What is the reason relaxin is measured in commercially available pregnancy tests for dogs?

Relaxin is a hormone that plays a crucial role during pregnancy, particularly in the later stages as the body prepares for parturition, or giving birth. In dogs, relaxin is produced by the placenta and is specifically involved in the physiological changes that facilitate labor. It helps to relax the pelvic ligaments, which allows for the birth canal to widen and make the passage of the puppies easier during delivery.

Therefore, measuring relaxin in pregnancy tests is significant because it serves as an indicator that the body is readying itself for the birthing process. The presence of relaxin signifies that the pregnancy is progressing, and the dog is approaching the time when she will give birth.

The other options do not accurately reflect the primary role of relaxin during canid gestation. Relaxin is not a fetal hormone nor does it directly indicate an increase in blood volume or prepare the body for lactation, although it might have some indirect effects related to these processes. Its main function relevant to pregnancy testing in dogs is related to preparing the body for parturition.