It is generally recommended to stop lying flat on your back after around 20 weeks of pregnancy. At this point, the growing uterus can put pressure on major blood vessels like the inferior vena cava, which can reduce blood flow to the heart and fetus, potentially causing symptoms such as dizziness and affecting fetal growth. Many experts advise switching to side sleeping, especially on the left side, for optimal circulation and fetal health. However, some studies suggest back sleeping might still be safe until about 28 to 30 weeks, but the overall consensus favors avoiding it in the second half of pregnancy to reduce risks such as stillbirth and low birth weight. In summary:

• Safe to lie on your back generally during the first trimester.
• Avoid back lying starting around 20 weeks (start of the second trimester), as uterus pressure impacts blood flow.
• Side sleeping, especially left side, is recommended for better circulation.
• Some findings say back sleeping may be safe until 28 to 30 weeks, but safer to avoid.
• If lying on your back causes dizziness or discomfort, that's a sign to stop immediately.

Overall, most healthcare providers recommend stopping lying flat on your back by 20 weeks into pregnancy and favor side sleeping for the remainder of the pregnancy for best outcomes.