progesterone hormone shots and posterm pregnancy

This is my 5th pregnancy and will be my 3rd child.  My first I started dialating and having contractions @ 30 weeks, she held off until 36 weeks, my 2nd @ 32 weeks my placenta ruptured for unknown reasons. 1 year later i had an ecotopic pregnancy and in 2005 i had an abortion, because my doctor told me that the pregnancy would likely end up a miscarriage because i had already started bleeding @ 7 weeks.  in Sep. 2007 my husband and i found out i'm pregnant, this time my doctor decided to start me on progesterone hormone shots @ 16 weeks until i reached 34 weeks and stopped taking them.  I am now 41 weeks and have been 5 centimeters dialated for a month now.  Do you think these shots could have anything to do with this? I had trouble finding info/studies/research on this drug being used during pregnancy and am very curious.  my docs say that it is completely safe and there are no known side effects.