Baby born three or more weeks before the due date. The length of a pregnancy is measured by gestational age. Gestational age equals the amount of time that has elapsed since the first day of the last menstrual period. A normal gestation lasts 40 weeks or 280 days. If delivery occurs before 37 weeks gestation, the baby is considered prematurely born. About 12.5% of all babies born in 2004 were born prematurely by this definition, an increase of 30% since 1981. Irregular menstrual periods or first trimester vaginal bleeding can confuse gestational age estimates. To improve the accuracy of gestational age estimates, some doctors use an ultrasound examination before about 20 weeks gestation. Ultrasound findings help your doctor to confirm or correct the gestational age estimate.
Birth weight is lower when a baby is born prematurely. Babies born with a weight of less than 2500 grams (about 5 lbs. 8 oz) are considered low birth weight babies. Generally, infants with a birth weight less than 2500 grams are premature; however, babies can be premature and still weigh more than 2500 grams at birth. They can also be born at term and weigh less than 2500 grams. There is no perfect system for categorizing infants; both measurements, birth weight and gestational age, are used.