Seasonal differences in progesterone (P4) concentrations and pregnancies per AI (P/AI) have been demonstrated in lactating dairy cows. Moreover, an association between uterine size and P/AI has been recently reported. Our objective was to evaluate seasonal differences in uterine size, circulating P4, and fertility in primiparous and multiparous dairy cows. Lactating Holstein cows (n = 704) were synchronized to receive timed AI (TAI) on d 81 ± 3 of lactation by using the Double-Ovsynch protocol (GnRH-7d-PGF-3d-GnRH-7d-GnRH-7d-PGF-56h-GnRH-16h-TAI). At the time of the last injection of PGF, uterine diameter was determined at the greater curvature using ultrasound, uterine length was determined by rectal palpation, and uterine volume was calculated. Circulating P4 at final PGF and GnRH were evaluated to assure synchronization of all cows used in the final analysis (n = 616; primiparous (P), n = 289; multiparous (M), n = 327). At the same time, respiratory rate (number of breaths per minute, BPM) and body temperature was determined in all cows. Fischer's exact test and t-test were used to analyze categorical and continuous variables respectively, and logistic regression analysis was used to calculate probabilities of P/AI related to uterine volume. There was a clear delineation in average respiratory rate (40 to 28 BPM) indicating a change of season (hot (H) n = 226 (112 P, 114 M), cool (C) n = 390 (177 P, 213 M). Uterine volume was greater in H than C season for all cows (150.3 ± 2.9 vs. 131.2 ± 1.9 mm³, H vs. C, P < 0.0001), and for P (134.6 ± 4.1 vs. 122.0 ± 2.8, P = 0.009) or M (165.7 ± 4.2 vs. 138.8 ± 2.6, P < 0.0001) cows. Circulating P4 was lower during H than C season in P (7.5 ± 0.3 vs. 8.1 ± 0.2, P = 0.08) and M (6.0 ± 0.3 vs. 6.8 ± 0.2, P = 0.02) cows. The P/AI was reduced in H vs. C for all cows (38.5% vs. 47.7%, P = 0.02), and for P (41.1% vs. 56.5%, P = 0.03) but not for M (36.0% vs. 40.4%, P = 0.21) cows. Logistic regression analyses indicated a relationship between uterine volume and P/AI for M cows in H (P = 0.03) and C (P = 0.03) seasons but for P cows only in C season (P = 0.02) with greater uterine volume associated with reduced P/AI. Thus, heat stress is associated with an increase in uterine size. Greater uterine size partially explains differences in fertility within parities and seasons.