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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.
Not useful for dating, the abdominal circumference of the fetus may also be measured.
The most accurate measurement for dating is the crown-rump length of the fetus, which can be done between 7 and 13 weeks of gestation.
In most countries, routine pregnancy sonographic scans are performed to detect developmental defects before birth.
This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth.
Obstetric sonography has become useful in the assessment of the cervix in women at risk for premature birth.
There is no difference, however, in perinatal death or poor outcomes for babies.
In normal state, each body tissue type, such as liver, spleen or kidney, has a unique echogenicity.