Early pregnancy decreases the proportion of progesterone receptor (PR) and estrogen receptor α (ERα)-positive cells. (A) Representative images of immunostaining for PR and ERα in mammary gland sections from age-matched virgin and parous mice in estrus. Scale bar, 50 μm. (B) Bar graph comparing the relative frequency of estrogen- and progesterone receptor-positive cells between mammary glands of virgin and parous mice. Data represent the mean ± SD (virgin mice, n = 6; parous mice, n = 5). For PR, P = 3.70E-07; for ERα, P = 0.003, by using two-tailed unpaired Student t test. (C) qPCR for progesterone receptor (Pgr), estrogen receptor alpha (Esr1), and the luminal marker keratin 8 (Krt8) genes in total mammary cell suspensions. Data are expressed as the mean ± SEM from four groups of a minimum of five parous and five age-matched virgin control mice.