Can ultrasound catch all birth defects
No. Ultrasounds can see many birth defects but they can not see them all. The fetal anatomy scan is usually done around 20 weeks. During this scan the technician will have a standard list of measurements to take and things to look for.
Sometimes the technician might see something called a "marker" and they are often referred to as a soft or hard marker. These markers can indicate a defect or they might be nothing at all. If you doctor finds a number of markers they might recommend additional fetal testing.
During routine ultrasounds some benign things way be seen as well. Your doctor will discuss the findings with you and they will discuss what further testing might be needed. Some birth defects can be completely ruled out by ultrasound. If there is something specific you are concerned about be sure to ask your doctor.