They claim that by his stage the unborn baby can already feel agonising pain and the procedure is therefore inhumane.

Many experts dispute these claims, however, and say that a fetus is naturally sedated and unconscious in the womb and cannot experience any senses.

Professor Stuart Campbell, who took the picture at his London clinic with 3-D and 4-D scanning equipment, said it did not necessarily show the unborn child had feelings - but it was certainly displaying human behaviour.

Proud parents at right: Louise and Sam Henry

'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.'

Professor Campbell, former head of obstetrics and gynaecology at King's College and St George's hospitals in London, said he did not know what caused the smile.

'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 baby is due in January and its parents, Louise and Sam Henry from Swallowfield, Berkshire, admitted they were stunned to see the smiling face during a routine scan.

Mr Henry, 40, who runs a financial advice business, said: 'Your primary concern at that stage is that you have a healthy baby, but to see the fetus smiling was absolutely fantastic.'

Eric Jauniaux, professor of obstetrics and foetal medicine at University College London said: 'There is no emotion or feeling at this stage. The evidence of pain and feeling is from 24 or 28 weeks.

'At 17 weeks the connection between the brain and the rest of the body is very limited.'

Dr Yehudi Gordon, who runs the private Viveka gynaecology clinic in London, said: 'All the emotional centres that we will ever have during our lives are fully formed by the time we are born.

'At 17 weeks the baby could be smiling or it could be the facial muscles getting together in preparation for sucking and feeding.'