The app, which went live yesterday morning and has already skyrocketed to number one in the entertainment category in the Australian app store enables users to "impregnate anyone you meet simply by taking a photo of them, marking up the image and pressing a button".

"Knocked App means you can meet someone and conceive with them with just the press of a button," it boasts.

It digitally alters images to give subjects the appearance of being nine months' pregnant.

Angus Mullane, from Appy Dude, who developed the app — the Sydney start-up's first — says already hundreds of users are posting pregnant photos of themselves on Facebook to "scare" their friends and family.

"We really just wanted to give people a laugh. It's had an amazing response," he said. "It's all over Facebook with teenage girls scaring their mums and stuff like that."

"It's really just about getting girls to have a laugh and see what they're going to look like with a bump and also kind of sending it to their mums and saying 'whoops'."

However, family planning groups have called for the app to be withdrawn from sale.

"I think it's absolutely dreadful," says Joanne Ramadge, chief executive officer of Sexual Health and Family Planning Australia.

"Teenage pregnancy is an extremely serious issue and to treat it in such a trivial way is not a considered or helpful approach ... Teenage pregnancy causes quite a lot of distress and problems for young people and their families and it should not be treated in such a light-hearted way."

Ms Ramadge said she was also concerned it could be used as a form of cyber-bullying.

"For someone who is out to bully this could be very easily used to make someone look as if they're pregnant and dispersed in a way that could be quite horrifying and devastating for some people."

Mullane defended the app as "tongue-in-cheek" though admitted it could attract some controversy.

"We definitely knew that some people would dislike it and some people would love it," he said.

He says Apple took two months to approve the app before it finally went live yesterday. He believes the average time for an app to be approved by Apple is 10 days. The review process includes ensuring the app meets specific guidelines around classifications such as offensive language and nudity.

Mullane compares the app to the controversial Ugly Meter — an app which snaps your picture, scans your face, then pronounces your ugliness on a scale of 1 to 10. The app has been criticised for its potential to damage self-esteem and aggravate bullying.

"It's the same kind of thing as Ugly Meter but it's just a joke and a bit of fun. It shouldn't be taken too seriously."