Ultrasound has been used for antenatal use since 1970’s, it can be used to image from 5 wks gestation up to term. 2 routine scans are now offered to the mother, one at 11 wks to check dates and screen for abnormalities by measuring the nuchal translucency and one at 20 wks to check the babies anatomy in more detail. Many abnormalities can be demonstrated by ultrasound, but luckily most pregnancies proceed happily once the risky 1st trimester has passed.
Very early scans are performed to check the baby is not ectopic and later scans are performed to check growth and placental site.
Fortunate for Antenatal u/s the baby is in a sac surrounded by fluid, fluid has no reflective boundaries within it and the difference between speed of sound through fluid and through soft tissues – this causes optimum reflections and ideal imaging.
The world health organisation has published an excellent if slightly wordy document on the physics of diagnostic ultrasound to improve users knowledge in unregulated areas, link to this below
BASIC PHYSICS IMAGING – WHO
Diagnostic Imaging and Laboratory Technology Essential Health Technologies