I believe that HPPA has, as part of its privacy stipulations, that employers cannot ask or otherwise inquire about personal medical information. I believe pregnancy (and the date of conception) would certainly fall under that umbrella.
That is an interesting question. Is the act of conception (or, rather, the act resulting in conception) medical information? I guess the act has medical implications, but so does drinking coffee or eating a cheeseburger, if you define it broadly enough.
What if an employer made public the fact that an employee went skydiving, the result of which was death (or a broken ankle, etc.)? I don’t think the act of skydiving is medical information, but then again, I like to think the law is logical.