I think that to take a gravid female from the wild, and be inclined to keep her babies shows a certain level of knowledge in the subject itself. My friend Abby said her friend caught a gravid garter snake and released it and its babies once they were born. To want to care for the snake, nonetheless identify it as gravid, requires a certain level of skill and knownledge in the first place.
Also, I think that the most crucial point to taking gravid snakes is making sure the area you're taking them from can endure the loss of both the ma and her babies. Henceforth, it's everything but responsible to go to an area with only a few garters and swipe the biggest, fattest female you can find. (The exception to this would be at some one's house or on there land, where the snake would either be killed by the home/land owner or meet Mr. Choppy the lawnmower.)