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Yes, chances of pregnancy are high, when you have unprotected intercourse, in the fertile period of the cycle.
In case a woman has a 28 day cycle, the unsafe or fertile days are from day 12 to day 16, during which time ovulation occurs and chances of pregnancy are high. Sex out of this period is safe and with less chances of pregnancy.
Men who use the pull out method must be able to know when they are reaching the point in sexual excitement when ejaculation can no longer be stopped or postponed. If you cannot predict this moment accurately, withdrawal will not be as effective. The pull-out method, is therefore not fool proof, and has a very high failure rate (27%), as a contraceptive method. Hence you should always use protection always to prevent pregnancy.
The best way to confirm pregnancy is with a urine pregnancy test, which can be done at 7 days after the missed period to have a highly accurate result.
In case, the test is positive, it would confirm pregnancy. In such a situation, you and your girlfriend should consult with a doctor for proper counseling and advice.
I hope this helps.