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Understanding the Stages of Prematurity

By BabyZone and ParentZone Editors
Understanding the Stages of Prematurity
In this article:
Estimating Prematurity
How Common is Prematurity?
How Premature is My Baby?
Frequently Asked Questions:

Prematurely born babies differ a lot depending on just how early they are born. Learning just how premature your newborn is can help you understand and better support her through the coming developmental stages.

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 a 2004 National Vital Statistics report, about 12 percent of babies born in the US each year, one in eight 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.)

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