Ms Darinne Ko had told officers from the Corrupt Practices Investigation Bureau (CPIB) that she was pregnant but had an abortion, a court heard Wednesday.
This was revealed by Mr Bay Chun How, a chief special investigator with the bureau who had been tasked to look into the case against law professor Tey Tsun Hang.
Mr Bay said he had learned of Ms Ko's pregnancy and abortion on April 2 last year when both Ms Ko and Tey were picked up for questioning by CPIB officers.
He said that he had been directed on March 30 last year to investigate Tey.
On April 2, he proceeded to the National University of Singapore to pick Tey up for questioning but he was not there.
Mr Bay then proceeded to Tey's home but went back to the bureau's headquarters midway because of a heavy downpour. Another team was sent instead.
When he arrived back at CPIB, he heard that Ms Ko was already cooperating with other officers. He was told that Ms Ko said Tey had indeed been given gifts and sex, and she had also "confirmed a relationship" between them.
Mr Bay is the seventh prosecution witness to testify in the sex-for-grades corruption trial involving Tey. Ms Ko, who was Tey's former student at NUS, was the first.
The 41-year-old professor is facing six charges of corruptly obtaining gifts and sex from Ms Ko, 23. The items she gave him include a Montblanc pen, an iPod and two tailored shirts. She also paid for a dinner Tey hosted.
Last week, Ms Ko testified that she had lost her virginity to Tey on a couch in his NUS law school office on July 21, 2010. They had sex again there four days later, on the eve of her 21st birthday.
She also said that at some point in their relationship she had believed she and Tey were in love.