I was on birth control in December and missed several days of taking the pill around the time I had unprotected sex. My last period was December 4th and today is January 30th. What's the chance of being pregnant ?

Pretty good. Have you taken an HPT?

About 45 percent.

First even taking the birth control pills, you can become pregnant. BC is not 100 percent. Why not use condoms if you knew you missed pills?
But missing pills make them even more ineffective.

After you are 2 week late, take a birth control test.

**by the way, depending on dates of ovulation, and just luck will determine pregnancy, there is no real way to know a percentage.