 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.