The age of a pregnancy is counted from the first day of your last menstruation. If your last period started on September 26th, it seems likely that you are about 13 weeks and 5 days pregnant on the 30th of December. The other option is that your last menstruation was not on the 26th of September, but on the 24th of October (the bleeding you experienced 3 days after sleeping with your boyfriend). If we conjecture that October 24th was the beginning of the last menstruation, then you would be 9 weeks and 5 days pregnant on the 30th of December.
An ultrasound scan does not calculate a pregnancy age according to the calendar (and thereby the last menstruation date)... but according to the embryo’s dimensions. Using an ultrasound, the embryo's dimensions are compared with statistically standard values and an approximate age is assigned. There is usually a difference between a pregnancy age calculated by the ultrasound apparatus and that calculated using the calendar.
Your ultrasound result (11 weeks and 1 day) is closer to 9weeks and 5days than to 13weeks and 5days. That’s why it seems more likely that your last menstruation was on the 24th of October rather than on the 26th of September. Therefore, we can guess that you probably conceived in the first half of November.
DISCLAIMER:
"Ask a Doctor" questions are answered by certified physicians and other medical professionals who volunteer
their time on eHealth Forum.
For more information about experts participating in the "Ask a Doctor" Network, please visit our
medical experts page.
You may also visit our Pregnancy , for moderated patient to patient support and information.
The information provided on eHealth Forum is designed to improve, not replace, the relationship between a patient and his/her own physician.
Personal consultation(s) with a qualified medical professional is the proper means for diagnosing any medical condition.
