Here's an interesting slap down of the officers from the opinion:
Even if the unconstitutionality of the alleged conduct was not clearly established through case law, the violation was so obvious “a reasonable state actor would know that his actions violated the Constitution.” Siebert, 256 F.3d at 654. The likely reason why there are so few analogous cases (Williams is the only factually similar case this Court has found), is that police officers know better than to detain, arrest, and jail someone in these sorts of circumstances.
In other words, cases with these particular circumstances don't come up often because most cops are smart enough to know that you don't detain a woman in labor.