Twins often are born during this time period. If they are well grown, they generally spend a week or two in the hospital and are unlikely to experience any long-term serious effects of their early birth. Quoted from "When you're expecting Twins, Triplets, or Quads" by Dr. Barbara Luke and tamara Eberlein.
My triplets were born at 33 1/2 weeks. Victoria weighed 3 lbs 14 ounces, Justin weighed 5 pounds 3 ounces and Naomi weighed 3 pounds. Justin was in the NICU for 14 days, Victoria 17 days in NICU and Naomi 24 days in NICU. Luckily they do not have any long term serious medical effects. They are, however, delayed in speech and receiving speech therapy to help them to catch up to other children their age.