3D scans are still pictures of your baby in three dimensions. 4D scans are moving 3D images of your baby, with time being the fourth dimension. Both types of scans are just as safe as a 2D scan, because the image is made up of sections of two-dimensional images converted into a picture.
These pictures show the kind of detail you can see on a 3D scan. With 3D and 4D scans, you can see your baby's skin covering the internal organs. You may see the shape of your baby's mouth and nose, or see yawn or stick its tongue out.
3D and 4D scans may nonetheless show more detail about a known abnormality. Because these scans can show more detail from different angles, they can help in the diagnosis of cleft lip. This can help doctors to plan the repair after birth. 3D scanning can also be useful to look at the heart and other internal organs.
After 30 weeks, your baby's head may go deep down in your pelvis, so you may not be able to see the face.
The best time to have a 3D or 4D scan is when you are between 26 weeks to 30 weeks pregnant. Before 26 weeks your baby has very little fat under the skin, so the bones of the face will show through the skin.
If fetus is lying facing outwards, with a good pool of amniotic fluid around her features, you should be able to see the face clearly. But if fetus is facing your back, or if there's not much fluid around, you won't see much. The same applies if you have a lot of tummy fat.