The remarkable image has been described as “fantastic” by the baby’s parents, Sam and Louise Henry.
Prof Stuart Campbell, who took the picture, exclaimed: “This is a joyful expression of the humanity of the foetus.”
The image could once again ignite the abortion debate. In England, Scotland and Wales, abortion is allowed up to 24 weeks of pregnancy.
Prof Campbell, former head of obstetrics and gynaecology at King’s College and St George’s hospitals in London, said: “I have seen a foetus 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”.
Prof Campbell commented that he did not know what caused the facial expression but said: “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.”
Dr Yehudi Gordon, who runs a private gynaecology clinic in London, said: “At 17 weeks the baby could be smiling or it could be the facial muscles getting together in preparation for sucking and feeding.”
In April David Cameron called for a reduction in the upper limit of abortion.
Mr Cameron said an upper limit of “20 or 22″ weeks would be sensible, in the light of modern science and technology.
Last year a new technology was revealed which allows mums and dads to hold a life-size model of their unborn child.
It works by taking data from ultrasound and MRI scans to build a life-size plaster model using a method called rapid prototyping.
In April it emerged that a baby boy who survived a botched abortion at 22 weeks was wrapped in a sheet and left to die by staff at the Rossano Calabro hospital in Italy.
The day after the abortion the baby boy, who still had his umbilical cord attached, was discovered moving and breathing by a Roman Catholic priest.
The baby was rushed to another nearby hospital where he died the following day, and his death caused outrage among pro-lifers.
A spokesman for the ProLife Alliance said: “There cannot be anybody in the world who is not horrified by a story like this nor anybody in the UK who would not support a massive reduction in the upper limit for abortion.”