Obstetric ultrasonography is routinely used for dating the gestational age of a pregnancy from the size of the fetus, determine the number of fetuses and placentae, evaluate for an ectopic pregnancy and first trimester bleeding, the most accurate dating being in first trimester before the growth of the foetus has been significantly influenced by other factors. Ultrasound is also used for detecting congenital anomalies (or other foetal anomalies) and determining the biophysical profiles (BPP), which are generally easier to detect in the second trimester when the foetal structures are larger and more developed. Specialised ultrasound equipment can also evaluate the blood flow velocity in the umbilical cord, looking to detect a decrease/absence/reversal or diastolic blood flow in the umbilical artery.
X-rays and computerized tomography (CT) are not used, especially in the first trimester, due to the ionizing radiation, which has teratogenic effects on the foetus. No effects of magnetic resonance imaging (MRI) on the foetus have been demonstrated, but this technique is too expensive for routine observation. Instead, obstetric ultrasonography is the imaging method of choice in the first trimester and throughout the pregnancy, because it emits no radiation, is portable, and allows for realtime imaging.
The safety of frequent ultrasound scanning has not be confirmed. Despite this, increasing numbers of women are choosing to have additional scans for no medical purpose, such as 3D and 4D scans. A normal gestation would reveal a gestational sac, yolk sac, and fetal pole. The gestational age can be assessed by evaluating the mean gestational sac diameter (MGD) before week 6, and the crown-rump length after week 6. Multiple gestation is evaluated by the number of placentae and amniotic sacs present.