Traditional, or 2D, ultrasound technology is still used as a routine diagnostic tool by obstetricians/gynecologists. During a 2D ultrasound, sound waves are sent through a transducer (that white thing that is placed on your abdomen), and reflected straight back. On the other hand, 3D and 4D ultrasound sends those sound waves at many different angles, and they are subsequently processed by a computer. Movements measured in rapid succession can then produce what looks like moving images, though they are in fact a series of still shots. These 4D ultrasounds are still relatively new, and they are used mainly for the enjoyment of expectant parents (more about this in our article - are there any medical benefits of 4D ultrasound?).
While they can be carried out at any stage in pregnancy, it is generally recommended that you have a 4D ultrasound after around 26 weeks gestation, and before 30 weeks. That way, your baby will already look like a real baby, but he or she will still have plenty of space to move around the uterus, so you can see your baby kick box in there! Deciding whether or not to have a 4D ultrasound is personal. These scans are rarely recommended for diagnostic of medical reasons, and serve to help you bond with your baby and perhaps to identify their gender, if 2D ultrasound has not already done that. Want to know more? Also look at our article about the safety of ultrasound during pregnancy.