Fetal heart ultrasound has to choose the right time for the examination to be done in one go. Fetal heart ultrasound test can observe the development of the fetus, for example, it can find out whether there is any malformation in the development of the heart and so on. In order to ensure that the fetal heart ultrasound passes the test in one go, it is often necessary to perform the test at the best time. For fetal heart ultrasound, it is important to have it done at the right time, and it is generally recommended to have it done between 22 and 28 weeks in mid-pregnancy, when satisfactory images can already be obtained. If the fetus is too small, the heart structure may be less developed, making it difficult to determine with a cardiac ultrasound. If the examination is performed late in pregnancy, the amniotic fluid is low and the fetus is too large, it may have poor positional changes, which is not conducive to diagnosis. Do fetal heart ultrasound, if the fetal position is not good, resulting in the examination can not be completed at once, at this time there is no need to panic, you can cooperate with the doctor to carry out appropriate physical activities, which will help to change the position of the fetus, fetal position change, continue to check. Before the examination, we should pay attention to avoid fasting, it is best to eat and drink enough to carry out the examination.