Pregnancy in a heifer after bilateral ovariectomy

A 24-month-old Nelore heifer became naturally pregnant after undergoing a bilateral ovariectomy. This unexpected event occurred after an experiment that was conducted to estimate the preantral follicle populations in calves’ ovaries. Fourteen female calves aged 8 months were bilaterally ovariectomized by laparotomy. The procedures were performed in a surgical suite under sedation and a paravertebral block. Both ovaries of each calf were carefully collected, identified, measured, weighted, photographed and processed for histology and follicle counting. Through all steps the ovaries integrity was assessed for visual and microscopy criteria. After those procedures, the calves were housed at an experimental farm with some bulls. Sixteen months later, the heifers were used in a study using intravaginal progesterone devices and evaluation of hormonal levels. Surprisingly, one heifer showed 6.11 ng/mL of serum progesterone (radioimmunoassay, Coat-a-Count) before the insertion of the intravaginal device. A transrectal ultrasound examination revealed an “ovary-like” structure, and a pregnancy was confirmed. The pregnancy progressed uneventfully, and the heifer delivered a healthy male calf. After weaning, the heifer was slaughtered; the reproductive tract was collected, photographed and evaluated. The presumptive ovary was histologically assessed. The parenchyma presented an unusual pattern, with predominance of connective tissue, with a low number of follicles at different stages, along with a corpus luteum. We describe this intriguing event to discuss possible explanations for this fact.