Generally, as long as you are being perfused with blood (your head and your kidneys are the easiest ones to monitor) then the baby is also being perfused. Monitor your urine output - if it stays normal for you then it is fine to have a lowish BP. You will notice a dizzy feeling / head spins if your BP is too low for you.

Where I work our priority is the mother. If she is ok and well perfused then so is the baby.