Full term is usually anywhere between 38 and 42 weeks.
The good news is that all babies are born on their birthday Both the dating scans for my boys were out by a week. I think this is more because not every woman has a 40week pregnancy.
Any calculation of a conception date is an estimate, even in a woman with regular 28-day menstrual cycles.
Except for in-vitro fertilization (IVF) and other assisted reproductive procedures, you cannot precisely calculate the exact day a sperm meets an egg and fertilization (conception) occurs.
This is where our wonderful ultrasound machines can make a crucial difference.
However, you can use various factors and knowledge about reproduction to help narrow down the date to within one to five days if your cycles are regular.