Fetal and Placental Growth during the First 90 Days of Gestation in Beef Heifers, and Effects of Maternal Nutrition

Early gestation is the critical period for successful establishment of pregnancy and placentation. Numerous factors are associated with altered conceptus growth and pregnancy loss, including maternal nutritional status. To document the patterns of fetal and placental growth, in Study 1 crossbred Angus heifers (n = 16) were estrous synchronized, bred via AI, and ovariohysterectomized (OVHX) at 5-day intervals from d 25 to 90 of gestation. To examine the effects of maternal nutrition, in Study 2 crossbred Angus heifers (n = 27) were synchronized and assigned to 2 dietary intakes at AI: CON, 100% of requirements; RES, 60% of CON; and were OVHX on d 16, 34, or 50 post-AI. The gravid uteri were cleaned of adnexa, opened with scissors, blunt dissected, and photographed. A ruler was included in each photo to indicate sizes. Fetal straight crown-rump (C-R) lengths (Study 1, all days; Study 2, d34 and 50) or embryonic disc long axis length (Study 2, d16) and surface area of the two to three largest placentomes (Study 1) were determined by image analysis of the photographs. For Study 1, fetal C-R length increased from 2.2 mm on d25 to 174.3 mm on d90, as described by a quadratic equation: y = 0.0223x² + 0.1018x - 16.077 (R² = 0.996, P<0.01), where y = fetal C-R length in mm and x = day of gestation. Placentome surface area increased from 91.4 mm² on d25 to 2277.9 mm² on d90, as described by an exponential equation: y = 14.999e^0.0546x (R² = 0.903, P<0.01), where y = placentome surface area in mm², e = the base of the natural log, and x = day of gestation. For Study 2, on d16 embryonic disc long axis lengths for CON vs. RES embryos were 1.084 vs. 0.587 mm (SEM = 0.396, P = 0.198), and on d34 and 50 C-R lengths for CON vs. RES were 1.20 and 4.15 vs. 1.17 and 3.79 cm (SEM = 0.154 [P = 0.46] and 0.114 [P = 0.12] for d34 and 50, respectively). These observations will serve as a useful ‘baseline’ for future studies of factors that can affect pregnancy establishment and fetal and placental growth. Additionally, we successfully applied our ovariohysterectomy method to study fetal and placental growth from d16 to 90 of gestation. Supported by the North Dakota State Board of Agricultural Research and Education, a USDA-NIFA-AFRI grant (2016-67016-24942), and additional local funds.