Gestational age is usually determined by the date of the woman's last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle.
Sometimes a woman may be uncertain of the date of her last menstrual period, or there may be reason to suspect ovulation occurred significantly earlier or later than the fourteenth day of her cycle.
Current evidence indicates that diagnostic ultrasound is safe for the unborn child, unlike radiographs, which employ ionizing radiation.
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.
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.
Some resources indicate that there are clear reasons for this and that such scans are also clearly beneficial because ultrasound enables clear clinical advantages for assessing the developing fetus in terms of morphology, bone shape, skeletal features, fetal heart function, volume evaluation, fetal lung maturity, Second-trimester ultrasound screening for aneuploi- dies is based on looking for soft markers and some predefined structural abnormalities.
Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones.
These markers are often not clinically significant and do not cause adverse pregnancy outcomes.
Features include the forehead at 10 o'clock, the left ear toward the center at 7 o'clock and the right hand covering the eyes at .
Some abnormalities detected by ultrasound can be addressed by medical treatment in utero or by perinatal care, though indications of other abnormalities can lead to a decision regarding abortion.
Perhaps the most common such test uses a measurement of the nuchal translucency thickness ("NT-test", or "Nuchal Scan").
Several randomized controlled trials have reported no association between Doppler exposure and birth weight, Apgar scores, and perinatal mortality.
One randomized controlled trial, however, came to the result of a higher perinatal death rate of normally formed infants born after 24 weeks exposed to Doppler ultrasonography (RR 3.95, 95% CI 1.32–11.77), but this was not a primary outcome of the study, and has been speculated to be due to chance rather than a harmful effect of Doppler itself.Traditional obstetric sonograms are done by placing a transducer on the abdomen of the pregnant woman.