"There are many variables involved in calculating a due date. There are all sorts of reasons that your due date might not have been ideal."

what if i knew the exact day of conception though?
Babies don't know the time, and they don't read a calendar. Growth in utero is very much the same as growth outside the womb. For instance - all babies grow teeth. They all grow their teeth at around the same time. But they don't grow them at exactly the same time.

Gestation is like growing teeth. All babies are ready to be born eventually, and at this point, labour will begin. This process takes about 40 weeks. But it doesn't take exactly 40 weeks. Some babies take 42 weeks. Some babies, honestly, take longer. Even at 42 weeks, the majority of fluid scans will show a normally-functioning placenta.

If you would like to look up research the examines due dates, look up the Mittendorf-Williams Rule. Instead of 40 weeks, they take into account a variety of factors, such as the cultural (and therefore, genetic) background of the parents, whether this is first or subsequent labour, and others, and from this calculate a due date that is often later than the Nagele's Rule date (the 40 week ruke).