@ZeroTron That's a great question. Many years ago I was reading a study about guys in the military. Couldn't get their wives pregnant despite great health markers. Now it's been awhile but IIRC they needed 15-20 nanograms/deciliter progesterone. The tail motor of sperm doesn't work without progesterone. So without further context, and the joke that's modern lab range of 0, that's my best guess.
Couldn't hurt to use pubmed and search around a bit, use sci-hub.st to try to find full papers.