Intrauterine position affects fetal weight and crown-rump length throughout gestation

Abstract Text: To investigate the effect of intrauterine position on fetal growth throughout gestation, data from 64 gilts (n = 784 fetuses) that were slaughtered at assigned days of gestation (d 43, 58, 73, 91, 101, and 108; n=8, 11, 11, 12, 11, and 11, respectively) on a project to evaluate fetal mineral deposition were used. Placental units were removed from the uterus, and position, sex, weight, and crown-rump length (CRL) of each fetus were recorded. Fetuses were classified into 5 categories for absolute intrauterine position: the ovarian end (OE) of the uterine horn, next to the ovarian end (NOE), the middle (M), next to the cervical end (NCE), and the cervical end (CE). Fetuses at the OE and NOE of the uterine horn were heavier (108.6, 109.3, 101.9, 103.6, and 105.0 g for OE, NOE, M, NCE, and CE, respectively; P = 0.06) and longer (12.8, 12.6, 12.2, 12.1, and 12.3 cm; P < 0.01) than those in the M at d 58 of gestation. Fetuses at the OE of the uterine horn were also heavier and longer than those at M and NCE at d 101 (1078.9, 1015.6, 945.5, 890.6, and 956.2 g, and 28.4, 27.4, 26.8, 26.3, and 26.8 cm; P < 0.01) and 108 (1410.3, 1453.4, 1318.0, 1254.1, and 1407.9 g, and 31.6, 31.4, 30.6, 30.1, and 31.1 cm; P < 0.01) of gestation. Fetuses at the CE were intermediate in weight and length. Male fetuses were heavier than female fetuses at d 43 (16.9 vs. 15.9 g), 58 (109.4 vs. 101.1 g), 73 (350.9 vs. 331.7 g), and 108 (1410.6 vs. 1298.6 g) of gestation (P < 0.05) and longer than female fetuses at d 58 (12.5 vs. 12.3 cm; P = 0.06), 73 (18.9 vs. 18.5 cm; P < 0.05), 101 (27.4 vs. 26.8 cm; P = 0.07), and 108 (31.1 vs. 30.5 cm; P < 0.05) of gestation. Fetal weight was highly correlated with CRL at all gestational ages (r = 0.778 to 0.955; P < 0.01). These results indicate that the absolute intrauterine position affects fetal growth such that each end of the uterine horn has heavier fetuses than the middle, and that male pigs grow faster than female pigs even prior to birth.

Keywords: fetal growth, gestation age, intrauterine position