How accurate are dating scans in pregnancy

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.Research shows that routine obstetric ultrasound before 24 weeks' gestational age can significantly reduce the risk of failing to recognize multiple gestations and can improve pregnancy dating to reduce the risk of labor induction for post-dates pregnancy.There is no difference, however, in perinatal death or poor outcomes for babies.

Also used is Doppler sonography which detects the heartbeat of the fetus.Obstetric sonography has become useful in the assessment of the cervix in women at risk for premature birth.A short cervix preterm is undesirable: At 24 weeks' gestation a cervix length of less than 25 mm defines a risk group for preterm birth, further, the shorter the cervix the greater the risk.The embryo should be seen by the time the gestational sac measures 20 mm, about five-and-a-half weeks.