By Caroline Parkinson
Health reporter, BBC News, Barcelona
Study suggests being told later in life can be traumatic
Children conceived using sperm donors should be told of their origins at an early age, research suggests.
A Cambridge University team talked to 165 children conceived this way, and found those not told until the age of 18 often felt shock and anger.
But a European reproductive health conference was warned once children know, they may want to seek out their biological parent.
A UK patient group urged parents to be open from children from the start.
The study is one of the first to compare the views of the offspring of donor insemination told of their origins during childhood with those who only found out in adulthood.
The offspring, aged 13 to 61, were all on the US Donor Sibling Registry, which helps offspring search for their biological parent and any siblings.
The majority live in the US, with 2% living in the UK.
They were all asked to fill out an online questionnaire.
The results showed around 60% of children being brought up in single parent or same-sex parent families were told about their origins before the age of three, compared with just 9% of children of heterosexual parents.
A third of children of children in heterosexual parents only told their children after they turned 18.
Donor offspring who were told after the age of three were asked how they felt when they discovered how they were conceived, with those who were younger excluded as they would have been too young to recall their response.
The earlier someone was told, the better, the study found.
Over two thirds of those told when they were over 18 reported feeling confused, compared to a third of those told when they were aged four to 11.
Similar differences were seen in the numbers who reported feeling betrayed or anger at being lied to, shock and numbness.
A woman aged 30, who found out how she was conceived in her late teens said: "I would have appreciated revelation of this information much earlier in my life.
"Learning of my biological identity at 17 years of age was a traumatic event."
However, a 13-year-old who found out aged four said: "I was so young I don't remember feeling much more than interested and curious."
Dr Vasanti Jadva, of the Centre for Family Research at the University of Cambridge, who led the work, said: "It appears it is better for children to be told about their donor conception at an early age.
"This finding is in line with research on adoption, which also shows that children benefit from early disclosure about the circumstances of their birth."
But she said it was important to recognise that telling children might trigger curiosity about their biological parent, and they may want to contact them."
Olivia Montuschi, of the UK's Donor Conception Network, said: "We advocate openness from the beginning, from before a child reaches five.
"We even encourage talking to babies about how they were conceived, not because they will understand, but so that parents can practice talking about the issue and get used to the language they want to use."
She added: "If parents do it this way, there is no big revelation. It just becomes one of the things a child knows about itself."
Details were presented to the European Society of Human Reproduction and Embryology conference in Barcelona.