Experts recommend that pregnant women in their second and third trimesters must avoid sleeping on their backs. Why? The sleeping position of the rear holds all the weight of the increasing uterus and child on your back, your intestines and your vena cava, the main vein that takes blood home to the core from your reduced abdomen. This stress can intensify backaches and hemorrhoids, lower digestion rate, interact with the flow of blood, and trigger potentially hypotension (causing lowering of blood flow), that could render you dizzy. Less than optimal circulation can also decrease fetal blood flow and decrease the oxygen and nutrients that your unborn child gets.
In conclusion: it’s not insecure to be on your back sometimes, but to be on your back for weeks and months is problematic.