The images, taken using revolutionary 3D and 4D scanning equipment, reveal the fetus smiling at least five weeks earlier than previously observed.
Stuart Campbell, who took the pictures, said: “This is a joyful expression of the humanity of the fetus.
“I have seen a fetus making a crying face at around 18 or 19 weeks, but not a nice smile. This is the earliest on record. It is just a delight.”
The pictures are likely to provoke debate about whether the pictures show a real smile and, if so, what that reveals about the developing fetus.
Professor Campbell, the former head of the departments of obstetrics and gynaecology at King’s College and St George’s hospitals in London, said the smile was at least a sign that fetuses can show “human characteristics” as early as 17 weeks.
“We cannot say that this is a fetus expressing emotion, but if you disturb a fetus at around 20 weeks, you can see it cry,” he said.
What caused expressions such as a smile was unknown, he added, but it was evidence of developing human behaviour.
“It is part of a sequence that involves yawning and making breathing movements and opening its eyelids and, of course, it makes a crying face.”
The parents of the fetus, Louise and Sam Henry, were “stunned” to find out at their scanning appointment that their healthy baby appeared to be grinning.
The pictures were all taken at Create Health Clinics in London and have added to fierce debate about fetal development. Abortion of healthy fetuses is legal in Britain up to 24 weeks.