Congratulations Betty, there are so many of us on Answerbank who are currently pregnant or have recently given birth.
The way pregnancy is worked out, is from the first day of your last period, although generally conception occurs around two weeks later .
Therefore, if the first day of your last period was on 20th December you would be 5 weeks plus 3 days into pregnancy, although the actual age of your Foetus would be around 3 weeks.
I always find this site interesting, as it has pregnacy calculators, take a look.
http://baby2see.com/development/week5.html