Based on your last period being around the week of December 9th and assuming a regular menstrual cycle, it is likely that conception occurred approximately two weeks after your last period. This would put the estimated time of conception around the end of December or early January. However, it's important to note that this is just an estimation and individual factors can vary. To get a more accurate determination of conception, you may want to consult with your healthcare provider who can consider additional factors such as ultrasound measurements.