This comparative analysis evaluated endocrine profiles and gestation length data of captive pregnant black rhinoceros (Diceros bicornis), white rhinoceros (Ceratotherium simum), and greater one-horned (GOH) rhinoceros (Rhinoceros unicornis). Hormone profiles were collected over three decades as part of pregnancy diagnoses. After the third month of gestation, the luteo-placental shift in progesterone production in pregnant rhinoceroses causes a significant increase in the concentration of faecal progesterone metabolites. We defined a laboratory-specific value of 1000 ng/g faeces as a threshold for incipient feto-placental progesterone production. Using this value allowed a comparison between species and revealed significant individual differences within a species. The mean ± SEM gestation days for reaching the 1000 ng/g faeces threshold were 89.5 ± 2.9 (range 56-138 days; n = 39) in black, 96.0 ± 2.6 (58-138; n = 39) in white, and 117.8 ± 5.3 (74-173; n = 19) in GOH rhinoceroses. For the calculations of gestation length, we complemented our results from three decades of reproductive monitoring with data from the literature, resulting in about 70 values for each species. Gestation length in the black, the white and the GOH rhinoceros was 460.6 ± 1.5 (range: 436 - 486), 503.8 ± 1.3 (range: 480 - 525) and 480.5 ± 1.1 (range: 453 - 505) days, respectively. Daylight length significantly affected gestation length, while the sex of offspring had no effect. On average, pregnancies with parturitions in spring and summer were one week shorter than those in autumn and winter. Although rhinoceroses are non-seasonal breeders, most parturitions in captivity occur in autumn and winter. We also analysed preconception endocrine profiles in the white rhinoceros. Conceptions in this species occurred after oestrous cycles of approximately 35 days (n = 18), 70 days (n = 3), 15 days (n = 1), after periods of ovarian inactivity (n = 5), and during a foal heat within one month after stillbirth parturition (n = 1). In conclusion, this study provides a comprehensive overview of gestational parameters in three rhinoceros species.