The number of ultrasounds needed throughout the pregnancy depends on the specific situation of the pregnant woman, and generally speaking, at least four ultrasounds should be done during pregnancy. The first ultrasound is performed at 10-14 weeks of pregnancy to determine whether the fetus is single or multiple, to understand the general situation of fetal growth and development, to measure the nuchal translucency and nasal bone, and to detect congenital abnormalities in combination with serological screening. In addition, it is easier to determine the gestational age of the fetus because there is less error in several indicators of ultrasound at this stage. The second examination is scheduled between 18 and 22 weeks of gestation, when the development of fetal tissues and organs can be more clearly understood, so as to know whether there is any abnormality in the fetus, and if there is any abnormality, it is more appropriate to terminate the pregnancy at this time (according to the obstetrician’s recommendation and relevant policies). The third examination is performed at 32-34 weeks of gestation, which may reveal some of the missed fetal anomalies, as well as further understanding of the fetal growth and development, position and amniotic fluid volume. The last examination is performed at 38-39 weeks of gestation to confirm the presence of fetal morphological abnormalities, monitor fetal growth and development, placenta and amniotic fluid, and estimate the birth weight.