Endometrial Thickening After Fertilization: Preparing for Implantation in IVF and Natural Cycles

After fertilization, the endometrium continues to develop under progesterone dominance to support implantation.

Timeline

• Ovulation (Day 0): Mature oocyte released; corpus luteum forms → secretes progesterone + estradiol.
• Fertilization (Day 0–1): In fallopian tube; zygote begins cleavage.
• Blastocyst Formation (Day 5–6): Enters uterine cavity.
• Implantation Window (Days 6–10 post-ovulation): Endometrium is maximally receptive (integrins, cytokines, pinopodes). Thickness typically 10–14 mm.
• Post-Implantation: Endometrium further thickens, vascularizes → decidua. hCG from trophoblast rescues corpus luteum.

Clinical Assessment

• Transvaginal ultrasound measures endometrial thickness (triple-line pattern favorable).
• < 7–8 mm associated with lower implantation rates.
• In IVF, estrogen priming (oral, transdermal, vaginal) optimizes lining before progesterone start.

Supporting a Healthy Endometrium

Balanced nutrition, adequate sleep, stress management, avoiding smoking/excessive alcohol, treating underlying conditions (PCOS, endometriosis, adenomyosis, fibroids).