Twins appear to kiss in the womb. A twin leans over and kisses the cheek of her sister in a heart-warming picture that would not be out of place in any family home.

Yet these siblings are a not even born and the astonishing images have been captured on a new ‘four-dimensional’ ultrasound scan of the womb.

The scans are a highly developed form of traditional ultrasound where very high frequency sound waves are used to produce images of what is inside the body.

As with older forms of ultra-sound, sound waves a emitted from a transducer, or probe, which is placed on the mother’s abdomen and then moved to ‘look at’ areas in the uterus. These sound waves bounce back off the fetus, helping to create a ‘picture’ of the child on a screen.

The new 4D scan uses the same frequency of sound waves w as in a normal ultrasound. But the sound waves are directed from many more angles, producing a ‘real-time’ video of the fetus as it moves and allowing scientists to say the images are in four dimensions.