A woman was given the wrong sperm donation at a fertility clinic after a "paperwork mix up", a report said.
The couple were trying for their second child at Simply Fertility in Chelmsford, Essex, when the mistake was made, according to the Human Fertilisation and Embryology Authority (HFEA).
The woman gave birth to a child as a result of the accident.
The clinic said it had taken measures to ensure it did not happen again.
The couple had originally undergone fertility treatment at the clinic in 2016, unsuccessfully using one donor but then successfully from a second donor, the report said.
When they returned to the clinic last year they asked for the same donor so as to give their child a full genetic sibling.
But the mistake by the clinic led to the original donor being used.
The mistake was realised after the embryo had been transferred to the woman, the HFEA said.
The HFEA's senior management team was informed as soon as he error was discovered, the report said, and it was explained to the couple who were offered support.
The couple decided to continue with the pregnancy.
A statement from the clinic said: "As a part of our investigation into this matter, we have identified how the mistake arose and we have put in place measures to ensure this does not happen again.
"We cannot discuss the details of the matter due to patient confidentiality but we have of course apologised sincerely for the distress caused to those concerned."
The error was highlighted as part of an HFEA report which showed that mistakes are rare - affecting less than 1% of fertility treatment cycles - but have risen by 6% in the last year and 18% in three years.