It is the story of a young woman who finds herself unexpectedly pregnant. The birth takes place against a dramatic background and makes its mark on history.

The plot of The Nativity Story is familiar enough. But the parallels between the lives of the Virgin Mary and the teenage actress who plays her in the first feature film to be premiered at the Vatican are causing controversy.

The Pope, upon whose doorstep the first showing takes place on Sunday, will not be attending. Nor will Keisha Castle-Hughes, the 16-year-old unmarried actress who plays Mary and who is expecting her first child in the spring.

Castle-Hughes, who said she was “thrilled” to be pregnant”, said she had made the film “in a state of grace”. Playing Mary, a “sweet, strong and courageous” figure, had been “a source of spiritual richness”.

A Vatican spokesman said yesterday that the Pope “never was going to attend. The surprise would have been if he had.” His decision had nothing to do with the film. He is 79 and preparing for a visit to Turkey on Tuesday.

Castle-Hughes, who became the youngest nominee for a Best Actress Oscar when she was shortlisted at the age of 13 for her role as the Maori girl Paikea in Whale Rider in 2002, will be in Australia filming. A part-Maori New Zealander, she also appeared as the Queen of Naboo in Star Wars: Episode III — Revenge of the Sith last year.

The father of her baby is Bradley Hull, her 19-year-old boyfriend of three years, whom she met in high school.

In the film, made in Italy and aimed at the Christmas market, Oscar Isaac, a Guatemalan actor who appears in the forthcoming Guerrilla, plays Joseph and the Iranian actress Shoreh Aghdashloo as Elizabeth, mother of John the Baptist.

The film, which follows the unexpected box-office success of Mel Gibson’s The Passion of the Christ three years ago, is directed by Catherine Hardwicke, noted for Thirteen, a raw account of modern female adolescence, and Lords of Dogtown, about surfers and skateboarders in California.

Christian websites in the US and Canada have questioned Castle-Hughes’s “suitability” to play the mother of Christ.

The film is made by New Line, the company that made the Lord of the Rings films, and is described as a chronicle of “the arduous journey of two people, Mary and Joseph, a miraculous pregnancy and the history-defining birth of Jesus”.

Father Melchor Sánchez de Toca y Alameda, deputy to Cardinal Paul Poupard, head of the Papal Council for Culture, which is hosting the screening, confirmed there were rumours that Castle-Hughes’s presence on Sunday had been “vetoed”, but said he “did not believe them”. He gave the film “8½ out of 10”, and said Castle-Hughes was “not expected to be a saint herself, only to do her work as an actress properly”.

Father Sánchez said he found “no major theological errors”, and the film’s graphic portrayal of Mary’s labour pains accentuated her humanity. But he added: “The interior religious life of Mary and Joseph is barely mentioned. I would have liked greater reflection on the trial of faith which is asked of them.”