Sleeping in your back

Mr

Hi everyone. Hope you can help? I’ve read so many conflicting things that say you shouldn’t lie on your back as it is dangerous to the baby and to you, but some say it’s not until the 3rd trimester and others say it’s from 17 weeks. My hips are so painful I’m struggling to lie on my side and I’ve always been a back sleeper so finding it difficult to get comfy. Has anyone been told definitively??? Thanks in advance.