According to the information you provided (delivery in September 13, 2009 and intercourse on October 14, 2008), you were supposed to have intercourse approximately mid-December, 2008 in order to conceive. If you know the date of conception you need to add one week and subtract three months to calculate the estimated date of delivery. Reverse the calculation to find out when you conceived if you know when you are supposed to have the delivery. The fact that you are 25th week pregnant now, also sets the approximate date of conception somewhere in the second half of December, 2009.
However, there is also a very small probability that the measurements were not okay(for example, the baby might be too small for its gestational age) and the only way to tell for sure who is the father of your child is the paternity test, which uses comparison of DNA both from the potential father and the baby.
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