One of the most common risks of a multiple pregnancy is preterm birth. On average, most twin pregnancies last 35 weeks, while pregnancies with triplets last only 33 weeks. Pregnancies with quadruplets last only 29 weeks on average. Low-birth weight (less than 5.5 pounds, or 2.5 kilograms) due to preterm birth or poor fetal development is also very common in multiple pregnancies. Babies weighing less than 3.34 pounds (1.5 kilograms) at birth are at a greater risk for lasting disabilities, including mental retardation, cerebral palsy, and hearing and vision loss. Lung problems and breathing difficulties are common in babies born before 34 weeks.