Hi, and thanks for your medical question on eHealth forum!
It is very difficult to predict the duration of pregnancy based on a urine test. If we consider Dec 11th as your last period date, then you conceived sometime toward end of Decâ25th or 26th or around that. This is most likely.
However, if you had an unusually light period, then considering the fact that some women do bleed on the day of their period in early pregnancy, you could have conceived about 14 days before Dec 11th. Hope this helps. Take care!