The NT scan must be done when you're between 11 and 14 weeks pregnant, because this is when the base of your baby's neck is still transparent. (The last day you can have it is the day you turn 13 weeks and 6 days pregnant.) It's usually offered along with a blood test in first-trimester combined screening options.
The anomaly scan, also sometimes called the anatomy scan, 20 week ultrasound, or level 2 ultrasound, is a pregnancy ultrasound performed between 18–22 weeks of gestational age. The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that this ultrasound is performed as a matter of routine prenatal care, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations
Ultrasound scans such as 3D and 4D are like real-time photos of your baby. In a 3D scan, multiple pictures of the baby are taken in 2D and then put together to create a 3D image effect. In a 4D scan, the pictures are taken in real-time, and you can see what your baby is doing inside you at that point in time, such as moving its legs and arms or even opening and closing its eyes. This is more like a live streaming or a video. The time component is the 4th dimension that is added to the 3D scan, and hence it is referred to as a 4D scan.