I can't tell you the percentage of your specific chances. A healthy couple with no known reproductive issues and ideal timing only have about a 25% chance per cycle of getting pregnant. The "withdrawl" method is the least effective of the birth control methods out there. If you guy want to "go bare" then you finding out what your personal reproductive cycle is will make things so much easier. You'd know at what point you need to avoid intercourse (or to use protection) and what days you are "safe".
Ok back to your dates. Since I don't know when you ovulate I am going to go with the textbook answer of cycle day 14.
A woman's fertile window is only five days (three days before, the day of and the day after ovulation) so going with text book that would make days 11-15 your fertile window.
Day 15 was 19August so that session(s) would be in that fertile window.
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