The Australian-developed iPhone app allows users to mark up the image by simply pressing a button.
The app, which has already skyrocketed to number one in the entertainment category in the Australian stores, digitally alters images to give subjects the appearance of being nine months' pregnant.
According to Angus Mullane, from Appy Dude, who developed the app, hundreds of users are already posting pregnant photos of themselves on Facebook to "scare" their friends and family, the 'Sydney Morning Herald' reported.
"Knocked App means you can meet someone and conceive with them with just the press of a button," the app boasts.
"We really just wanted to give people a laugh. It's had an amazing response. It's all over Facebook with teenage girls scaring their mums and stuff like that," Mullane said.
"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," he added.
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But family planning groups have called for the app to be withdrawn from sale.
"I think it's absolutely dreadful," said 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," she noted.
Ramadge cautioned that 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," she stated.
Though Mullane defended the app as "tongue-in-cheek," he admitted it could attract some controversy.
"We definitely knew that some people would dislike it and some people would love it," he said.