Your due date is always dated back to your last period ,,, so if period came on 1st June but not on the 1st July you would on the date your period is due be classed as aprox 4weeks. So it works out perfect for you, hope this helps.
You got pregnant from the sex after your period.
When a doctor tells you a gestational age (GA) count, he or she begins the count on Day 1 of your period, or assumed day 1 if you don't know when it was. (An ultrasound will do this computation also, the same way using the GA count.) The doc does not think you got pregnant on the first day of bleeding, but that is the traditional day the count begins because it is a visible signal. Back before ultrasounds existed, it was the only reliable place to start the clock. The count goes like this: Day 1 first day of bleeding, day 14 assumed to be day of conception, day 280 estimated due date. It's two weeks for period to conception and 266 days from conception to full term, for a 280-day gestational age.
Sperm can live up to five days in your system after having sex, some research says longer than that. So you could have conceived several days after you had the sex.
As far as I know pregnency start wiv first day of ur last period so when u missed ur perid u alredy 4/5wks preggo hope it help x