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