Hi, thanks for your question. Withdrawal is one method of contraception that is used world wide.  There are two considerations in the scenario you describe. 1. If your partner had ejaculated within a couple of hours of you having the sex you did, there can be some live sperm in his pre ejaculate that can then be introduced to your vagina when he puts his penis inside and you can get pregnant from this. 2. If any of the semen came in direct contact with the outside of your labia and vaginal opening then it is possible to become pregnant. If  this is the case, to prevent pregnancy you could take Plan B the emergency contraception. This is best taken within 24 hours or as soon as possible and can be taken up to 5 days after the risk. You can get Plan B free from any youth clinic or buy it over the counter at a BC pharmacy near you ( about $35) 

To read more about withdrawal as a contraceptive method see: www.optionsforsexualhealth.org/birth-control-pregnancy/birth-control-options/natural-methods/withdrawal