The objective of this study was to determine the effect of estrous expression, detected by an automated activity monitor, on progesterone concentrations and its association with fertility. Estrous expression (n = 665 events) from 290 lactating Holstein cows were recorded. Cows were monitored continuously by an activity monitoring system attached to the cow’s neck. The threshold level to be considered an alert on the monitor was set at SCR’s index of 35 or approximately an 80% increase in physical activity. The peak of activity was determined for each event and classified as either high (index ≥80) or low (index 35–79) using the median. Blood sampling was harvested upon detection of estrus (0 d) and analyzed for P4 and E2. A subgroup of 171 events were also sampled at 7, 14, and 21 d post-AI and analyzed for P4. Animals had their ovaries scanned by ultrasound at each collection. Pregnancy was performed at 31 ± 3 d post-AI. Data were analyzed by ANOVA and logistic regression using proc MIXED and GLIMMIX of SAS. Animals that had a higher increase in physical activity had higher P/AI when compared with animals with lower activity (30.7% [48/380] vs 23.6% [64/259]; P < 0.05). Animals with low activity had higher concentration of P4 and lower concentrations of E2 upon detection when compared with animals with high activity (1.0 ± 0.2 ng/mL vs. 0.3 ± 0.2 ng/mL; P < 0.01 and 4.6 ± 0.3 pg/mL vs. 6.7 ± 0.2 pg/mL; P < 0.01). Follicle diameter did not differ between animals with high or low peak of activity (P = 0.41). Higher concentrations of P4 at 7 d, 14 d and 21 d post-AI were found in animals that had high estrous expression detected at the time of alert (7 d – High: 3.4 ± 0.2 pg/mL vs. Low: 2.7 ± 0.2 pg/mL; P < 0.05; 14 d – High: 4.9 ± 0.4 pg/mL vs. Low: 2.9 ± 0.4 pg/mL; P < 0.01 and 21 d – High: 6.8 ± 0.3 pg/mL vs. Low; 5.4 ± 0.3 pg/mL; P < 0.01). Size of corpus luteum on 7, 14, and at 21 d post-AI did not differ between animals that expressed high or low activity. In conclusion, animals that had higher expression of estrus had greater P/AI and a P4 profile at and post-AI normally associated with improved early embryonic development.