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If you have had sex with your girlfriend on the last day of her menses, then she was in her safe period and ovulation had not occurred . so even if you would have ejaculated inside her, the chances of pregnancy are minimal. As sperms can stay alive for 72 hours , but ovulation would occur by day 14-16 in a 28-30 day cycle, so she is safe.
I hope i have answered your query in detail.
wishing you health ,
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