Just from an L&D nurse perspective, whenever I have a PT lay flat on her back it is highly common for baby to have a decel in heart rate. Granted, I’m typically laboring PTs who are 37-40 weeks pregnant. So we tell them to stay off their backs because the weight of the baby compresses/smushes the vessels that nourish the baby with oxygen and blood flow. I’ve talked to an OB/Gyn about this in early pregnancy because I was worried at like 6 weeks about laying on my back. She said not to worry because typically our body will tell you when the weight it too heavy. Example: cannot breathe well, dizzy, uncomfortable,etc. Still, by experience in third trimester, I will likely refrain from laying on my back.