A normal gestation lasts 40 weeks or 279 days. If delivery occurs before 37 weeks gestation, the baby is considered prematurely born. 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 can also indicate whether a baby is born prematurely.
However, babies can be premature but 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 and both measurements, birth weight and gestational age, are used. Infants born more prematurely than 32 weeks gestation routinely require long hospital stays and usually require care in an Special Care Nursery . These infants are also at risk for long term problems.