Relationships of light and heavy fetuses to uterine position, placental weight, gestational age, and fetal cholesterol concentrations.

With emphasis on increasing litter size to improve productivity and profitability, lightweight pigs born will increase in number as litter size increases and may be problematic due to reduced neonatal survival of lightweight pigs. To study developmental aspects of lightweight fetuses, fetal and placental weights were evaluated at three stages of pregnancy (30, 70, and 104 d) from white composite gilts (n = 88, 123, and 135, respectively) in relation to uterine position of light and heavy fetuses. Fetal cholesterol concentrations were analyzed at the beginning (d 70) and end (d 104) of the in utero growth phase (last third of gestation). At 30 d of gestation, no differences were noted between fetal weight and position within the uterine horn, but at 70 d and 104 d of pregnancy, heavy fetuses were found at the tubal ends and light fetuses at the cervical ends of the uterus. Using the criteria of +/- 1 SD or +/- 2 SD of the mean and lower and upper 10 or 20% of fetal weights for each gestational age, light and heavy fetuses in conjunction with their placentas were different from population means at all stages of gestation. Cholesterol concentrations were lower in light fetuses (d 104) and increased in heavy fetuses. In 28 litters (d 104) selected for extremes in within-litter fetal weights, concentrations of IGF-I increased with fetal weight (P < .01), which suggests that growth and development of lightweight pigs may be limited by critical endocrine components.