Blood sampling and progesterone assay
Blood samples were collected by a puncture of the median coccygeal vein or artery using evacuated tubes, 7 d after TAI. Samples were refrigerated (4oC) for approximately 4 h, and were then centrifuged (3000 X g for 15 min) and stored at –20 °C. Serum progesterone (P4) concentrations were determined (in unextractedsera) using an antibody-coated-tube RIA kit (Coat-A-Count®, Diagnostic Products Corporation, Los Ange-les, CA, USA). The intra-assay coefficient of variation was 2.6%, and assay sensitivity was 0.006 ng/mL.