Hi Anonymous and thanks for your question.
The most accurate way to determine paternity is through a paternity test after a baby is born.
At this time, the method the clinic most likely used was to look at your menstrual cycle and your most likely time of ovulation, which is when you would be most likely to become pregnant.
The highest risk of pregnancy is during the second and third weeks of the menstrual cycle. While a normal cycle is considered anywhere between 21 and 35 days, the average is about 28 days. Ovulation (the time when a woman is fertile) occurs mid month, this averages to around day 14 every month, give or take several days before and after. If a woman has unprotected sex around this time, she may get pregnant.
There is no reason to be ashamed. We hope you will seek the support you need to decide your next steps and move forward. We wish the best for you.