Intrauterine position and sex of adjacent fetuses in litter bearing species have been implicated in physiological and behavioral differences in males and females. Our objective was to establish growth curves for fetal and placental weight gain as influenced by sex status of flanking fetuses under crowded uterine conditions. Gilts were subjected to unilateral-hysterectomy-ovariectomy surgery at 160 d of age and mated at approximately 280 d of age. Gilts were assigned to be harvested at d 45, 65, 85, or 105 of gestation. A total of 297 pregnancies were evaluated in 4 contemporary groups. Position in the uterus relative to the cervix, fetal weight, and placental weight were recorded at harvest. Data were coded to test when each fetus was adjacent to 0, 1, or 2 opposite sex fetuses after removal of the first and last fetuses in each litter to remove bias of ends not being flanked on both sides. After this editing a total of 343, 651, and 303 fetuses were included in the analyses for 0, 1, or 2 opposite sex flanking fetuses, respectively. Nonlinear analyses fitted a logistic function (Wt = Ae^tk) to the fetal and placental weight data to establish unique growth curves for each flanking sex status code. Two parameters were estimated representing the initial weight at day zero (A) and a slope parameter (k) with respect to time of gestation (t). Gauss-Newton method was used in the NLIN procedure to generate solutions. A 4 df F-test was calculated to compare the ability of code-specific functions to account for more variation than the pooled function. The null hypothesis of a single pooled function to describe fetal weight was rejected (F = 2.49; P < 0.05) but not for placental weight (F = 0.80; P > 0.1). Code specific growth curves indicated placental weight separated only in the latter stage (d 105) of gestation whereas fetal weights separated as early as d 65 of gestation. Fetal growth development in pigs is influenced by sex status of adjacent fetuses, with increased growth by those surrounded by the same sex, and could be a potential source of variation in behavioral and reproductive differences later in life.