You can't find a definite date for impregnation. However, if you are pregnated at all (since there might be other reasons for stopping a period), you can detrmine a range of dates most likely during which it occured. The beginning of the range is at least after seven days of your monthly in December because it is unlikely that you would ovulate again before that (the so-called safe "period principle" is based on this fact). The end of the range should be the days just prior to your expected date of January menstruation (was that 10th January?).
