Fig. 1.

Profiles of major hormones in the blood during the estrous cycle (a) and pregnancy (b) in pigs. a. During the estrous cycle estrogen concentrations increase prior to estrus by the coordinated actions of gonadotropin-releasing hormone (GnRH), follicle stimulating hormone (FSH), and luteinizing hormone (LH) and decrease on the day of estrus. Progesterone concentrations increase rapidly on the day of estrus until d 12–d 14 and decrease rapidly from d 15 of the estrous cycle due to regression of the corpus luteum induced by prostaglandin (PG) F_2α (PGF) from the endometrium. b. During pregnancy estrogen concentrations decrease from estrus, maintain low concentrations with brief increases on around d 12 and d 25–d 30 of pregnancy, and increase prior to parturition. Progesterone concentrations increase from estrus to reach maximum concentrations on d 12–d 14, then decrease slowly until d 30, and remain fairly constant throughout pregnancy until near term. Developmental processes that occur in the female reproductive tract and morphological changes of preimplantation embryos and early stage conceptuses to corresponding days of pregnancy are indicated on top. Elongating conceptuses on around d 12 of pregnancy secrete estrogen and interleukin-1β2 (IL1B2), and the implanting conceptuses produce maximum levels of interferon-δ (IFND) and IFN-γ (IFNG) on around d 14–d 16. The endometrium and conceptus produce PGs on d 12, and the endometrium produces PGF to induce parturition at term