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Only if semen or pre-cum comes in direct contact with vagina, pregnancy can result. Oral sex usually does not result in pregnancy, unless the semen or pre-cum comes accidentally in contact with vagina. If you are not planning for pregnancy, consider visiting a gynecologist for further assistance to get your partner started on suitable regular birth control measures. She should be regular with its use to have the benefits and avoid pregnancy. Avoid having unprotected sexual contact.
Take care.