The Danish film was shown last Monday at midnight, as part of Channel 4's 'Banned' season, which looks at issues of censorship, taste and decency.
The film, made by controversial director Lars von Trier, had previously been shown on TV with pixellation to obscure the graphic sex scenes.
A number of viewers have complained to Ofcom about the incident. The media regulator has said it is examining the complaints and will make a ruling in its next complaints bulletin.
Ofcom's programme code says "portrayals of sexual behaviour must be defensible in context".
Channel 4 has defended the screening saying the sex scenes in the art house movie are fleeting and in context with the images in the rest of the film.
The broadcaster also said there was an explicit warning about the film's graphic content before it aired.
"We have received no complaints since the film was broadcast," a Channel 4 spokesman added.
Channel 4 has recently come under fire from screening a live exorcism, and Derren Brown's 'Russian Roulette' programme last year prompted more than 100 viewer complaints.
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