Understanding the Stages of Prematurity

A premature or preterm birth occurs when a child is born at or before 37 weeks gestation (most pregnancy last about 40 weeks). According to the march of Dimes, about 12.8 percent of babies born in the US each year—1 in 8 births—are premature.

Estimating Prematurity

Irregular menstrual periods or first trimester vaginal bleeding can confuse gestational age estimates. To improve the accuracy of gestational age estimates, many doctors use an ultrasound examination before about 20 weeks gestation to help confirm or correct the gestational age estimate.

Another way to estimate prematurity is the baby's birth weight. In general, the more premature infants are, the less they will weigh.

There are several terms used for this:

• Low-birth-weight (LBW) indicates that a baby weighs less than 2,500 grams (about 5 lbs., 8 oz.) at birth
• Very-low-birth weight (VLBW) indicates that a baby weighs less than 1,500 grams (about 3 lbs., 3 oz.)

Although it is easy to measure the birth weight precisely, it is sometimes difficult to know how birth weight fits with gestational age. Generally, infants with a birth weight less than 2,500 grams are premature. However, babies can be premature but weigh more than 2,500 grams at birth. They can also be full-term and weigh less than 2,500 grams. So, both measurements, birth weight and gestational age are used. Of the two measurements, higher gestational age best predicts the outcome. (Still, many published, medical studies on the outcomes of prematurely born infants use birth weight to estimate prematurity.)