Intrauterine crowding and placental insufficiency are main reasons for prenatal losses in swine. Two studies were conducted to examine: 1) conceptus development and in vitro steroidogenic capability of three regions of the placenta (middle, inner, polar) at d 30, 50, 70, and 90 of gestation; and 2) the effects of intrauterine position and fetal sex on conceptus development and in vitro steroidogenic activity of the placenta and endometrium at d 40, 60, 80, and 100 of gestation. All variables were examined in gilts that were uterine intact before hysterectomy (n=19) and in gilts unilaterally hysterectomized-ovariectomized (UHOX) before breeding (n=17) to induce intrauterine crowding. Placentas were combined according to the sex of the fetus associated with the placental unit (except at d 30). Placentas were sectioned into middle, inner and polar regions. Placental tissues were incubated, and release of progesterone (P₄) and estrone (E₁) was determined. Fetal survival rate was greater (P< .01, .05, .001 at d 50, 70 and 90, respectively) from intact versus UHOX gilts. Placental length and weight, fetal length and weight, and allantoic fluid volume were greater in intact compared to UHOX gilts. The polar region of the placenta released less P₄ than the other regions at d 50, 70 and 90. Uterine status (P< .005) affected P₄ release only at d 90. Sex of the fetus did not affect placental P₄ release. Region of the placenta affected E₁, release at d 30 (P< .01) and d 50 (P< .06). Uterine status did not affect E₁ release. Sex of the fetus affected (P< .001) E₁ release only at d 50. In the second study, a total of 45 gilts was used. Placentas and endometrium were combined based upon the intrauterine position of the associated fetus. Placental and endometrial tissues were incubated and release of P₄ and E₁ was determined. Uterine status (intact or UHOX) did not affect the variables measured. Intrauterine position affected fetal and placental weights (P< .02 and .01, respectively) at d 40 of gestation. No significant effect of intrauterine position was detected on placental and endometrial P₄ release. At d 100 of gestation, placentas associated with fetuses bordered in utero by fetuses of the same sex released more (P< .01) E₁ compared to placentas associated with fetuses bordered by fetuses of the opposite sex. Estrone release by the endometrium was not significantly affected by intrauterine position. Only trace amounts of testosterone and dehydroepiandrosterone sulfate were measured in the fetal fluids at all days of gestation. Intrauterine position had no effect on P₄, E₁ or androstenedione concentrations in fetal fluids. Collectively, the results indicated a) a differential release of P₄ and E₁ by the three regions of the placenta at certain days of gestation, b) no compensatory increase in placental steroidogenic activity per unit of tissue when total placental mass was reduced, and c) a limited effect of intrauterine position on placental and endometrial steroidogenesis at various days of gestation. / Ph. D.