You should stop lying on your back during pregnancy around 20 to 28 weeks, with most advice recommending avoiding back sleeping after 20 weeks. This is because, as the uterus grows, lying on your back can put pressure on major blood vessels like the inferior vena cava and aorta, which can reduce blood flow to your heart and the fetus. This reduced blood flow may cause dizziness, nausea, low birth weight, or increase the risk of stillbirth. The safest and recommended sleeping position after this stage is lying on your side, preferably the left side, to optimize circulation and fetal oxygen supply. It is important to start every sleep on your side, including naps, from the third trimester onward (around 28 weeks), but transitioning away from back sleeping is generally advised starting at 20 weeks of pregnancy.