A vaginal swab taken from a woman who was allegedly raped by her biological son was never tested for semen because of a mix-up.

The High Court heard this on the sixth day of an ongoing rape trial which, so far, has not thrown up conclusive physical evidence against the accused.

The 33-year-old man is charged with raping and molesting his mother, 56, after he returned home at about 2.30am on Oct 4, 2013.

He is the second of the woman's three sons from her first marriage, and lived with her and his stepfather in a one-bedroom flat.

His lawyer said earlier that his mother and stepfather had conspired to accuse him of rape, in a bid to make him leave the flat.

Yesterday, Singapore General Hospital (SGH) gynaecologist Tan Wei Ching testified about her medical examination of the woman, just hours after the alleged rape.

The issue of the vaginal swab taken from the woman to test for semen arose when Dr Tan was questioned by defence lawyer Andy Lem about an e-mail sent by the Health Sciences Authority (HSA) in May 2014 asking about the whereabouts of the swab.

Dr Tan said that the swab, which was placed in a wet container, was sent from SGH to HSA on Oct 4 - the day of the alleged rape and the day the swab was taken - but was rejected by HSA as it was in an "inappropriate medium".

Three days later, the swab was returned to SGH's accident and emergency (A&E) department. But Dr Tan said she was unaware of this.

Further investigations showed that after receiving the swab, SGH's A&E department forwarded it to the hospital's microbiology laboratory.

Mr Lem said it was "a bit surprising" that Dr Tan was notified by HSA only about six month's later, in May 2014. "I'm surprised too," she replied. Dr Tan was not asked in court why the swab was placed in a wet container.

On the first day of the trial, a DNA analyst from HSA had testified that it was the practice to use dry containers as moisture could contaminate the swab.

Dr Tan was also questioned about her examination of the alleged victim. In her report, Dr Tan said she did not detect any bleeding, open wounds, bruising or lacerations on the woman's body.

Mr Lem suggested that, given the woman's account of the sexual assault, it was more likely than not that she would have suffered physical injuries. Dr Tan said it was possible but said the absence of injuries did not mean that the assault did not take place.

Gynaecologist Tan Wei Ching said that the swab, which was placed in a wet container, was sent from SGH to HSA on Oct 4 - the day of the alleged rape and the day the swab was taken - but was rejected by HSA as it was in an "inappropriate medium".