I've had this exact problem since I was only about one month pregnant. I honestly don't think it has anything to do with the uterus pressing on anything as my uterus was not much bigger at that time. I'm also 23 weeks along now. The numbness happens when standing too long, sitting too long (~30 minutes or more), or laying on my back. If standing I kick my leg out until the numbness goes away. Otherwise I slap and rub it, which is rather effective. Sure hope this is only pregnancy related! I've been thinking of seeing a chiropractor for it. Annoying, but it doesn't hurt, so I keep putting it off. Interesting to see others with the same exact problem.