Female C57BL/6 (B6) mice were mated to BALB/c males and administered RU486 (1 mg/kg) or vehicle (control) on 1.5 and 3.5 dpc. Pregnancy and fetal outcomes were assessed in treated mice at 9.5 and 18.5 dpc and at birth. (A) Schematic of experimental design. (B) Pregnancy rate (% mated mice with ≥1 implantation site at 9.5 dpc or fetus at 18.5 dpc). (C) Number of normal implantation sites or viable fetuses per pregnant dam, number of abnormal or resorbing implantation sites (fetal losses) per pregnant dam, and fetal viability as percentage total implantation sites per pregnant dam were measured. See Supplemental Figure 1 for images of uteri recovered at 9.5 dpc. (D) Representative photomicrographs of viable implantation sites from control pregnant dams and abnormal implantation sites from RU486-treated dams on 9.5 dpc, stained with Masson’s trichrome. Asterisk indicates decidua. Pound sign indicates degenerating fetal tissue. Scale bar = 1 mm. See Supplemental Figure 2 for additional histology of implantation sites. (E) RU486-treated dams exhibit an elevated fetal resorption rate on 18.5 dpc. Resorption sites are indicated by arrows; letter C indicates cervix. Scale bar = 20 mm. (F) Fetuses of RU486-treated dams are visibly growth restricted. Scale bar = 12 mm. (G) Fetal and placental weights and fetal weight/placental weight ratios were measured in viable fetuses on 18.5 dpc. (H) In a separate cohort, mice were allowed to deliver, and number of viable pups per dam was quantified. For panels B, C, G, and H, treatment group is indicated in legend. (B) n = 36–49 mated females/group; data analyzed by χ² test. (C and H) n = 9–16 pregnant dams/group; data shown as mean ± SEM with individual mice indicated by symbols, analyzed by unpaired t test; (G) n = 70–110 fetuses or placentas/group; data shown as violin plots with median and quartile values marked, analyzed by linear mixed model ANOVA with mother as subject. *P < 0.05, **P < 0.01, ***P < 0.001.