It is generally safe to sleep on your back at 9 weeks old, but it is always best to consult with your healthcare provider for personalized advice. Sleeping on your back can help alleviate pelvic pain, as it takes pressure off the area. However, if you are experiencing any complications or have specific medical conditions, your doctor may recommend a different sleeping position. Additionally, using pillows or supports under your knees and lower back can provide extra comfort and support while sleeping on your back. Remember to prioritize your comfort and seek professional guidance when needed.