According to US Magazine, Bristol Palin has revealed the circumstances of her child's conception:
When Bristol Palin lost her virginity, it wasn't the romantic experience she had envisioned.
In her new autobiography, Not Afraid of Life: My Journey So Far, the 20-year-old single mom reveals that, while drunk for the very first time, she lost her virginity to Levi Johnston during a camping trip.
Palin says she woke up alone in her tent, with no recollection as to what happened. Johnston, meanwhile, "talked with his friends on the other side of the canvas."
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When she confronted him about their sexual encounter, Johnston, now 21, told Palin what she wanted to hear: they wouldn't have sex again until they were married.
But, Palin writes, they became intimate again shortly after. When she learned she was pregnant in 2008, Palin was on birth control pills prescribed to treat her cramps; it took eight home pregnancy tests to convince her that she was expecting. ...
This revelation makes the book interesting. It will be worth reading to find out whether any context is provided which would motivate the inclusion of this information.
The AP article linked-to by US Magazine looked a little bogus -- The Ponca City News: Our AP Features -- but I did find what looks like a more genuine AP article, "Bristol Palin calls her ex 'the gnat' in new book."
Personally, I don't think Bristol Palin should do penance for becoming pregnant, and I don't fault her for seizing the opportunities that have come her way.
Update: The New York Post has another story, "Levi? Puke! Bristol Palin reveals the joy of teen love." That story differs from the AP story in some ways. Perhaps what the AP reported isn't a story of conception, but rather her story of the loss of her virginity. It's not something that can be clarified without reading the book.