For 4D ultrasound appointments, you need to bring your previous ultrasound checklists. If you have had a non-invasive DNA or amniocentesis test, you need to bring the checklist for your doctor to look at. The doctor will estimate the size of the fetus based on the time of your last menstrual period and when you had the ultrasound, and then determine with you when you need to have a 4D ultrasound. If it is too big, many organs may not be seen clearly when you have a 4D ultrasound due to the reduced amniotic fluid volume. If it is too small, some tiny malformations cannot be seen clearly when doing 4D ultrasound because the fetus is smaller. So the doctor must calculate the time to do the 4D ultrasound according to the specific situation.