Serologic testing for pregnancy in bitches is complicated because of female dogs' normal endocrine physiology. Bitches do not produce a pregnancy-specific hormone similar to human chorionic gonadotropin, the compound assayed in early pregnancy tests for women. In addition, progesterone is not a useful indicator of pregnancy because all bitches secrete progesterone for two months after heat, regardless of whether they were bred or not.

However, serum relaxin concentrations are increased in pregnant bitches three or four weeks after breeding. A test kit for canine relaxin is available. The manufacturer recommends that if the results of the relaxin test are negative at three weeks after breeding, the test should be repeated one week later. Because relaxin concentrations do not fall immediately after pregnancy loss, false positive results may be seen for an undetermined amount of time after spontaneous pregnancy loss.¹ In pregnant bitches, the rate of increase in relaxin concentration is not correlated with litter size.²

Because the relaxin test does not confirm pregnancy any earlier than abdominal ultrasonography nor does it provide information about fetal viability or litter size, abdominal ultrasonography is my preferred method for diagnosing pregnancy in bitches. However, the relaxin test may be useful in practices that do not have convenient access to ultrasonography or when working with canids that cannot be handled regularly.

Margaret V. Root Kustritz, DVM, PhD, DACT

REFERENCES

1. Carlson DA, Gese EM. Relaxin as a diagnostic tool for pregnancy in the coyote (Canis latrans). Anim Reprod Sci 2006;e-publication.

2. Einspanier A, Bunck C, Salpigtidou P, et al. Relaxin: an important indicator of canine pregnancy [German]. Dtsch Tierarztl Wochenschr 2002;109:8-12.