If you have a typical 28 day cycle your most likely time of ovulation would be around the 19th sex on the 15th and 17th does indeed leave you open to pregnancy then,
however also for 36 hours after ovulation.
So no hun with the information you gave we can not guess who the father is.
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what is it with people sleeping around with "other guys".
Has no-one heard of fidelity or self worth?