Calculating due dates is no easy feat, and is a slightly controversial topic. But usually, due dates are calculated by finding the first day of your last period and adding 40 weeks to that.
The major caveat with due dates is that it’s all based on a 28-day menstrual cycle. Various studies have found that only about 10-20% of women have 28-day cycles, which means that due dates might be a bit off for a very large number of women.
What’s more, most pregnancies don’t last exactly 40 weeks - they last somewhere between 37 and 42 weeks (nice and precise, eh). In the end, less than 5% of people give birth on their due dates.
The bottom line? Due dates are really handy for knowing roughly when your baby will arrive so, you know, you’re not on a flight across the world when your waters break. But they should always be taken with a pinch of salt.