The best time to do a 4D scan is when the baby has developed some chubby cheeks which helps to make good images. This is usually between about 27 and 34 weeks. People offering 3D/4D scans usually differ slightly in their choice of best time. Some providers recommend the starting time as early as 24 weeks and the ending time as late as 35 weeks. Theoretically speaking, one can do a 4D scan anytime between 12 and 40 weeks. The challenge on smaller babies is that they have not got fat on their bodies and you then get images of very skeletal looking babies. The challenge on larger babies is that they are bigger and more squashed, with less fluid around them, making it difficult to get good images. The bigger the baby the nicer the 3D/4D images but the more difficult it becomes to get those good images. Smaller babies are easier to get images on because there is lots of water and space to move allowing the oppotunity to get clear facial shots but obviously more skeletal looking. Amniotic fluid (the water around the baby) is the key to getting good images. Lots of water in front of the babies face acts as a window allowing for sharper images.
The advantage of doing an early 3D/4D scan is that the entire baby can fit onto the image. Some mommies like to do two 3D/4D scans during their pregnancy, an early one showing the whole body and a later one to show the nice chubby facial views. In the above images you can see the skull bones are not yet fused.
