Gestational age can be calculated by using a woman's last menstrual period or by ultrasound (u/s). The LMP, when reliable is an accurate way to determine the age of a pregnancy. Ultrasound dating is equally if not more accurate when performed early in pregnancy (1st trimester). U/S dating becomes less accurate the farther along a pregnancy advances.
It is very common for pregnancies to advance beyond the due date. This is called a post-term pregnancy and specifically refers to pregnancies beyond 42 weeks (Normal is 40 weeks).
In the absence of an obstetric intervention, ~ 3-14% of pregnancies will go beyond 42 weeks and 2-7% beyond 43 weeks.
A woman can only conceive around the time of ovulation. However, this window of time may change especially in a woman who does not have a regular cycle.
In your case,your LMP dating and ultrasound dating are identical, so using your LMP dating is fine.
you always class the 1st day of being pregnant as your last period, the ultrasound date is pretty accurate but not 100% i wouldnt worry if your not sure. If you go over this date, it makes no difference to your conception date as it is fairly common for a first pregnancy to do this. you should be able to get pregnant anytime...but even so your conception date with be your last period after the event!!