Laying on your back

Kit • Army Ranger wife 3/31/14, pharm student, 1st time mommy to be 8/8/15 🎀💞
When they say no laying on your back is that for everything?? Lol I'm sorry if that sounds stupid but  that term is just so vague. Lol what is the problem with laying on your back. I thought it was don't lay on your stomach while pregnant? 😣😣😣😣 
Ps I'm laying on my left side as I write this. Lol