Ruth Harper
An arrest warrant said that Katharine Ruth Harper had sexual encounters with a 15-year-old boy, and sent him nude photos of herself Handout

Police in the US have charged a pregnant 27-year-old teacher with having an improper relationship with a 15-year-old pupil.

Police received an anonymous tip-off that Katherine Ruth Harper, who taught English at Tidwell Middle School, Fort Worth, was having sex with the teenager, who was a pupil at nearby Byron Nelson High School.

The pair had met when Harper was the teenager's tennis coach and she sent him a number of nude images via smartphone.

Police said the pair had communicated 76 times in June and July last year, with their sexual encounters occurring during that time, sometimes involving alcohol.

The arrest warrant says the boy admitted to the sexual encounters after being questioned by his school principal.

Harper is eight months pregnant but the warrant did not specify if the teenager, who cannot be named, was the father, Fox News reported.

Harper faces the second-degree felony charge of having an improper relationship with a student, and has been released from Denton County Jail after posting $15,000 (£12,000) bail. She has also been placed on administrative leave by her school.