During many pregnancies, a doctor will order an ultrasound to check the status of the fetus, including its age, size, growth and health. The ultrasound uses high-frequency sound waves to project a picture of a fetus onto a monitor. The ultrasound technician or physician takes specific measurements of the fetus to determine its gestational age, which helps the doctor and the mother calculate the due date and verify that a pregnancy is progressing well. The most accurate estimate of gestational age is calculated during the first trimester.
From Five to Eight Weeks Gestation
Step 1
Measure the gestation sac in three dimensions. Note that all measurements described here are performed by an ultrasound technologist who uses the ultrasound technology to measure the fetus as accurately as possible.
Step 2
Add the three measurements and divide by three to determine the mean sac diameter (MSD).
Step 3
Add 30 to the MSD. The total equals the approximate gestational, or menstrual, age in days. The gestational age is the number of days from the first day of the mother's last period.
From Seven to 13 Weeks Gestation
Step 1
Measure the fetus's crown-to-rump length (CRL). This measurement is taken from the top of the head to the bottom of the fetus.
Step 2
Compare the answer to the chart at the website baby2see.com (see Resources) to determine the gestational age and weight of the fetus.
Step 3
Calculate the biparietal diameter (BPD) if the pregnancy is at least 13 weeks along. This measurement is the circumference of the head.
Step 4
Compare the result with a BPD chart at the website baby2see.com to determine the fetus's age (see Resources).
Tips and Warnings
- Call ahead before undergoing an ultrasound. If you are early in your pregnancy, you may need a full bladder so the technician can get a good image of the baby (see Resources). Early ultrasounds may be conducted using a transvaginal ultrasound (see Resources).
