Influence of initial length of uterus per embryo and gestation stage on prenatal survival, development, and sex ratio in the pig.

The effects of uterine length per embryo and stage of gestation on prenatal survival, development, and sex ratio were determined by systematically restricting embryos to 5, 10, 15, 20, 25, 30, 35, 40, 45, or > 50 cm of initial uterine length per potential embryo and examining the reproductive tracts at d 17, 23, 29, 35, or 41 of gestation. At d 3 the mean length of a uterine horn was 169 +/- 35 cm, with a range of 80 to 255 cm. The mean total number of corpora lutea (CL) was 12.4 +/- 2.34. Ovulation rate tended to be correlated with the length of uterine horns (r = .38; P > .1). Prenatal survival was highly correlated with initial assigned uterine space from 5 to 25 cm/CL (r = .95; P < .05). Prenatal survival was not affected by space (r = .53; P < .1) when space was > 25 cm/CL. In all groups, regardless of space available, some prenatal loss occurred before d 17. In sections with < 25 cm/CL, a second loss took place between d 29 and 35. Before d 35, there was no significant effect of restriction on either fetal length or weight, but after d 35, fetuses from sections with > 25 cm/CL were longer and heavier than those from sections with less space (P < .05). Greater uterine space was occupied by larger fetuses (P < .05). Male fetuses occupied more uterine space than female fetuses did in both crowded and roomy sections (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)