Anne Shortall was not pregnant at the time she was beaten to death by murder accused Roy Webster, the Central Criminal Court has heard.
When gardai told Mr Webster this, he said: "I f***ing knew it."
Detectives put it to him in interview that this had been determined in a post-mortem examination on Ms Shortall.
The jury also heard Mr Webster (40) told gardai he did not intend to kill Ms Shortall and hit her with a claw hammer in a "panic reaction" when she threatened to "blow the lid" on their one-night stand.
In the interviews, Mr Webster said he hit Ms Shortall three to four times on the head and felt like he was having an out-of-body experience or "watching a horror movie".
Mr Webster, a father-of-two, from Ashbree, Ashford, Co Wicklow, denies murdering Ms Shortall (47) on April 3, 2015, at the Murrough, also in Wicklow.
He has pleaded guilty to manslaughter, but that plea was not accepted by the prosecution.
Detective Garda Fergus O'Brien said the accused was interviewed after his arrest at Wicklow Garda Station on April 7 and 8.
Mr Webster, a cabinet-maker, told gardai he was on jobseeker's allowance because work had dried up.
He said he was out on December 20, 2014, in the Forge pub, he got talking with Ms Shortall and each said "I fancied you".
Things got "flirty" and they "ended up having a kiss there", then went back to her apartment, where "one thing led to another and she said "let's go upstairs", Mr Webster told gardai. He said they had sex.
She later told him she was pregnant and an abortion would cost £6,500 (€7,400).
On Friday, April 3, he drove her in his van down to a quiet place called the Murrough.
He asked had she taken a pregnancy test and she said she had not. She said "so you don't even have the money with you now" and started shouting.
He told gardai she got out of the van and said: "Don't you f***ing worry about it, I'll sort it out myself."
He said he got out to reason with her and she started threatening "if I didn't cough up she was going to start ringing my wife and ringing my house".
"There was a bit of me pleading with her, 'please don't, please don't', I said 'I have a wife and child and a newborn baby at home, please don't ruin that for me'," he told gardai.
He said she replied: "I don't give a s**t."
His head was spinning and he could "just see my whole world crashing down - this one had my back to the wall".
"I swung open the side door of the van, and I grabbed the first thing, it was a hammer. I hit her on the head with it," he said in his interview.
She fell back into the van, and, still conscious, told him "you f***ing pr**k, I'll f***ing ruin you", so he hit her again, his statement continued.
He said there was "blood pouring out of her head… I couldn't believe how much."
Mr Webster told gardai he closed the van door, cleaned his hands with white spirit and drove on.
"She was in the back of the van at this stage, she wasn't talking or moaning or moving," he said.
He told gardai he taped Ms Shortall's head first as there was blood coming out of it and he "thought the tape might stop the blood".
"I also put tape on her hands to stop her flailing around," he said.
"I suppose subconsciously I didn't know whether she was dead or alive."
He went home, leaving the body in the van.
He "played with the young one, fed the baby" watched TV later and fell asleep on the couch.
The next morning, they went to Arklow to do some baby shopping then returned home.
"Anne was still in the van. I thought, 'Jesus I am going to have to take her out'," he said.
He lifted her out and put her in the workshop.
The next day, he stayed in and had a "pyjama day", watching movies with his kids.
In the second interview, Mr Webster said he hit Ms Shortall because she was "threatening my family and my livelihood".
The trial continues.