Oh honey. First, if it an honest question, you are much too young, and too uneducated sexually to be preforming oral sex.
Second, even if semen could enter your womb through your stomach, the acid in your stomach would destroy all traces of potency.
Third, really honestly contemplate your knowledge on this subject, and don't do anything before you are both ready and prepared (As in think about taking steps to prevent pregnancy before you start having sex) What -some- people have said is false. You can't just get pregnant by having a man ejaculate inside of you. (One sperm can be enough to get you pregnant, even pre-cum is enough!) For example, if there is pre-cum (or sperm) on his hands, and he then masturbates you, you can also become pregnant that way( Even if you are a virgin! This is a huge misconception, the hymen is a layer of tissue, but it does not filter out sperm). Sperm is live (potent) in the vagina for about six hours, however if it collects within the cervix, anywhere from between 3 and 7 days.