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If exactly you had 27 days regular menstrual cycle, then there will be no chance of getting pregnant. Even if you had unprotected sex or even ejaculation was done at the time of intercourse.
For better understanding, ovulation will occur exactly 14 days before menstruation. The fertile period will be exactly during that week of ovulation.
So, you do not need to worry much.
If you had complaints of irregular menstruation then this will not count.
Then better to go for blood investigation to confirm.