Pregnancy Conception Date: Estimating Based on Cycle & Medication

Question
I was hoping you could help me figure out which day is the most likely that I conceived. I am concerned about this because I have bipolar disorder and panic disorder and was taking a lot of medicine at the time including Lithium Carb and Xanax. I am currently 21 weeks pregnant. The first day of my last MP was Dec 20. On Dec. 22 I was still spotting a little. I had intercourse on the night of the 22nd. The next time I had intercourse was between Dec 26 and 30. My husband did not ejaculate in me when we had sex on the 22nd but did sometime between the 26 and 30. Last time I got pregnant it was right after I finished my period...like the very next day. So far, I've had two ultrasounds. One at eight weeks said that the day of conception was Dec. 25. One at seventeen weeks said I conceived on Dec 23. Knowing all of this, when did I most likely conceive? I hope this wasn't too confusing!  
Regards, Jessica

Answer
Hello Jessica. You cannot possibly conceive that close to your period- it takes 14 days for your ovaries to ripen an egg after your period starts. You conceived this baby at the very earliest on or about Jan. 3. Ultrasound is a very poor predictor of due dates etc and should not be relied upon for such info.Hope that helps- Cheri.