Comparative Approach to Mechanisms in the Maintenance of Early Pregnancy

Abstract

The sexually mature female pig (Sus scrofa, domestica) has recurring estrous cycles of 18–21 days that are not affected by season. The onset of behavioral estrus is characterized by elevated plasma levels of luteinizing hormone (LH surge) and elevated plasma estrogen (estradiol and estrone) concentrations that precede onset of estrus by 24–48 hr. Ovulation occurs 38–42 hr after onset of estrus, with gilts (virgin females) releasing 10–16 ova. Corpora lutea (CL) are well formed by day 4 or 5 of the estrous cycle, and plasma progesterone concentrations increase from about 5 ng/ml on day 4 to 30–40 ng/ml between days 12 and 14. Luteal regression begins on about day 15 in nonpregnant females, and plasma progesterone concentrations decline to basal levels (1 ng/ml or less) by day 17–18. This cycle repeats itself unless interrupted by pregnancy or events leading to endocrine dysfunction. These general comments concerning the porcine estrous cycle are based on data presented by Guthrie et al. (1972) and Anderson (1974).