The uterine lining, also known as the endometrium, is a tissue inside the uterus that plays a role in reproductive health. Its function is to prepare for and support a potential pregnancy each month. It undergoes significant changes throughout the menstrual cycle, thickening to create a nurturing environment for an embryo.
Normal Thickness Throughout the Cycle and Pregnancy
The thickness of the uterine lining fluctuates in response to hormonal signals during the menstrual cycle. During menstruation, the lining is at its thinnest, measuring between 2 to 4 millimeters (mm) as it sheds. Following menstruation, in the early proliferative phase (days 4-7), it begins to thicken, reaching 5 to 7 mm. By the late proliferative phase (days 11-14), the lining continues to grow, reaching 11 mm.
As the cycle progresses into the luteal or secretory phase (days 14-28), the uterine lining becomes thickest, averaging 12 to 13 mm and reaching 18 mm. This increased thickness is important for embryo implantation, where a range of 7 to 14 mm is considered optimal for conception. While 6 mm is considered a good chance for pregnancy, implantation can occur with a lining as thin as 4 mm.
When the Lining is Too Thin
A uterine lining is considered thin if it measures less than 7 mm. A thin lining can affect fertility, as it may not provide sufficient support for a fertilized egg to implant and grow. This can lead to difficulty conceiving or an increased risk of miscarriage. It may also result in light or irregular menstrual bleeding.
Several factors can contribute to a thin uterine lining. Hormonal imbalances, particularly low estrogen levels, are a common cause. Conditions like Polycystic Ovary Syndrome (PCOS) can also lead to a thin endometrium due to hormonal disruptions. Poor blood flow to the uterus, influenced by factors like smoking or certain medications, can impair the lining’s development. Past uterine surgeries or infections, such as Asherman’s Syndrome, may also cause scarring that prevents the lining from thickening properly.
When the Lining is Too Thick
Conversely, a thick uterine lining, known as endometrial hyperplasia, occurs with excessive growth. This condition is associated with an imbalance of hormones, particularly too much estrogen and insufficient progesterone. Factors can include certain medications, obesity, or conditions like PCOS.
A thick uterine lining can manifest through symptoms such as heavy or prolonged menstrual bleeding, or bleeding between periods. For postmenopausal individuals, any vaginal bleeding warrants medical attention. While most cases of endometrial hyperplasia are not cancerous, some types, particularly atypical hyperplasia, carry a higher risk of developing into endometrial cancer. Uterine polyps, which are overgrowths of endometrial tissue, can also cause a thick appearance and similar bleeding irregularities.
When to Consult a Doctor
Consult a healthcare professional if you experience persistent changes related to your menstrual cycle or reproductive health. Symptoms such as heavy or prolonged menstrual bleeding, bleeding between periods, or any vaginal bleeding after menopause warrant a medical evaluation. Unexplained pelvic pain or difficulty conceiving also indicate a need for assessment.
A doctor can diagnose uterine lining irregularities through tests, often starting with an ultrasound. They can then provide guidance and discuss management options based on your circumstances and health goals.