When Alien was released here in 1979 it was given an "X" rating under the old cinema classification system, but not because of its chest-bursting horror. It was due to censors thinking the alien eggs would be a bit too weird and sexy for the innocent kids of the day.
The censor notes pulled out of the BBFC's archives reveal concerns over the "dark undercurrent" running through the film, with the eggs and weird alien reproduction scenes described as being potentially "disturbing in a non-specific way" to teenage audience.
The censor added: "Occasionally the image is explicit as when the leathery egg opens up to reveal a glistening pulsating membrane which erupts into a squid-like creature," although it doesn't sound particularly sexy when described like that. [io9]