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Experiment 1 Pregnancy rate was 65% for gilts in this experiment and did not differ among treatments (Table 1). No significant differences were observed between control and estradiol valerate-treated gilts for any of the other traits measured in this experiment. The number of live conceptuses was decreased (P = 0.05) for progesterone-treated gilts compared with control gilts. Although the number of dead fetuses was numerically greater in the progesterone-treated gilts, the effect of progesterone treatment on the number of dead fetuses (mummies) was not significant. Placental weights and fetal hematocrits did not differ between progesterone-treated and control gilts. In contrast, fetal weights, after fitting placental weight as a covariate, were increased (P < 0.01) in the progesterone-treated gilts compared with control gilts. No differences were observed in the number of large and small fetuses or placentas between control and progesterone-treated gilts. Experiment 2 Pregnancy rates were 75% and 58% (P < 0.05) for control and progesterone-treated gilts, respectively (Table 2). For pregnant gilts, progesterone treatment had no effect on the number of piglets born alive or stillborn or the number weaned alive or dead at weaning (Table 2). Piglet birth weight, weaning weight, and Day-56 weight was also not affected by progesterone treatment. Gestation length was 0.5 days shorter (P = 0.05) in progesterone-treated gilts compared with control gilts. Similar to experiment 1, progesterone treatment had no detectable effect on the number of large or small piglets. Frequencies of piglet stillbirths and death by the time of weaning are illustrated in Figure 1. The incidence of both stillbirths (P < 0.05) and loss before weaning (P < 0.01) increased if piglets weighed < 1 kg at birth. TABLE 1. Least squares means (±SEM) for traits from UHO gilts that received no treatment (control), estradiol valerate treatment on Days 11 and 12, or progesterone treatment on Days 2 and 3 of pregnancy and were killed at 105 days of gestation. a Progesterone group less than control (P = 0.05). b Progesterone group greater than control after using placental weight as a covariate (P < 0.01). TABLE 2. Least squares means (±SEM) for traits from intact gilts that received no treatment (control) or were treated with progesterone on Days 2 and 3 of pregnancy. a Progesterone group less than control (P < 0.05).