By far, the most common concern I hear from pregnant women is that they have read they should sleep on their left side (not just their side, their left side), but they awaken in a panic in the middle of the night when they find themselves on their backs.
I realize the books all say the same thing, but we have to use some common sense here. Mother nature isn’t always kind, but there is usually some rhyme or reason to the design. I ask my patients what people did before they had all those books to read.
The truth is that some women can’t lie on their backs without feeling lightheaded or dizzy. This is because, in their particular case, venous return to the heart is decreased by the weight of the pregnant uterus and therefore blood flow out of the heart to the vital organs (like the brain and the uterus) is also decreased. These women know instinctively not to lie on their backs because it is very uncomfortable for them in that position.
Often in active labor, we see that when a woman lies flat on her back while she is having strong contractions, her baby may respond with heart rate changes which we can fix by turning her onto one side or the other. But active labor is a special circumstance and women are not generally comfortable flat on their backs in labor either.
So sleep however you are comfortable while you are pregnant, you will be awakened in the night soon enough for a good reason!