Chances of being pregnant ?

Ovulation takes place between 12 and 14 days before your next period. You should not measure it from your last period, as the time between your period and ovulation can vary but the time from ovulation to the next period is very constant. This is because it is the time it takes for the egg to move down the fallopian tubes after ovulation and them be excreted when the uterus finds out it is not fertilized. If you have an average cycle of 28 days it works out to 14 days after your last period started but if your cycle is not 28 days it is on another day.

If your last period ended on July 16 it probably started around July 12. For a 28 day cycle it should place ovulation around July 25. Only a pregnancy test in another 18 days will tell if your period goes missing.