Dear Ms. Ultrasound,
I know that they usually do an internal ultrasound early on in the pregnancy. How far along do you have to be to have a normal external ultrasound?
When doing an early ob ultrasound exam, there are a few factors to take into consideration.
The abilities of the ultrasound equipment. There are ultrasound machines that cost $20,000 dollars and there are machines that cost $200,000. The resolution differences might show a late five week sac on the expensive machine vs. a seven week sac on the cheaper machine.
The mother's body type also plays a part in identifying an early gestational sac. The larger the body type, the lower the image resolution.
The rule for ultrasound is, the closer the ultrasound probe (transducer) is to the anatomy you would like to image, the better the picture. (Little skinny people make the best ultrasounds). This is why the internal probe produces such good pictures. It is right up against the anatomy. A gestational sac can be identified from five weeks on with this probe.
Sorry there is no solid number for me to give you on the external ultrasound dating capabilities. With that said, by at least seven weeks most externals will show something.
-- Jane Foley RDMS