Hello and thank your for using HCM for your questions! Short answer: You do not need to worry. Detailed answer: Your story is very common for many couples engaging in sexual relationship. If we do the math (supposing your girlfriend has a regular 28 days cycle) then you had sex in a pretty safe period of the menstrual cycle (previous cycle must have started by 25th of November, her mid-period should have been by 9th of December and taking +-3 days when one should avoid unprotected sex it means that you should have avoid it between 6-12 of December) and moreover you used a condom. Given this information from your side, I would say that it is highly improbable that your girlfriend is pregnant. When I make this statement I rely more on the fact that you used condom than based on the math of menstrual cycle, because maybe your girlfriend has a longer cycle and all calculations will differ. Please note that condom is safe in 99% of cases, as it depends on how you use it, it could break, etc. But I don t think this is the case. The delayed period could be due to stress. You could use a quick pregnancy test, which is safe and cheap, in order to be sure. But I am confident that your partner is not pregnant. I hope I answered your question and good luck!