I'm not aware of the progesterone others have recommeneded, so that may be a good thing. I do know that proper nutrition can be key to a healthy pregnancy. Dr Tom Brewer made it his life's work to study toxemia, and found it was completely related to malnutrition. As a result, he implented a diet that consisted of fruits, veggies, 80-100 gr of protein a day, salt to taste, plenty of water, grains, etc, and had a ZERO rate of toxemia. Now, I know that is not what you are specifically worried about, but it certainly can't hurt! I know that poor nutriton can also cause premature rupture of membranes and a series of other things. Go to blueribbonbaby.org or bradleybirth.com and read up on his diet. You can even get spreadsheet to help you keep track. IT's a good life habit to get into anyway. And good nutrition is one thing you CAN control in your pregnancy which is nice.
Check out his site and see if he has anything to say on preterm labor, too I just never looked specifically for that.
Best of luck and blessings
A.