When I was about 3 months pregnant I knew I was going to want something to help support my belly and my knees more. I have slept with a pillow between my knees for a long time now, but I wanted something that was going to do both, not one pillow for my knees and one for my belly.
I ended up getting this Boppy pillow because I had heard good things about them from my friends, and they had good reviews online. I didn't want the fully body Boppy pillow because I sleep with a special water pillow and didn't want to give that up. Instead, I got the half size one called the Cuddle pillow.
I never took into consideration that because I sleep on a Sleep Number bed (air chambers), I might not need the extra support under my belly, at least not at first. So when I was 3 months pregnant, it was actually uncomfortable on my belly. It was pushing my "swollen tummy" up more than supporting it as it was. It felt better to just let my belly lay on the air mattress.
I didn't really start using the pillow until I was just over 5 months pregnant. One thing I didn't like at first was that the pillow was so bulky. But I flipped the pillow around and used the part that should go under your belly between my knees. This felt much better. It lifted my leg and just gave enough support under my belly. The more I use it though, the flatter it is getting. It still feels ok under my belly, but the part between my knees is getting too flat.
I think this pillow would do someone sleeping on a regular spring mattress much more. Springs don't give and conform to your body like an air mattress does. When I lay on the floor during the day I do need that extra support and the pillow works much better.