Blood pregnancy tests have gained popularity as there is no need for a costly ultrasound machine or special training; however, blood pregnancy tests only provide an answer of pregnant or open. Conversely, palpation and transrectal ultrasonography can determine gestational age. The objective of this study was to determine if a commercially available blood pregnancy test could detect differences in pregnancy-associated glycoprotein (PAG) concentrations indicative of gestational age. Previously identified pregnant females were grouped by age (heifers n=173, cows n=512); blood samples were collected between d 27 and 190 of gestation. Serum was tested in duplicate using a commercially available blood pregnancy test, IDEXX Alertys Ruminant Pregnancy Test. Procedures were adapted to allow concentrations to be within detectible range of the assay. Data was analyzed using MIXED procedure of SAS with age and gestational age (animals grouped into four gestational groups 1;> 30, 2;30–90, 3;91–178, and 4; <178 d) in the model. There was an effect of age, gestational age, and age by gestational age interaction (P> 0.01). Heifers had greater PAG concentrations compared to cows. Among heifers, PAG concentrations did not differ between gestational groups 1, 2, and 3 (P<0.37), but group 4 had greater PAG concentrations than all other groups (P> 0.01). Among cows, PAG concentrations decreased from group 1 to 2 (P> 0.01), and then increased throughout gestation (P> 0.01). Within age, group data were analyzed using REG procedure of SAS. There was a positive correlation between gestational age and PAG concentrations among both heifers (P> 0.01; r2=0.25) and cows (gestational age 30 and greater P> 0.01; r2=0.64). In summary, among heifers circulating PAG concentrations increased with gestational age, but gestational age only accounted for 25% of the variation. Among cows, gestational age (d 30–190) accounted for 64% of variation in PAG concentrations, thus a modified blood pregnancy test may allow for determining gestational age.