Relaxin as a hormonal aid to evaluate pregnancy and pregnancy loss in bottlenose dolphins (Tursiops truncatus)

This study was conducted to critically evaluate weekly and monthly circulating concentrations of immunoreactive relaxin throughout pregnancies that resulted in live births, stillbirths, and abortions in aquarium-based bottlenose dolphins. A relaxin RIA was used to analyze serum collected during 74 pregnancies involving 41 dolphins and 8 estrous cycles as well as 8 non-pregnant dolphins. Pregnancies resulted in live births (n = 60), stillbirths (n = 7), or abortions (n = 7). Relative to parturition (Month 0), monthly changes (P < 0.0001) in relaxin was indicated by relatively low concentrations during early pregnancy (Months −12 to −9) which subsequently increased (P < 0.05) during mid- (Months −8 to −5) to late (Months −4 to −1) pregnancy; relaxin was highest (P < 0.05) at the time of parturition. Post-parturition (Month 1), concentrations decreased (P < 0.05). During the first 4 weeks post-ovulation, relaxin concentrations were not different between pregnant and non-pregnant dolphins (status-by-week interaction, P = 0.59). Status-by-month interaction (P < 0.0002) involving different pregnancy outcomes was due, impart, to an increase in relaxin during early pregnancy (P < 0.05) that was comparable among dolphins with live births, stillbirths, and abortions except concentrations were lower (P < 0.05; 52%) at mid-pregnancy in association with pregnancy loss. Thereafter, concentrations increased (P < 0.05) during late pregnancy in dolphins with stillbirths but not in dolphins with abortions. In conclusion, this study provided new information on the pregnancy-specific nature of relaxin, critical evaluation of the fundamental characteristics of relaxin during pregnancy and pregnancy loss, and clarification on the strengths and limitations of relaxin as a diagnostic aid to determine pregnancy status and assess maternal-fetal health in bottlenose dolphins.