Delayed embryonic attachment, circulating pregnancy-associated glycoproteins, and pregnancy loss in heifer embryo recipients.

This study investigated pregnancy loss (PL) in embryo recipients based on pregnancy-associated glycoproteins (PAGs) and impact of P4 supplementation on pregnancy per embryo transfer (P/ET) and PL. In experiment 1, 1003 fresh IVF embryos were transferred into Holstein heifers. Overall P/ET was 42.3 % at 28d (424/1003), 39.1 % at 33d (392/1003), 37.0 % at 47d (371/1003), and 35.5 % at 63d (356/1003). Overall PL was 16.5 % from 28 to 63d, 7.5 % 28-33d, 5.4 % 33-47d, and 4.0 % 47-63d. Using an observational retrospective cohort experimental design, each PL heifer was matched to a non-pregnant and pregnant heifer. Embryonic attachment (Eatt) at 21d (PAGs>0.6 ng/mL) was similar for Pregnant (80 %), PL 33-47d (76.2 %), and PL 47-63d (80 %), but lower for PL 28-33d (50 %). The PAGs at 28d were lower for PL 28-33d vs. Pregnant (10.1 vs. 15.3) and tended lower for PL 33-47d vs. Preg (12.4 vs. 15.9). In experiment 2, Holstein heifers received ET (n = 540) and were randomized to P4 supplementation (PlusP4) from 14 to 19d or untreated (Control). PlusP4 was greater than Control for Eatt at 21d (64 vs. 55 %) and for P/ET at 28d (52 vs. 39 %). Heifers were divided into terciles for PAGs at 21d. Upper PAGs tercile had greatest P/ET (81 %), medium PAGs tercile was intermediate with PlusP4 greater than control (58.5 % vs. 38.6 %), and lowest PAGs tercile was lowest P/ET with PlusP4 greater than Control (12.2 % vs. 2.3 %). Thus, embryo recipients with suboptimal placentation, indicated by PAGs (at d21 or d28) had greater PL d28-33 and d33-47. Also, early P4 supplementation (d14-19) increased P/ET in these heifers.