Around implantation, the endometrium is in its secretory phase and is usually about 7-14 mm thick on transvaginal ultrasound. In fertility treatment, at least 7 mm is commonly considered a favorable threshold, although pregnancy can still occur with a thinner lining.

Implantation typically occurs about 6-10 days after ovulation. Thickness alone does not determine implantation. Hormonal timing, endometrial pattern, blood flow, and embryo quality also matter. The receptive window is commonly around cycle days 19-24 in a typical 28-day cycle. - Goodman & Gilman’s The Pharmacological Basis of Therapeutics, section “Effects of Cyclical Gonadal Steroids on the Reproductive Tract”

A recent systematic review on thin endometrium is indexed as PMID 38745960.