Thank you for your query at AskTheDoctor.com
I understand your concern.
For pregnancy to occur, intercourse is mandatory with ejaculation inside the female vagina. Rubbing pre-cum on the vulva can't lead to pregnancy.
Ejaculation inside a female vagina causes the semen to ascend up due to the vaginal wall contractions. If there is a viable ovum ( during ovulation ), this sperm tries fusing with the ovum to cause fertilization and possible pregnancy.
Hence, there is no way pre-cum can lead to pregnancy.
Hope this was helpful,
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