Hello, and I hope I can help you today. First trimester ultrasound is the most accurate way to date for pregnancy. However, most ultrasounds calculate a conception date estimating an average woman who has a 28 day cycle. There is also a window of time where conception is likely to take place, which in your case, could be anywhere from January 17 through the 31st. I am calculating that counting back from your due date because the measurement of the fetus in early pregnancy by ultrasound can correctly predict the date of conception within five days for after it may have occurred. By convention, we generally do not change the due date for pregnancy unless it is greater than five days off from what is predicted based on the woman s last known menstrual cycle. So in summary, it is most likely your ex-boyfriend is the father of your child. I hope I was adequately able to answer your question today, and that this information was helpful. Best wishes and good luck, Dr. Brown.