Our objective was to describe the mechanism and timing of pregnancy losses (PL) in cows after the first timed artificial insemination (TAI). A total of 136 cows that experienced PL were included in a matched case-control study. Cases were obtained from 3,164 cows from 4 dairy farms enrolled in a prospective cohort study. Cows with pregnancy-specific protein B (PSPB) ≥ 0.3 ng/mL on d 25 after TAI and were open based on transrectal ultrasonography 27 to 32 d (PG1) were defined as early PL (n = 49 cows). Cows that were pregnant at PG1 but open at subsequent evaluations were considered later PL (n = 87 cows). Cows that remained pregnant during the study period (from TAI to 90 d after TAI) were defined as Controls (n = 266 cows). Two Control cows were matched to each PL cow based on days in milk and parity. Progesterone (P4) and PSPB were measured weekly from 10 d before TAI until a cow was diagnosed open or remained pregnant and reached 90 d after TAI. Week of PL and cause of PL (embryonic death [ED] or corpus luteum regression [CLR]) was determined from weekly PSPB using a cutoff of the lowest PSPB concentrations of Controls (from 25 to 88 d after TAI) and weekly P4 concentrations. Data were analyzed by t-test, chi-squared and linear regression. For early PL, 30.6% (15/49), 16.3% (8/23) and 53.1% (26/49) were due to ED, CLR and undefined causes, respectively. For cows with later PL, 37.9% (33/87), 48.3% (42/87), and 13.8% (12/87) were due to ED, CLR and undefined causes (P = 0.04). Mean average days for occurrence of PL based on PSPB concentration cutoff for ED was 48.6 (ranging from 28 to 88) and for CLR was 45.3 (ranging from 24 to 86) (P = 0.29). For early PL, P4 concentrations on d 25 were 5.8 ± 3.4 ng/mL and on d 32 were 6.6 ± 2.3 ng/mL (P > 0.05). PSPB concentrations on d 25 were 0.6 ± 0.6 ng/mL and on d 32 were 1.9 ± 1.5 ng/mL (P < 0.05). In conclusion, the occurrence of pregnancy loss due to ED occurred with more frequency in early PL near 32 d, whereas later PL were more likely due to CLR. Supported by AFRI Competitive Grant no. 2010–85122–20612.