Our expert says:
Not really a psychiatric question, but I'll have a go. Yes. It only needs one sperm. SO it is very possible in the way you describe, that some sperm will be left in place, and when you "start gain", they could be transferred back into you, and make their way to causing pregnancy. Even if he withdraws before orgasm, there can be sperm in the precum that he produces earlier, and that can be enough to achieve pregnancy.
Basically, you're describing a form of Coitus Interruptus, which is well knwon to be capable of causing pregnancies.
Of course see your doctor or clinic to check whether you are indeed pregnant, and get better information about available methods of contraception.
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