From the AP:

NEW YORK — Bristol Palin and Levi Johnston say they’re engaged – and they haven’t told her parents yet.
Palin, the daughter of 2008 vice-presidential candidate and former Alaska Gov. Sarah Palin, is featured on the cover of Us Weekly with Johnston, holding their son, 18-month-old Tripp.
Bristol Palin tells the magazine that she finds the idea of telling her mother about her engagement “intimidating and scary.”

I would think so.