American Sniper just had a blisteringly successful opening weekend in the UK, and its $105 million (£69m) US box office takings means Clint Eastwood's film is likely to be the highest grossing war film of all time. But people have begun to notice something suspicious in the Iraq-based movie: plastic babies.

There's a scene in the Best Picture-nominee in which real-life Sniper Chris Kyle (Bradley Cooper) and his wife Tyra (Sienna Miller), pass the baby to eachother during a coversation. Problem is, the baby is quite clearly a doll.

Telegraph film critic Robbie Collin chose not to include the plastic babies in his review, but did notice them, saying: "they’re conspicuously wobbling rather than moving, which makes the crying sound effects seem a bit eerily detached."

Meanwhile, in The Sunday Times, Camilla Long writes: "I have never seen so many terrible fake babies in one film."

Back in December movie site HitFlix picked up on plastic-babygate, saying: "It's so obvious, and neither one of them looks like they are comfortable holding it. The weight's all wrong, and it shows from the way they have to try to liven it up with their own body language. Cooper in particular looks like he's just plain never held a baby." The writer, Drew McWeeny, speculates that maybe Eastwood was so keen to sign off on the film he didn't want to worry himself about the terrifying robot baby.

NewsOK's review read: "Cooper and Miller deliver the performance of their careers during an argument over what's supposed to be their infant daughter, but instead end up cradling a jarringly fake plastic baby and trying to pretend it's the real thing. There's just no excuse for that kind of bush-league nonsense."

It's so obvious - the weight's all wrong. Cooper in particular looks like he's just plain never held a baby

Plastic babies have been a hot topic of conversation among Reddit users, who have agreed that the doll looks, in no particular order: "super fake", "like a Cabbage Patch Kid", "like it weighed less than a pound", and "really flimsy". One user summed it up simply, with the words: "FAKE RUBBER BABY".

But mostly, people are curious as to why the plastic babies are there at all.

'It's like a Cabbage Patch kid'

There are restrictions on filming babies. In California, where shooting for American Sniper took place, infants must be at least 15 days old and have a work permit and a doctor's note to start working. Day rate can range from around $150 to nearer $800, depending on whether the baby is a background or principal actor, and infants under six months can't work for longer than 20 minutes per day. Then there's the obligatory nurse, studio teacher and parent to be accommodated for.

Budget had been an issue for Steven Spielberg, who was the first director for American Sniper, but dropped out when he felt he couldn't make the film he wanted with a mere $60 million.

Perhaps Eastwood felt that he could cut some costs by using a doll. HitFlix writer Drew McWeeny, speculates that maybe Eastwood was so keen to sign off on the film he didn't want to worry himself about the terrifying robot baby.

Reddit users have suggested that Cooper has a "crippling fear of babies", alternatively that "Chris Kyle really liked dolls" or that "Eastwood got tired of yelling at the baby actor maybe."

Film writer Shawn Levy has written a plea for us all on Twitter:

The plastic baby mystery remains unsolved, but their creepy presence is captured on film for all to see, for eternity.

UPDATE: Plastic baby mystery potentially solved

Since the world's press has woken up to the mystery of American Sniper's plastic babies, some form of explanation has emerged from Jason Hall, the film's screenwriter and executive producer.

Back in December, he responded to journalist Mark Harris' tweet that "That plastic baby is going to be rationalised by Eastwood auteur cultists until the end of days."

Hall replied: "hate to ruin the fun but real baby #1 showed up with a fever. Real baby #2 was no show. (Clint voice) Gimme the doll, kid."

However, the plot thickens: this tweet was reported yesterday in The Hollywood Reporter, but appears to have been deleted from Hall's feed since. The Hollywood Reporter were also unable to get a response from the film's representative.

Where was Real Baby #2? Did Real Baby #1's fever go away? We're still after answers.