The effects of P4 concentrations during follicle growth on fertility responses in dairy cows were evaluated. In study 1, pregnancy per artificial insemination (P/AI) increased (P < 0.01) in cows initiating the timed AI program in diestrus compared with anovular and cyclic cows without a corpus luteum (CL; 36.9, 25.1, 28.1%); however, pregnancy loss was smaller (P = 0.08) in cyclic cows without CL, followed by cows in diestrus, and anovular cows (10.0, 13.5, 15.0%). In study 2, concepti on d 15 after AI were longer (P < 0.05) in anovular compared with cyclic cows (47.8 ± 8.8 vs. 9.4 ± 5.8 mm). Concepti transcriptome analysis depicted 417 genes differentially expressed in response to cyclic status, mostly involved with embryonic development. In study 3, cyclic cows were induced to ovulate the dominant follicle from the first wave (FW; P4 = 1.2 ± 0.3 ng/mL), first wave supplemented with P4 (FWP4; P4 = 4.3 ± 0.3 ng/mL), or second wave (SW; P4 = 5.4 ± 0.2 ng/mL). Cows in FW had (P < 0.05) larger ovulatory follicles (19.6 ± 0.6, 15.6 ± 0.6, 15.2 ± 0.5 mm), greater estradiol concentrations during proestrus (8.0 ± 0.6, 7.0 ± 0.7, 5.9 ± 0.6 pg/mL), and a faster P4 rise after AI. Concepti on d 17 after AI were longer (P = 0.05) for FW compared with FWP4 and SW (16.6 ± 2.3, 9.8 ± 2.2, 9.6 ± 2.0 cm); however, major changes on transcriptome were not observed. Treatment did not affect any biological process in the endometrium on d 17 of gestation. In studies 4 and 5, a single ultrasound evaluation identified a low-fertility cohort based on the absence of CL at the initiation of the timed AI program. Increasing P4 concentrations to at least 2.0 ng/mL improved (P ≤ 0.01) P/AI in cows lacking CL similar to those in diestrus in the 5-d timed AI (No CL = 28.6, 2CIDR = 43.7, Diestrus = 47.3%) and the Ovsynch protocols (No CL = 28.9, 2CIDR = 37.2, Diestrus = 33.9%). Low concentrations of P4 during the development of the ovulatory follicle impair fertility and a minimum of 2.0 ng/mL seems to be needed to restore P/AI. Lack of differences on conceptus and endometrium on d 17 suggest that effects occur early in gestation.