Vaginal Progesterone

Vaginal progesterone takes advantage of the "uterine first-pass effect" — drugs absorbed through the vaginal mucosa achieve higher concentrations in the uterus relative to blood levels. Common preparations include Crinone 8% gel (once or twice daily), Endometrin suppositories (2–3× daily), and micronized progesterone capsules (Prometrium, used off-label vaginally).

Started 3–5 days before frozen embryo transfer and continued through 8–10 weeks of pregnancy if successful, vaginal progesterone supports the luteal phase by stabilizing the endometrial lining. It avoids the pain and local reactions of intramuscular injections.

Blood progesterone levels are characteristically lower with vaginal delivery than IM injection but do not reflect endometrial concentrations; monitoring focuses on clinical response.