Week 13th. When should I start sleeping on the left side and when to buy maternity pillow?

Hope this helps : https://sg.theasianparent.com/safe-sleeping-positions-pregnancy?utm_source=question&utm_medium=recommended

Sleeping on the left is always more comfortable so you can start that already. Same for a maternity pillow too I’d say.

You can already start to sleep using maternity pillow and on the left side which is highly recommended.