At the age of 21, aeronautical engineer Lauren Burns was told a family secret that turned her world upside down.

Barbara Burns revealed that Lauren was conceived in a Melbourne clinic using donor sperm. The man she knew as her father was infertile.

Mrs Burns says the culture at the time was to 'take the child home and forget how it was conceived. Just pretend it was your child and nobody knew.'

Helped by her mother, Lauren embarked on a long and arduous search to discover the identity of her donor father – a man she knew only as 'C11' – from the first letter of his surname and his height, 5'11".

Four fruitless years later, Lauren had one final card to play. What she managed to finally discover was truly astonishing.

'Searching For C11', a two part Australian Story special, untangles a complex ethical and emotional web involving not just Lauren Burns but two other young women...