This isn't an extensive article, but recently Pregnancy Weekly reports that stetching may prevent preeclampsia, or pregnancy induced hypertension. It suggests that light stretching releases a special kind of protein called transferrin, which protects against stress on the body.
I usually do a few yoga or pilates stretches before bed to ward off evil leg cramps and generally unwind. After reading so much about relaxin release and how it's important not to overstretch (valid advice but yet another pregnancy risk), it's nice to read about stetching in a positive light.
From what I can see preeclampsia is still much of a common wonder to the medical world and there are no clear cut answers on how to avoid it. But if stetching might stack the odds in your favor of keeping it at bay while allowing your muscles a well earned breather, why not?