How can a pregnant woman combine the Prophetic Sunnah regarding sleep with doctors' recommendations to sleep on her left side?

It has not been established in the Sunnah that sleeping on the back is the best sleeping posture. Rather, what is established is that sleeping on the right side is the best, because it is from the guidance of the Prophet, peace and blessings be upon him, and it is more beneficial for the heart and aids in performing Qiyam al-Layl (night prayer). As for sleeping on the back, it is among the worst postures due to the health harms it causes. And sleeping on the left side brings health, comfort, and deep sleep, and there is no harm in it if it is beneficial for the fetus's health.