To improve the detection of ovulation and early diagnosis of pregnancy on dairy farms, a disposable electrochemical immunosensor was developed using indirect competitive immunoassays and screen-printed carbon electrodes. After optimisation of the assay conditions, the linear range was from 0.16 to 50 ng/mL. The limit of detection was 0.16 ng/mL. There was no cross-reactivity with testosterone, 17-β-estradiol and estriol. The intra-assay coefficient of variation was less than 13% and interassay coefficient of variation was < 9%. The percentage recovery was from 95.8% to 115.7%. Stability lasts 4 days at 4 °C. The results of preliminary application trials indicate that the method could have practical potential.