Final answer:
A bovine fetus the size of a big cat would be in the later stages of the third trimester, roughly estimated at around 250-280 days into the gestation period, nearing the end of its development similar to the 38th week of human pregnancy.
Step-by-step explanation:
When a bovine fetus is the size of a big cat, it is approximately in the late stages of its development. Considering the gestation period of cows is about 280 days, and the size of a big cat could be similar to the size of a full-term bovine fetus, it is likely nearing the end of the pregnancy cycle. By the 38th week, or roughly 266 days, comparing closely to a human gestation period, the fetus in humans is fully developed, and this can be considered roughly analogous to the development of a bovine fetus, although exact correspondences in size and development stages could vary slightly. Thus, while there is not a precise day provided as each fetus can grow at slightly different rates, a bovine fetus the size of a big cat would likely be in the later stages of the third trimester, which could be roughly estimated at around 250 to 280 days into the gestation period.