British doctor Stuart Campbell said images captured using a so-called 4D scanner can produce detailed images of unborn babies exhibiting facial expressions.
"Yawning is quite common inside the uterus, and then at 26 weeks you begin to see them blinking, which is astonishing because it is dark in there inside the uterus and at that time the baby starts to show expressions on the face like smiling, crying expressions, sucking their thumb, all sorts of things."
Campbell says he has oberved such expressions since he first started using the revolutionary scanner in 2001.
What cannot be determined is what causes the smiling and other expressions and what they signify.
It was previously thought babies learned to smile after birth by copying their mothers.
Campbell pioneered the use of the scanner in the UK at the private Create Health clinic in London.
While traditional ultrasound scans show a baby's internal organs, the 4D scan can look for any abnormalities on the extremities and face, such as a cleft palate.
Kate Blackwell, who paid $425 for the scan, couldn't believe her eyes.
"Now that I've seen what it looks like, I can really see it is a real baby, which sounds ridiculous but when you feel something moving inside the whole time, now I've seen its face I can really feel it's my child."