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 accuracy of using your LMP to date your pregnancy depends on having regular cycles and assumes you ovulated (released an egg) on day 14 of your cycle and conceived right away - none of which may be true.
Because few women know the exact day they ovulated or conceived, an ultrasound done in the first trimester of pregnancy has been shown to the be the most accurate way to date a pregnancy.
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. If your baby is larger, his measurements won't line up with others at the same gestational age.
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The estimated gestational age is likely to appear incorrect because the measurements would lump your baby in with babies further along in development.
In addition to the ultrasound, the doctor has other methods of estimating gestational age.
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.