Also, the cat/cow stretch that Ianita described is a good one for taking tension off the low back. You don’t need to tighten your abs, especially at the end of your pregnancy, to get the stretch through the muscles along the length of the spine.
Here’s a pretty good visual.
The way I cue these poses is to lead each movement from the tailbone. Inhale, lifting the tailbone up, which tilts the pelvis forward, which then encourages the spine to follow, flowing from the low back, to the middle back, to the neck, up to the crown of the head. Feeling is loose and relaxed. Heart comes open and forward slightly, but not with force.
Exhale, drawing the tailbone down, which will tilt the pelvis back, then flow through the spin until the middle back floats up and the crown of the head tilts down. You might be able to feel them, but there is no need to pull in the abs tightly during pregnancy (some variations of this pose, people exhale as much as possible and draw the belly button up to the spine)