The boy, who cannot be named, was just 16 when he became friends with the victim, then 12, on Facebook in early 2011.

The girl created the Facebook account because ''she was bored and wanted to meet guys''.

But she lied to the boy about her age, telling him she was also 16, and had sex with him twice before revealing her true age.

The boy became angry, telling her she was too young and that he could ''get charged''.

But two weeks later, their relationship resumed at the girl's request.

A doctor refused to give her a contraceptive implant, but she began taking the pill. She regularly forgot to take it and eventually stopped the medication altogether.

The pair had sex seven times before September 2011, a day before the boy, who had turned 17, told her he was leaving to go to the US.

The girl, then 13, told him she had a present for him and, when asked what it was, said ''going away sex''. About a month later, she went to a medical centre with stomach pains and was told she was pregnant.

She was interviewed by police and the 17-year-old was arrested.

He was sentenced for having sexual intercourse with a child in the ACT Supreme Court in September last year but was not convicted of the offence.

The judge instead placed him on a 12-month good behaviour order.

The Crown appealed that sentence, saying the judge erred by not convicting the teen.

The Crown said the judge should not have allowed the likely consequences of a conviction, which included placing him on a sex offenders' register, to override considerations of punishment and deterrence.

The Crown also said the consequences of convicting the teenager were an ''irrelevant consideration'' and should not have been taken into account.

But the appeal was dismissed by the full bench of the ACT Court of Appeal, which found there was only one appealable error made in the original sentencing decision.

The court found that error did not justify interfering with the original sentence.

It found the judge was entitled to regard the boy's criminal responsibility as falling at the ''lower end of the scale''.

''As his honour remarked, [the boy] did not initiate the sexual activity, he did not prey on [the girl] or exploit any advantage he had over her,'' the judges wrote.

''There was no breach of trust. There was no coercion or even enticement.''

The judges acknowledged that those found guilty of intercourse with a child would ordinarily be dealt with severely and that deciding not to record a conviction would be unusual.

''But this was no ordinary case,'' the judges wrote. ''By his decision not to record a conviction, his honour resolved to give [the boy] a chance to begin his adult life without a criminal record and without being stigmatised as a sex offender.''