Most births of single babies occur at 39 weeks. But, the average length of a twin pregnancy is 35 weeks. About 50 percent of twins are born preterm, or earlier than 37 weeks.

"Preemies" tend to weigh less than 5 pounds or 2,500 grams, according to the March of Dimes. This increases their risk for many significant health problems. Some may be temporary, such as jaundice, anemia, and difficulty breathing. Others have lasting effects, such as persistent respiratory problems, mental retardation, cerebral palsy, and vision and hearing loss.

To lower the chance of early births, women who are pregnant with twins typically see their doctors more often. They also are monitored for signs of preterm labor. Their doctors may treat preterm labor with bed rest and medications to stop contractions. Near the 20th week of pregnancy, physicians may recommend regular ultrasounds to monitor the growth rate of the twins to ensure they are growing at about the same rate. During the third trimester, the physician may suggest a non-stress test and/or a biophysical profile (non-stress test with an ultrasound) to assess the well-being of the fetuses.

If you're carrying twins, closely follow your doctor's guidance about diet, exercise, and rest, and avoid harmful substances such as tobacco smoke and alcohol. And, prenatal care is important. Make sure you keep all your doctor's appointments. Learn the symptoms and signs of preterm labor and know what to do if you experience them. Do all you can to lessen your risk of having your babies too soon.