I agree with AnnieBrooke.....sounds like the baby is the new bf's given the timeframe.
Get a DNA test done.
not trying to be mean but you may have to go get a dna test done. I tried to figure it out for you but it kinda runs together. sorry and good luck
Here is from a conception calculator I checked:
First Day of Last Menstrual Period: September 10, 2011
Probable Date of Ovulation: September 24, 2011
Possible Dates of Conception: September 20 to September 28, 2011
Due Date: June 16, 2012 (40 weeks)
Sounds much likelier to be the new boyfriend's baby than the old boyfriend's baby. Get a DNA test, I assume the baby is here by now?