Since you just typed, “comfortable pregnancy,” and your post is titled alleviating back pain, I have an idea of what you are looking for, but could you be more specific?
1) First and foremost, does the client have permission from doctor to exercise / stretch and if so, are there any movements specifically contraindicated by their doctor?
2) What phase of pregnancy?
3) Where is the pain? Typically, it’s lower lumbar in the erector spinae team of muscles, but it’s better for me to ask than to assume.