A forensics blunder allegedly pointing to the wrong suspect in the case of an intellectually impaired teen who fell pregnant was ''pretty unfortunate'', a court has heard.
The Crown alleges subsequent testing strongly identified the girl's father, not her brother, as the likely parent of her unborn child.
But the father's lawyers have flagged an intention to try and get key DNA evidence thrown out, arguing it was based on an assumption.
The 50-year-old man is on trial in the ACT Supreme Court accused of committing incest on his then 15-year-old daughter.
Her pregnancy came to light in 2010 when the girl's mother became concerned something was wrong.
The case was reported to police and the pregnancy was later terminated.
Her father, who cannot be named for legal reasons, was later charged with incest.
But he has pleaded not guilty, and is on trial in a case hinging on DNA evidence.
Melbourne-based forensic scientist Nicholas Vandenberg conducted tests at the request of the Australian Federal Police.
He concluded it was 45 million times more likely the accused fathered the unborn child than another, unrelated male picked at random from the Australian Caucasian population.
Mr Vandenberg yesterday said the relative chance of the accused fathering the child, as opposed to another random man, was 99.999998 per cent.
But barrister Shane Gill flagged an intention to ask for Mr Vandenberg's evidence to be excluded on the grounds it was based on assumptions and its prejudicial nature outweighed its evidentiary value.
Mr Vandenberg also said his analysis ruled out the girl's older brother, who also had an intellectual impairment.
But the court heard a preliminary report suggested the brother was in fact the father of his sister's unborn child.
Forensics expert Joannah Lee was called in to review the initial report and concluded, based on the DNA profiles, the brother could be ruled out but not the father.
Under cross-examination from Mr Gill, she said it was unclear what went wrong.
''Do you agree that it's pretty sloppy practice, a result that names [the brother] as the father,'' Mr Gill asked.
''It was pretty unfortunate, yes,'' she replied.
Ms Lee said in her 14 years in DNA analysis she had never seen a similar mistake. She said one possible explanation was ''contextual bias'', and early on investigators considered it ''extremely unlikely'' the accused had fathered a child with his daughter.