Pregnancy Sex Foreplay Issue?

I heard that breast pumping before the birth can cause premature labor. Well, what about my breasts being sucked on? When my husband and I have sex, we both love for him to suck on my boobs as a form of foreplay. Now that I'm getting into the second half of my pregnancy I worry about it stimulating premature labor. At what point should we stop if it's a hazard? Any helpful suggestions or comments? I am a FTM.