It's called human chimerism http://news.yahoo.com/for-yahoo-health-dad-learns-that-unborn-twin-162421331.html
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So the story writes as if they were doing some sort of artificial insemination or something. Imagine if they did it the "old-fashioned" way, and then he finds out it's not his baby. Talk about some major trust issues popping up.
That would be terrible. If it happened to me, and we didn't do the research to find out it was a unborm twin (how easy is that to find out?, not too easy I would imagine), I'd have broken up with my partner and she'd be left to raise a baby alone... Yikes.