Ultrasounds are used to date pregnancy. Chances are you may have gotten pregnant from sex while on your period. Possibly what you thought was a period was really pregnancy related spotting and you are actually about 2 weeks more pregnant then you thought! Go see a doctor!
If you want to find out if you are pregnant in general, take a pregnancy test. If you get a positive result, indicating that you are pregnant, you will need to go to the doctor. The doctor will be able to tell you exactly how long you have been pregnant, and you can figure out if it happened during yourperiod.
Chances of becoming pregnant on your period is rare. If your period is regular and if it is stopped coming month, there are rare chances of becoming pregnant.
Usually the symptom of pregnancy of a lady having a regular menstrual cycle is the stoppage of the menstruation. If you got an unexpected stoppage then you should conduct a urine test and if the urine test is positive then you can make sure that you are pregnant and to confirm it further you should visit a gynaecologist.
Taking a pregnancy test during the appropriate times, along with seeing your gynecologist to have a pelvic exam or ultra sound, are the only ways to truly test for pregnancy.