an important part of the female reproductive system that plays a major role during the menstrual cycle, pregnancy outcome, and in postmenopausal women. The endometrial thickness range is changed during these stages.  

• While you are in a menstrual cycle, the endometrium grows to a thick, blood vessel-rich, glandular tissue layer which result in a change in the normal thickness of endometrium. This occurs in the anticipation of the implantation of a blastocyst in the uterus. 

• The endometrium thickness range is increased in size and number of glands and blood vessels during pregnancy. During pregnancy, the fusion of vascular spaces forms the placenta, to supply oxygen and nutrition to the embryo and fetus. 

• In postmenopausal women, the endometrial thickness must be no thicker than 4-5 mm. If endometrial thickness ranges more than this, endometriosis occurs.

Endometrial Thickness Chart - What is Normal Size of Endometrium in mm?

The thickness of the endometrium ranges depending on the phases of women. The normal endometrial lining thickness chart that is accepted is measured through transvaginal ultrasound or transvaginal ultrasonography. The normal size of the endometrium is measured in mm.

An endometrium normal size can also vary based on factors such as hormonal imbalances, obesity, or polycystic ovary syndrome (PCOS). These conditions can result in thicker or thinner endometrial linings outside of the normal range. Other factors that can affect endometrial thickness include medication use, such as hormone replacement therapy (HRT), or intrauterine device (IUD) placement.

Here is the endometrial thickness chart in different stages of women:

Endometrial Thickness Chart Normal Size (in mm) During Premenopausal Stage

• The normal size of endometrium in mm during the menstrual phase of pre-menopausal women ranges from 2-4 mm. During the early proliferative stage, the endometrium normal size (in mm) increases to 5-7mm. The endometrial thickness range during the last proliferative or pre-ovulatory phase is up to 11 mm. During the secretory phase, the endometrial lining thickness is 7-16 mm.

• The normal size of endometrium in mm during the pre-menopausal stage varies at different stages of the menstrual cycle. 

The thickness of the endometrium in the ovarian cycle phases like menstrual phase, follicular phase, luteal phase, ischemic phase is as follows:

• The endometrial thickness if present less than 14mm is considered normal in the menstrual cycle. While a woman is in hormonal therapy, the endometrial thickness reaches up to 15mm. 

• The endometrial thickness range following dilatation and curettage or spontaneous abortion is less than 5 mm. If the endometrium is thicker than 5 mm after abortion or delivery, consider it as remnants of conception or labor.

• The endometrial thickness will be decreased with the long-term use of oral contraceptive pills.The menstrual cycles and ovarian response are responsible for clinical pregnancies. 

Endometrial Thickness Chart Normal Size (in mm) During Postmenopausal Stage

The endometrial thickness normal range can also differ for postmenopausal women.  The normal endometrial thickness in mm should be less than 5 mm for post-menopausal women. If the postmenopausal endometrial thickness varies from this measurement further evaluation is needed.

The endometrial thickness range during vaginal bleeding (not on tamoxifen):

• The upper limit of normal endometrial thickness is < 5mm

• If the endometrial thickness range is > 5 mm, there can be a risk of carcinoma (~ 7%), and 0.07% risk, if the endometrial thickness range is < 5mm.

• On hormonal replacement therapy, the normal size of endometrium in mm: upper limit of normal endometrial thickness in mm is 5mm.

The normal size of endometrium in mm- with no history of vaginal bleeding:

• The acceptable normal size of endometrium in mm is suggested as 8-11 

• If the endometrial thickness range is > 11 mm, there can be a risk of carcinoma (~ 7%), and 0.02% risk, if the endometrial thickness range is < 11 mm.

The normal endometrial thickness with tamoxifen:

• The normal size of endometrium in mm with tamoxifen is < 5 mm. 

If the endometrial thickness is increased without any vaginal bleeding or uterine bleeding a transvaginal ultrasound or a consultation with a gynecologist is reasonable.

Endometrial Thickness Chart Normal Size (in mm) During Pregnancy

A normal endometrial thickness for pregnancy is required for having a normal pregnancy. It helps the embryo to get implanted and also gets the nutrition it needs to grow. 

The endometrium thickness in pregnancy thickens during the later phase of pregnancy. The normal endometrial thickness for pregnancy is around 8 to 15 mm. 

During pregnancy, the endometrial thickness increases to support the growing fetus. Ultrasound is the most common method used to measure endometrial thickness during pregnancy. The endometrial thickness varies depending on the gestational age during pregnancy. The endometrial thickness during pregnancy increases gradually throughout the first trimester, and then plateaus in the second and third trimesters.

The endometrial thickness varies from person to person and there are many factors that affect it. It has been seen for a lady to conceive a baby and for the pregnancy to continue, the endometrial thickness needs to be at least 15mm or more.

Endometrial Thickness Using MRI

Normal endometrial thickness and size is well assessed on MRI. The measurement of endometrial thickness should be taken at a mid-sagittal slice, the same as the transvaginal ultrasound assessment plane. 

Until now you have learned about how thick the endometrium should be, the endometrial thickness chart should be in different phases. But how to measure the endometrial thickness?

How to Measure Normal Endometrial Thickness? 

The thickness of the endometrium should be measured on the long axis or sagittal plane, ideally on transvaginal ultrasonography through the endocervical canal. The measurement is the biggest echogenic area from one basal endometrial interface across the endometrial can all of the other basal endometrial surface. Proper care should be taken to not include hypoechoic myometrium or intrauterine fluid in this measurement.

Measurement of Endometrium Thickness Range

Ultrasound diagnostic procedure is used to measure the normal endometrial thickness or for diseased endometrial thickness. Transvaginal ultrasound is also used to examine the situation when you experience any vaginal bleeding. 

An MRI is recommended by obstetrics and gynaecology doctors if your position of the uterus or other problem makes it difficult for an ultrasound. 

4 Major Conditions That Can Cause A Change In The Endometrial Thickness Range

The common cause of changes in normal endometrial thickness is due to variation in the hormones such as estrogen and progesterone. These hormones regulate the menstrual cycles and pregnancy of women. The change in these hormones causes disease symptoms like vaginal bleeding or uterine bleeding,endometrial hyperplasia, endometrial cancer, endometrial receptivity.

1. Vaginal bleeding

Vaginal bleeding during menstrual cycles is normal. Abnormal vaginal bleeding is different from the menstrual cycle that comes monthly. The abnormal vaginal bleeding includes bleeding that happens before puberty or after menopause. The causes of abnormal vaginal bleeding include:

Cancers and precancerous factors 

• Endometrial cancer

• Cervical cancer

• Ovarian cancer

• Vagina cancer

Endocrine factors

• Hyperthyroidism

• Hypothyroidism

• Polycystic ovary syndrome

• Change in birth control pills or menopausal hormone therapy

Fertility and reproduction factors

• Sexual intercourse 

• Imbalance hormone levels 

• Ectopic pregnancy 

• Miscarriage 

• Pregnancy outcomes 

• Vaginal atrophy

Infections

• Endometritis

• Gonorrhea

• Virginities

• Pelvic inflammatory disease

Medical conditions

• Celiac disease

• Thrombocytopenia

• Von Willebrand disease

Medications and devices

• Forgotten or retained tampon

• Tamoxifen side effects 

• Intrauterine device

Noncancerous growths and other uterine conditions

• Adenomyosis

• Cervical polyps

• Uterine polyps 

• Endometrial polyps

• Trauma blunt trauma or penetrating injury to the vagina or cervix, sexual abuse.

Treatment of vaginal bleeding

Depending on the underlying cause the treatment of vaginal bleeding is determined. The necessary for the treatment is also decided based on the severity. In many cases, the cause of vaginal bleeding such as thyroid, liver, kidney, or blood clotting problems is treated with medications.

Medications for the treatment of irregular vaginal bleeding include

• If the cause of vaginal bleeding is lack of ovulation, use of progesterone to be taken regularly or an oral contraceptive with progesterone is suggested.

• If the cause of vaginal bleeding is a precancerous change, progesterone medications are prescribed to reduce endometrial cancer to avoid surgery.

• In postmenopausal women or women at the stage of menopause, an oral contraceptive is prescribed for a regular vaginal bleeding pattern and to relieve hot flashes.

• If vaginal bleeding is due to polyps or due to benign tumors, a surgical procedure is advised as medication cannot treat this.

• If the infection is the cause of vaginal bleeding, antibiotics are used. If the bleeding is observed during pregnancy, an evaluation is conducted and if reported as endometriosis medications or surgery is carried out.

• If the medications are unable to control the vaginal bleeding, then a surgical procedure is necessary.

• Hysterectomy is occasionally required when hormonal medications cannot control excessive vaginal bleeding.

2. Endometrial hyperplasia

Endometrial hyperplasia is a condition where an excessive proliferation of endometrial cells occurs. Endometrial hyperplasia is caused due to increased levels of estrogen and due to insufficient levels of progesterone-like hormones that will counteract the estrogen effect on the proliferation of endometrium tissue. An endometrium normal size is crucial in diagnosing endometrial hyperplasia.

Endometrial hyperplasia may occur due to obesity, polycystic ovarian syndrome, tumors that produce estrogen, or intake of compounds for estrogen replacement therapy. Endometrial hyperplasia may cause or coexist with endometrial cancer.

The endometrial lining thickness will change during endometrial hyperplasia. The thin uterine lining will become unusually thick because of the increased cell number.

Endometrial hyperplasia is classified into 4 types as per World Health Organization; these include -

• Simple hyperplasia without atypia

• Complex hyperplasia without atypia

• Simple atypical hyperplasia

• Complex atypical hyperplasia

Later the classification of simple and complex was removed and was distinguished only as presence and absence of atypia.

Endometrial hyperplasia- Irregularity and cystic expansion of glands and budding of glands which leads to change in the appearance of individual glands. This type of endometrial hyperplasia will develop into endometrial cancer eventually. In a retrospective cohort study, 1.6% of patients with endometrial hyperplasia symptoms have eventually developed endometrial cancer.

Atypical endometrial hyperplasia- simple or complex architectural changes with atypical changes England sales that include cell stratification, tufting, loss of nuclear polarity, enlarged nuclei, and increase in meiotic activity. All these changes in the atypical endometrial hyperplasia or similar to true cancer cells but do not show any invasion into the connective tissue which is the defining characteristic of cancer. In a retrospective cohort study, 22% of patients with atypical endometrial hyperplasia have shown endometrial cancer occurrence.

Endometrial hyperplasia is diagnosed through ultrasound or transvaginal ultrasonography, biopsy, hysteroscopy. It is essential to monitor the endometrial thickness and endometrium normal size in mm to diagnose and treat endometrial hyperplasia.

The thickened endometrium treatment depends on the severity and type of hyperplasia. For simple hyperplasia, hormonal therapy is usually the first line of treatment. This may involve taking progestin to balance out the effects of estrogen. In contrast, complex hyperplasia with atypia, a more severe form of the condition, often requires surgery, such as a hysterectomy, to remove the uterus.

You need to consult a healthcare provider if you experience:

• Vagina bleeding

• Uterine bleeding

• Painful urination

• Missed menstrual cycles

Endometrial thickness treatment in endometrial hyperplasia

Endometrial thickness treatment involves hormonal therapy, such as cyclic or continuous progestin therapy, or hysterectomy.

3. Endometrial cancer

Endometrial cancer is cancer that occurs in the uterus causing thick endometrial lining. It usually occurs due to the abnormal growth of cells that can invade or spread to other parts of the body. The first symptom of endometrial cancer is vaginal bleeding that is not associated with the menstrual cycle. 

Endometrial cancer mostly occurs in post-menopausal women. Symptoms like pain with urination, during sexual intercourse, or pelvic pain or seen in women with endometrial cancer. Endometrial cancer sometimes is also referred to as uterine cancer.

Endometrial cancer is diagnosed by endometrial biopsy or by taking samples during dilation and curettage procedure. Abdominal hysterectomy is a leading treatment option for This endometrial thickness treatment. This involves the removal of the uterus, fallopian tubes, and ovaries on both sides.

Vaginal bleeding, uterine bleeding, and abnormal menstrual cycles are common symptoms. The diagnosis is done through transvaginal ultrasound to examine the endometrial stripe thickness range in women with postmenopausal bleeding. 

Endometrial cancers or classified into the following types 

• Carcinoma - type I and type II

Type I endometrial carcinoma occurs before the menopausal period and at the time of menopausal in women.

Type II endometrial carcinoma occurs in older, postmenopausal women and is not associated with endometrial hyperplasia.

• Endometrioid adenocarcinoma

In endometrioid adenocarcinoma, the cancer cells grow in patterns reminiscent of normal endometrium. Many new glands are formed from columnar epithelium with abnormal nuclei.

• Serous carcinoma

Serous carcinoma is a Type II endometrial tumor that is common in postmenopausal women with atrophied endometrium. It appears with many atypical nuclei, papillary structures, and rounded cells instead of columnar cells.

• Mucinous carcinoma

It is a rare form of endometrial cancer and occurs in less than 1-2% of all endometrial cancers. Well-differentiated columnar cells organized into glands with characteristic mucin in the cytoplasm can be seen.

• Mixed or undifferentiated carcinoma

Mixed carcinomas have both- type I and type II cells. These include malignant mixed Mullerian tumors derived from the endometrial epithelium.Undifferentiated endometrial carcinoma has a prognosis more than grade III tumors. These tumors show sheets of identical epithelial cells with no identifiable pattern.

• Other carcinomas 

Non-metastatic squamous cell carcinoma and transitional cell carcinoma for the rarest carcinomas that occurred. The common genetic causes are uncategorized and there are no guidelines on how to treat this type of cancer.

• Sarcoma

Sarcomas or uncommon endometrial cancers begin in the non-glandular connected issue of the endometrium. These are generally nonaggressive and if they reoccur can take decades to get cured.

Treatment of endometrial stripe thickness in endometrial cancer

Thickened endometrium treatment in the case of endometrial cancer include surgery to remove the uterus, fallopian tubes. Radiation therapy with high energy is another option for thickened endometrium treatment in endometrial cancer.

Other options to thickened endometrium treatment include chemotherapy with powerful drugs and hormone therapy to block the hormones that cause the proliferation of the cells.Targeted therapy is another option to treat endometrial stripe thickness in endometrial cancer which involves drugs that attack specific weaknesses of the cancer cell. Immunotherapy will help to boost the immunity to fight against endometrial cancer.

4. Endometrial receptivity

Endometrial receptivity is a complex process that helps the embryo in providing the opportunity to attach, invade and develop into a new individual for the continuation of the species.

The window of implantation is 3 to 6 days within the secretory phase. Endometrial receptivity plays a major role to preclude normal implantation, fertility, or pregnancy outcome. The ovarian response is also important for clinical pregnancies besides endometrium.

Importance of endometrial thickness and endometrial receptivity on pregnancy outcome

A retrospective cohort study was conducted to understand the relation between endometrial thickness or thin uterine lining symptoms and clinical pregnancies that are caused due to unstimulated menstrual cycles. 

Even though it is proven that endometrium is important for the development and maintenance of pregnancy outcomes, it is unclear which endometrial factors are relevant. During transvaginal ultrasound evaluation, endometrial thickness, echo pattern, and endometrial perfusion are evaluated. 

The significance of endometrial thickness has been investigated in in-vitro fertilization treatments [IVF- clinical pregnancies] and reproductive technology with ovarian response.

The association of pregnancy outcome with endometrial thickness in unsimulated menstrual cycles with fresh embryo transfers was described in this retrospective cohort study. The study evaluation was adjusted to factors that influence the pregnancy outcome and the endometrial thickness.

This retrospective cohort study of endometrial thickness has shown the following data

• In all the studies, the association of endometrial thickness with pregnancy outcome rate was performed only with high dose IVF (clinical pregnancies) stimulations, reproductive technology like cryopreserved embryos, or low dose Intra Uterine Insemination (IUI) stimulations.

• The retrospective cohort study has confirmed the reduced pregnancy outcome with endometrial thickness of ≤ 7 mm. The reason for the reduced pregnancy outcome in patients undergoing GnRH stimulated IVF therapy compared to endometrial thickness > 7 mm is unclear.

• In contrast, a lower pregnancy outcome is observed in women with thick endometrium lining. But an increased pregnancy outcome is not confirmed with particularly thick endometrium lining of > 11 mm, > 13 mm, > 14 mm with GnRH stimulated IVF therapies.

• Also found that there is no significant correlation between endometrial thickness and pregnancy outcome in hormone-stimulated reproductive technology (IUI) treatments.

• Lower pregnancy outcomes with thin uterine lining symptoms could be inherited if it significantly affected fertility. Other factors can influence lower pregnancy outcomes due to thin uterine lining such as exposure of the uterus to radiation, multiple curettages.

• In the end, this prospective cohort study has concluded along with thick endometrium lining that thin endometrium is also associated with lower pregnancy outcomes in unsimulated menstrual cycles.

What is endometriosis?

• Endometriosis is a disease of the female reproductive system where the presence of normal endometrial mucosa is seen in locations other than the uterine cavity. It usually occurs on the ovaries, fallopian tubes, tissue around the uterus and ovaries, and in rare cases, it may occur in other parts of the body too.

• Symptoms include pelvic pain, heavy periods, pain with bowel movements, and infertility. The cause of endometriosis is not known but can affect pregnancy outcomes or clinical pregnancies.

• A biopsy is the surest method of diagnosis. 

Treatment for endometriosis

As there is no cure for endometriosis, treatment of pain and treatment of endometriosis-associated infertility are the only management procedures. Pharmacotherapy is initiated for pain management based on the symptoms and examination of endometrial stripe thickness by transvaginal ultrasound. 

To restore or preserve fertility, endometriosis is treated exceptionally with reproductive technology like surgery, fertility medication, or IVF (clinical pregnancies). Usually, surgery is done through the laparoscopic method. During the fertility treatment, ultra-long pre-treatment with the GnRH agonist has a higher chance of resulting in pregnancy with endometriosis when compared to short pre-treatment.

Take-Home Points

• Normal endometrial thickness in healthy women will change according to the age and phases of life.

• Endometrial thickness normal size in mm in post-menopausal women is < 5 mm.

• Endometrial thickness normal size in mm during menstrual cycles is around 14 mm and if someone is taking tamoxifen, endometrium normal size in mm is less than 5mm.

• Abnormal vaginal bleeding can be due to endometrial hyperplasia, endometrial cancer, pregnancy outcome, abortion or vaginal injury.

• Endometrial cancer causes thick endometrial lining. Endometrial hyperplasia when left untreated can lead to endometrial cancer.

• Consult a gynecologist if you observe any abnormal vaginal bleeding or uterine bleeding.

• Medications, hormonal therapy, surgery can be done to correct the endometrial thickness in women.

• Endometrial thickness exhibits a relationship with pregnancy outcomes in fresh embryo transfer cycles.

• Thin uterine lining and thick endometrial lining are associated with the ovarian response but not important predictors of clinical pregnancies.

References

• NCBI

• Frontiers

• Fertility and Sterility