It's impossible to ovulate 1 week before your period and get pregnant. That would mean you have a SEVERE luteal phase defect and would not be able to substain a pregnancy.
All women not matter what their periods are like ovulate 11-17 days BEFORE their period. The luteal phase is UNCHANGING. It is RARE, for a luteal phase to change from month to month and usually only happens happens when there is a hormone deficancy.
It is, however, perfectly normal to ovulate on different days of your cycle because the follicular phase CAN and does change for many women.
It is also nearly impossible to actually "get pregnant on your period", HOWEVER if you typically have a short cycle or have occational short cycles you could get pregnant from intercourse the week of your period since the sperm can live inside you for up to a week. If you have a short cycle and ovulate RIGHT after your period ends you can get pregnant.
at 7:58 PM on Dec. 27, 2010