Also used is Doppler sonography which detects the heartbeat of the fetus.
Doppler sonography can be used to evaluate the pulsations in the fetal heart and bloods vessels for signs of abnormalities.
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women have routine obstetric ultrasounds between 18 weeks' and 22 weeks' gestational age (the anatomy scan) in order to confirm pregnancy timing, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations and multiple pregnancies (twins, etc).
Although 91% of fetuses affected by Down syndrome exhibit this defect, 5% of fetuses flagged by the test do not have Down syndrome. Usually scans for this type of detection are done around 18 to 23 weeks of gestational age (called the "anatomy scan", "anomaly scan," or "level 2 ultrasound").Performing an ultrasound at this early stage of pregnancy can more accurately confirm the timing of the pregnancy and can also assess for multiple fetuses and major congenital abnormalities at an earlier stage.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.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.