How Precise is an Ultrasound for Determining the Exact Age of a Baby?

An accurate gestational age gives your prenatal care provider a basis for measuring the baby's development and growth. He is better able to identify problems with your baby's development with this information. Using the date of the last period gives a general idea of that information, but abnormalities in the cycle or inaccuracies in the date affect the calculations. Ultrasounds commonly are used to determine the age of a fetus.

Technician using an ultrasound (Image: zilli/iStock/Getty Images)

Calculating Conception

The ultrasound machine and software allows the technician to see the baby's development and measure the baby. In the first trimester, the ultrasound measures the fetus from head to the pelvis, since the baby is so small. The head, abdomen and thigh are the measurements used to estimate the gestational age after the first trimester. Data on measurements for different gestational ages allows the machine to calculate an age for your baby by comparing the data. The level of the amniotic fluid also is measured and used in the calculations.

Timing is Everything

The timing of the ultrasound affects the accuracy of determining the gestational age. Early ultrasounds generally are more precise. Later in the pregnancy, babies grow at much different rates, and babies at the same gestational age are more likely to vary in the measurements taken in the ultrasound. Much of the baby's development occurs in the first trimester at certain points, making it easier to pinpoint the age based on how much the baby has developed. The measurements also are more accurate in the beginning, when babies' growth rates usually are similar.


Some babies are naturally larger than average. If your baby is larger, his measurements won't line up with others at the same gestational age. The estimated gestational age is likely to appear incorrect because the measurements would lump your baby in with babies further along in development.

Other Methods

In addition to the ultrasound, the doctor has other methods of estimating gestational age. Hormone levels in the blood give the doctor an idea of how far along a woman's pregnancy is. The ultrasound combined with the hormone levels and the date of the woman's last period give an accurate estimation, especially if all of the methods give a similar time frame. Later in the pregnancy, the obstetrician or midwife uses measurements of the expectant mother's abdomen. Called the fundal height, the measurement used goes from the top of the uterus to the pubic bone. This measurement in centimeters usually corresponds to the point in the pregnancy, so a woman at 24 weeks pregnancy should measure around 24 cm.

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