Pregnancy Concerns & History of Substance Use: Seeking Guidance

Question
I am 26 years old and expeting my first child. My estimated due date is December 9, 2005. I have battled a crack cocaine addiction for about 3 years. On March 1, 2005 I was unfaithful to my boyfriend of 5 years due to being high on the drug. I was about three days into my period at this time and also used a condom.I have gotten help for my addiction and been sober ever since that night. I had intercourse with my boyfriend on March 12 and 17 2005. I had a positive pregnancy test on April 4, 2005. Today is September 30, 2005 and I am 30 weeks pregnant based on an ultrasound I had done in May. I feel awful about what I have done and am worried that this baby may not be my boyfriend's. So my question is do you think this baby was concieved at the time I cheated, which was March 1.

Answer
If a sonogram showed that you are 30 weeks pregnant on September 30, your last period must have been March 3 and you would have ovulated on March 16. You could have become pregnant if you had sex on March 15,16,17, ot 18. Therefore, your boyfriend is the father. That would make your due date around December 9. You could not have conceive on March 1 because you had not ovulated yet.