Gestational Age and Why It Matters

Your Baby from Conception to Birth: Gestational Age Explained

Nurse and premature baby
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When people ask you how far along you are in your pregnancy, they are asking the gestational age of your baby. Usually, when you confirm your pregnancy, your doctor or nurse will estimate the number of weeks from conception by counting from the first day of the mother’s last menstrual period. A normal pregnancy lasts between 38 and 42 weeks. Gestational age will be used to determine your baby's developmental milestones in the womb, as well as their health risks at birth.

Infants who are born before 37 weeks gestation are considered premature; infants who are born after 42 weeks gestation are considered postmature.

Corrected Age vs. Gestational Age

When parents of premature babies are comparing their babies to charts of normal infant development, they should use their gestational age as well as their birth age. A baby born at 30 weeks (10 weeks early) can be expected to crawl 10 weeks later than a baby born at 40 weeks gestation. This delay in developmental milestones for babies born early is why preemies have what is called a corrected age, or adjusted age. The American Academy of Pediatrics calculates a baby's corrected age as your preemie's actual age in weeks minus the number of weeks he or she was premature.

Gestational Age, Abnormalities and Birth Defects

In addition to estimating when important developmental milestones may occur, knowing your child's gestational age can help you determine if your baby is at risk for abnormalities.

Babies with down's syndrome and other chromosomal abnormalities do not grow at the same rate of normal babies. Also, babies born with birth defects usually show some abnormal growth for their gestational age. The use of ultrasound technology throughout pregnancy is meant to monitor growth as compared to gestational age to ensure that all is well.

Lower Gestational Age at Birth

Babies born at a lower gestational age than 35 weeks are at a higher risk than preemies before after and thus may be treated differently at birth and throughout their childhood to ensure that the baby is developing as it should. Gestational age is also very important in difficult pregnancies or for babies who are in distress in the womb. Doctors will determine the viability, or survival rate of the infant based on the gestational age. Just the same, doctors will make decisions about how reasonable inducing labor is depending on the gestational age of the child. Also, when moms suffer with dangerous pregnancy conditions such as gestational diabetes, preeclampsia, and other complications, when to deliver the baby is based on the age of the fetus.


Newborn Intensive Care: What Every Parent Needs to Know. 3rd Edition. 2010 American Academy of Pediatrics.

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