The endometrium, or the inner lining of the uterus, plays a critical role in the female reproductive system. Each month, under the influence of reproductive hormones, this lining undergoes changes to prepare for the potential implantation of an embryo. These cyclical changes cause the endometrial thickness to vary, increasing or shedding depending on the stage of the menstrual cycle and whether pregnancy occurs.

Why Endometrial Thickness Matters

A properly thickened endometrial lining is essential for a fertilised embryo to successfully implant and grow. If the endometrium is too thin, implantation may fail, or early miscarriage might occur. On the other hand, an excessively thickened lining could also interfere with normal reproductive processes or indicate underlying conditions. Therefore, monitoring endometrial thickness is important not only for understanding menstrual health but also when addressing fertility concerns or undergoing assisted reproductive treatments like IVF.

Hormonal Influence on Endometrial Changes

Two primary hormones, estrogen and progesterone, regulate the endometrial cycle:

• Estrogen promotes the thickening of the endometrium during the first half of the menstrual cycle.
• Progesterone stabilises and prepares the lining for embryo implantation in the second half.

If pregnancy does not occur, hormone levels fall, and the thickened endometrium is shed during menstruation.

Endometrial Thickness Through Life Stages

The thickness of the endometrial lining naturally varies depending on age, hormonal status, and reproductive phase:

• Before puberty: The endometrium is thin and inactive due to low hormone levels.
• During reproductive years: The lining thickens and sheds cyclically.
• Post-menopause: The endometrium becomes thin again and typically remains under 5 mm in thickness, unless hormone therapy or pathology is present.

Endometrial Thickness During Menstrual Cycle 1. Menstrual Phase (Days 1–5)

• Shedding of the lining occurs if there is no pregnancy.
• Endometrial thickness is at its thinnest, measuring approximately 2–4 mm.

2.  Proliferative Phase (Days 6–14)

• Under the influence of estrogen, the lining begins to rebuild and thicken.
• By the middle of this phase, the thickness typically reaches 5–7 mm.

3.  Pre-Ovulatory Phase (Late Proliferative Phase)

• As ovulation approaches, the lining continues to grow and can measure up to 11 mm.

4.  Secretory Phase (Post-Ovulation, Days 15–28)

• Progesterone causes further development, making the lining ready for implantation.
• This is the peak thickness phase, with measurements typically reaching 12–16 mm.

Note: During any part of the cycle, an endometrial thickness up to 14 mm is generally considered normal for reproductive-aged women.

Endometrial Thickness and Fertility

For women trying to conceive naturally or through treatments like IVF, endometrial thickness is a key factor:

• Too thin (<6 mm): May indicate poor estrogen levels or uterine health issues, leading to lower implantation rates.
• Too thick (>16 mm): Might suggest hormonal imbalance, polyps, or endometrial hyperplasia, which could hinder implantation.

Studies suggest that a thickness of 7–10 mm during embryo transfer in IVF offers the best chance of successful implantation.

Endometrial Thickness in Pregnancy

If conception occurs, the endometrium evolves into the decidua, which supports the embryo. As pregnancy advances, the lining thickens further to provide nourishment and support for the growing fetus. Even at early stages, a thickness of 6 mm or more is associated with a higher chance of successful pregnancy.

Endometrial Thickness After Menopause

In postmenopausal women, hormone production significantly declines, leading to a thin endometrial lining. In healthy individuals not on hormone replacement therapy (HRT), the lining typically measures less than 5 mm.

If bleeding occurs after menopause and the endometrial thickness is found to be greater than 5 mm, further investigation is required to rule out endometrial hyperplasia or cancer.

Measuring Endometrial Thickness

Transvaginal ultrasound (TVS) is the most common and effective method to assess endometrial thickness. It is non-invasive, accurate, and widely used during fertility evaluations or to investigate abnormal bleeding.

In some cases, if TVS is not feasible due to anatomical reasons or other health factors, an MRI may be recommended for a clearer view.

Signs of Abnormally Thick Endometrium

An endometrium that becomes thicker than usual may cause noticeable symptoms, such as:

• Vaginal bleeding after menopause
• Spotting between periods
• Heavy or prolonged menstruation
• Irregular cycles (either too short or too long)
• Absence of menstruation (amenorrhea)

These signs may indicate conditions like endometrial hyperplasia, polyps, fibroids, or in rare cases, endometrial cancer.

Treatments for Abnormal Endometrial Thickness

If too thick:

• Progestin therapy: Helps counteract excess estrogen and may prevent the need for surgery.
• D&C (dilation and curettage): In cases of severe hyperplasia or suspected cancer.

If too thin (especially relevant in fertility):

• Estrogen supplementation to promote growth.
• Medications and supplements to improve blood flow to the uterus (like sildenafil or vitamin E).
• hCG injections: Mimic natural hormonal signals to support the lining.
• PRP therapy (Platelet-Rich Plasma): An emerging technique to rejuvenate thin linings.

Note: These treatments may have variable success, and consultation with a fertility specialist is necessary to customize care.

When to Consult a Doctor

You should see a gynaecologist or fertility expert if you experience:

• Unusual bleeding patterns (heavy, absent, or postmenopausal)
• Spotting between cycles
• Persistent pelvic pain or pressure
• Difficulty conceiving
• Early pregnancy losses

Early evaluation can help detect hormonal imbalances, uterine abnormalities, or early signs of endometrial disease.

Ideal Endometrial Thickness for IVF Success

For couples undergoing in vitro fertilization (IVF), optimal endometrial thickness is crucial. Most fertility experts agree:

• 7–10 mm is the ideal range for embryo transfer.
• Anything below 6 mm significantly reduces implantation chances.
• The lining should also appear trilaminar (three-layered) on ultrasound for best outcomes.

Conclusion

Endometrial thickness is a key marker of reproductive health, affecting natural conception, fertility treatment success, and overall uterine well-being. While fluctuations in thickness are normal during different life stages, abnormalities should not be ignored. Monitoring the endometrium through ultrasound, especially during fertility planning, can greatly increase the chances of a healthy, full-term pregnancy.