Pregnancy and twinning rates in Thoroughbred mares following the administration of human chorionic gonadotropin (hCG)

To determine the effect of hCG administration to cycling Thoroughbred mares, on pregnancy and twinning rates and the number of serves in the treated cycle.

A retrospective case control approach was conducted involving 2119 mare ovulatory cycles, on 1110 mares over a 7-year period. Data were collected by 1 of the authors during routine stud work at 3 commercial Thoroughbred farms in the Waikato region of New Zealand. The hCG (1500 IU) was administered by intravenous injection to selected mares 24 h before the expected time of breeding. Mares were scanned for pregnancy (singleton or twins) 14 days after the onset of dioestrus or detection of ovulation. Multilevel logistic regression analyses were used to identify the risk factors associated with the outcomes of interest while simultaneously controlling for possible confounding factors.

Treatment with hCG tended to improve the odds of pregnancy (p=0.06), produced a 3-fold increase in the odds of twins (p<0.001), and increased the odds of a mare having a single serve in the treated ovulatory cycle (p=0.036). The first ovulatory cycle of a season in which a mare was bred was associated with a lower odds of pregnancy (p=0.02), and a lower odds of twins (p=0.003), when compared with subsequent cycles. Lactating mares were less likely to be diagnosed with twins (p=0.005), and were more likely to have a single serve (p<0.001), in any one ovulatory cycle than non-lactating mares.

This report supports the role of hCG as an important therapeutic tool in veterinary management of broodmares for optimal reproductive performance. Mares treated with hCG must be managed in the knowledge that they have an increased likelihood of twins.