Does Levi's Story Add Up?

Gryphen casts doubt on Levi Johnston's claim that Sarah Palin nagged him and Bristol to allow the governor to fake a pregnancy for Tripp as a way to keep the baby and save her daughter's reputation. But how, Gryphen asks, could that be possible given the timeline of both Trig's and Tripp's births? Good question - and the first real dent in Johnston's credibility.