A jealous teenager stabbed a mum-of-two to death because she believed the woman was pregnant with her boyfriend's child.

The 17-year-old had threatened to kick the unborn child out of Tracy Ann Meade's womb, the Old Bailey heard.

She phoned her aunt before the murder and said: 'I'm going to stab her. Do you know where her heart is?' Miss Meade, a 39-year-old welfare assistant, was lured to a quiet area behind a block of flats in Warminster Road, South Norwood, on May 31 last year were she was stabbed in the heart.

The girl then went to the cinema with her boyfriend, who was 18 at the time, and demonstrated how she had killed his former lover.

The teenager, who cannot be named, admitted murder and is now facing a life sentence.

Wearing a smart black trouser suit and white blouse, she wiped away tears in the dock as prosecutor Sallie Bennett-Jenkins told the court the killing was "born out of jealousy".

She said: "This was a chilling act - a killing that had been carefully premeditated by the defendant who had spoken of her intention to kill."

The court heard Miss Meade had met the girl's boyfriend at the Phil Edwards pupil referral unit in South Norwood, where she worked.

They began an affair a year after he left the school and he moved in with her.

But he became increasingly violent to Miss Meade and she would show friends the bruises he had caused to her face and body.

He was jailed for a year for dangerous driving and began an affair with the teenager, who was 16 at the time, shortly after his release in May 2006.

The teenager had babysat for the couple when they went away for a weekend, the court heard.

"In the days following his release it became clear to Miss Meade and others that a relationship was developing with the girl," Miss Bennett-Jenkins said.

"That caused Miss Meade great distress."

She added: "By May 25 Miss Meade was particularly distressed because she believed she was pregnant and the boyfriend was the father.

"The teenager was also aware of this fact and had made threats to Miss Meade concerning the pregnancy."

Three days later the girl told a friend that Miss Meade did not want the child.

"Miss Meade told a friend that the teenager, when she had found out that Miss Meade was pregnant, had threatened to kick the baby out of her," Miss Bennett-Jenkins said.

"The victim was still deeply in love with the boyfriend and gave him an ultimatum - return to her by June 1 or the relationship was over."

On May 31 she was stabbed to death.

The teenager met Miss Meade on the pretext that she wanted to collect some things belonging to the boyfriend from her flat.

Miss Meade picked up her love rival in her car at about 7pm and they drove off.

Miss Bennett-Jenkins said that at some point the girl "collected and armed herself with a knife from her flat."

She also rang her aunt, saying: "I'm going to stab her. Do you know where her heart is?"

Miss Bennett-Jenkins said: "She then asked again if the heart was on the left or the right."

At about 9.30pm the girl was seen running from Warminster Road.

Police later found Miss Meade's car in a secluded alleyway near garages behind a block of flats.

Miss Bennett-Jenkins said: "It may be that we will never actually know the reason that Tracy Meade went down that small, dark and relatively isolated path to the garage area.

"One inference is that she was directed to the scene by the defendant, who was armed with the knife.

"Whatever it was that led Miss Meade to that location, it was there that she was stabbed.

"Miss Meade was lying face down on the floor a few metres away from the car. There were obvious stab wounds to her body and it was clear that she was beyond assistance."

A postmortem showed she died from haemorrhaging following multiple stab wounds to the heart.

The teenager took £70 from the dying woman as well as documents relating to her boyfriend, who she arranged to meet afterwards, telling him: "I've done it."

After the killing she was spotted catching a 410 bus with her boyfriend looking "cool, calm and collected", the court heard.

Miss Bennett-Jenkins said the couple went to a Vue cinema in Croydon to watch X-Men: The Last Stand.

She added: "The defendant accepts that it was she who suggested that they go together to the cinema.

"CCTV on the bus has captured what may be thought of as astonishing images.

"The girl was filmed 'demonstrating what she had in fact done to Miss Meade. She is raising her hand then placing her hand to the area of the chest, which is in fact where Miss Meade was stabbed.

"There was 'an expression almost of disbelief' on the face of the boyfriend."

But it was the 19-year-old who led police to the killer after getting caught breaking into Tracy Meade's flat hours after her death.

When police arrested the girl she claimed two men called Andre and Jasper had ordered her to carry out the killing after she refused to hold drugs for them.

Police spent 400 hours investigating the bogus claims before they realised the girl had acted alone.

Her boyfriend is facing charges of perverting the course of justice after it was alleged he lied to police about her movements but there is no evidence he knew anything about the murder, said Miss Bennett-Jenkins.

Mr Jim Sturman QC, defending the teenager, said: "She behaved that night in a disgraceful and chilling manner and she is very, very sorry for that."

Judge Gerald Gordon will sentence the teenager later today.