Gestational dating sonogram
Traditional obstetric sonograms are done by placing a transducer on the abdomen of the pregnant woman.One variant, a transvaginal sonography, is done with a probe placed in the woman's vagina.After 13 weeks of gestation, the fetal age may be estimated using the biparietal diameter (the transverse diameter of the head, across the two parietal bones), the head circumference, the length of the femur, the crown-heel length (head to heel), and other fetal parameters.Dating is more accurate when done earlier in the pregnancy; if a later scan gives a different estimate of gestational age, the estimated age is not normally changed but rather it is assumed the fetus is not growing at the expected rate.
The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5mm, but may not be visible until the embryo reaches 7mm, around 7 weeks gestational age.
In some countries, routine pregnancy sonographic scans are performed to detect developmental defects before birth.
This includes checking the status of the limbs and vital organs, as well as (sometimes) specific tests for abnormalities.
Randomized controlled trials have followed children up to ages 8–9, with no significant differences in vision, hearing, school performance, dyslexia, or speech and neurologic development by exposure to ultrasound.
Several randomized controlled trials have reported no association between Doppler exposure and birth weight, Apgar scores, and perinatal mortality.