The lining of the uterus is referred to as the endometrium. It’s one of the few organs in the body that grows and shrinks over a person’s reproductive years. The body prepares the endometrium to host an embryo every month as part of the menstrual cycle. During the process, the endometrial thickness increases and decreases. The hormones estrogen and progesterone trigger these cycles of endometrial growth and shedding through menstruation if a pregnancy does not materialize. Preparation for implantation, maintenance of pregnancy if implantation happens, and menstruation in the absence of pregnancy are the some physiological activities of the uterine endometrium. This means it  plays a critical role in human reproduction and survival.

The endometrium’s typical thickness varies throughout a woman’s life, from childhood to sexual development, fertile years, and after menopause. It is at its thinnest during menstruation, according to research, when it measures between 2–4 millimeters (mm) in thickness. In pregnancy, endometrial thickness is crucial. An endometrium that is neither too thin nor too thick is linked to the highest odds for a healthy, full-term pregnancy. This enables the embryo to effectively implant and obtain the nourishment it requires. As the pregnancy continues, the endometrium thickens.

References:
Dresden, D. (2019, November 19). What to know about endometrial thickness. Medicalnewstoday.com; Medical News Today. https://www.medicalnewstoday.com/articles/327036

‌Gurevich, R. (2020). The Endometrium and Its Role in Your Fertility. Verywell Family. https://www.verywellfamily.com/understanding-the-endometrium-1960066

Southern Illinois University. (2022). Endometrium. Siu.edu. https://histology.siu.edu/erg/RE020b.htm