A lovely calming breath for pregnancy is, what I call, the sufi breath.
Breath in to the count of 4 breath out for 4 then
in 4 out 5
in 4 out 6
in 4 out 7
in 4 out 8
Repeat anything from 2 to 20 times. You will find your body relaxing and releasing tension and stress and your mind will have something to focus on with the counting so will not wander off to the grocery list or how you're going to decorate the nursery.