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). In crowded sections with assigned space < 25 cm/CL at d 41, males were lighter than females (male 10.17 g, female 11.56 g). In sections with assigned space > 25 cm/CL, males were heavier than females (male 12.91 g, female 12.05 g) (P < .05). Sex of embryos at very early stager, of gestation was determined by using a probe of a DNA segment of the Y-chromosome. When embryos were restricted to < 10 cm initial uterine length, 42% were male (P > .1), and when embryos were restricted to > 15 cm, sex ratio of male:female was 49:5 1.