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If you had periods between 11th and 17th and have an average cycle of 30 days then you would have ovulated around the 16th day or 26th of Oct. So if you had sex until 28th of the month then you do cover the fertile period. Hence you can be pregnant. However, you can get a positive test only on or after the expected due date if you miss the period.
Also since you have irregular periods you need to go back a few months and calculate the average duration of your cycle. Count back 14 days from the first date of period to know the ovulation date. So 3-4 days before to 2-3 days after is the most fertile period. The day is counted from the first day of the period.
It would be good if you could consult a gynecologist. Hope this helps. Take care!
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