Sleeping on your back while pregnant

Maria ā€¢ I`m happily married, & I am a new mommy to a beautiful baby boy & with my new little girl. My life is now complete šŸ’ššŸ’œ
I am 27 & 2 days pregnant and when I go to bed I sometimes lay on my back to sleep. But everyone is telling me that it's bad laying on your back when you are pregnant. I was wondering why is it bad? I am a first time mom to be, I don't want to sleep on my back if it hurts my baby. and when I ask why is it bad, nobody knows why they just tell me that it is.Ā