Pete and Alisha Arnold, both 30, say they launched www.birthornot.com because they are unsure if they want to be parents.
They believe the controversial poll so the public could 'make a difference in the real world'.
The couple, from Minneapolis, have uploaded regular scan images of the foetus, which is a perfectly-healthy boy they have nicknamed 'Wiggles'.
The subject matter is so provocative that some are questioning whether the site has been set up deliberately to upset anti-abortionists.
However, the couple strenuously deny this.
Alisha is now 17 weeks pregnant and web users have until December 7 to cast their vote - just two days before the 20-week cut-off line for a state-legal abortion.
Mary Spaulding Balch, director of state legislation for National Right to Life, said: 'This is shocking.The first thing that came to my mind when I heard of this was the Roman Colosseum when the mob picked who lived and who died.
'They are talking about a baby that is 17 weeks old, it has a beating heart, its brain is working and nerve endings throughout its body.
'By 20 weeks when the vote is up it will be developed enough to feel excruciating pain. What they are doing is beyond comprehension'.
Eric Scheidler, executive director of the Chicago-based Pro-life action league, said: 'I am horrified at the depths to which they have sunk and their failure to comprehend the dignity and value of human life.
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