If the first day of your last period was on the 24th of July, then you can’t be pregnant from sexual intercourse that occured before that date. The age of a pregnancy is calculated not from the date of conception, but from the first day of your last period (usually about 2 week before conception). If you count your pregnancy age from the first date of your last period, your pregnancy age was about 8 weeks and 2 days on the 19th of September. The ultrasound indicated an age of 7.5 weeks. Obviously, there is a difference between the age determined according to the last menstruation and that determined by the ultrasound. That is because the ultrasound doesn’t count the days but measures the embryo’s dimensions and compares them with the standard dimensions for an average pregnancy. Some women don’t remember when their last menstruation happened and then an ultrasound examination is the only way to assume the age of a fetus. A difference of less than 1 week (like in your case) is not a problem.
The embryo’s heartbeat can’t be heard before the 7th week of pregnancy.