Hello...sarah.my period started on 8th October...

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Regarding your question

How to avoid pregnancy after an unprotected intercourse ?

You should have used the  Emergency contraception pills which are the best solution in your case.
According to the recommendations of the World Health Organization (WHO), in this case, one of the following is taken:
1) Levonorgestrel 1.5mg: one time
Provided that the drug is taken within 5 days of the date of intercourse. The efficacy of this contraceptive drug is 55% -98%. According to the date of taking the medicine.
The more the medication is taken after a shorter period of intercourse, the higher its efficacy. Meaning that if it was taken after hours of intercourse better than taking it after days of intercourse.
2) Ulipristal acetate 30mg for one time:
And the efficiency of this drug to 98% to prevent pregnancy if taken within 3 days of intercourse and not after 3 days.
These drugs prevent pregnancy by preventing them from fertilizing the sperm from the sperm, or preventing them from exiting the egg from the ovary.
What is the probability of pregnancy if intercourse took place after 7 days of period starts ?
 the probability is low because the most fertile window in the menestrual cycle is between day 10-20 , but you can't tell if you might get pregnant or not even if it's slightly possible
 
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