Plasma progesterone (P4) levels in pregnant rats on day 18 and 21 without any treatment (controls) and on day 21 after treatment from day 18 until delivery with vaginal P4 (15 mg, bid), s.c. injections of P4 (4 mg), topical P4 in fish oil (15 mg, bid). Asterisks indicate P <0.05 compared with controls. Note the physiological P4 withdrawal from day 18 to day 21 in non-treated rats, that is prevented by s.c. and topical P4, but not by vaginal P4. From Kuon et. al. Am J Obstet Gynecol. 2010;202:455 e1–9.