Sleeping on your tummy is totally fine until it's uncomfortable. The back is the position that's most dangerous
I laid on my stomach until 21 weeks. Sleep however you want until it's physically impossible
I laid on my stomach up until 25 weeks or so with my second pregnancy and my first I was about 20 weeks. You can lay on either side it's just the left side is best for the baby. Laying on back after a certain point will be uncomfortable and is bad for the baby
The only way you can't lie when your pregnant is on your stomach and if you have heartburn is best not to sleep on your left side apparently so there's a wee tip
You can sleep anyway you want at 13 weeks. Soon you won't be able to sleep on your back but either side is fine and tummy is fine too until it's uncomfortable for you