Thanks for posting your query.
Congratulations, you are pregnant and expected date of delivery is counted from first date of last menstrual period plus 7 days.
When you got periods on 12th Dec
2012, there might be possibility of ovulation on 26th Dec 2012 and this ova got fertilized and pregnancy
Now the uterine cavity
is occupied by developing fetus and the growing process is continuing ;thats why this month you don't get periods again.
You should consult with gynecologist and should go for thorough check up.
You should also go for ultrasound
imaging for confirmation of pregnancy and its intrauterine location.
Dr. Mayank Bhargava