OK - I was trying to channel the "Coffee Tawk" lady there with the title, in case you didn't get that.
Most of humanity accepts the notion that we gestate for 40 weeks...but is that really true? Not necessarily.
Check out this article, The Lie of the EDD: Why Your Due Date Isn't when You Think
Here's just a smidge:
...There is one glaring flaw in Naegele's rule. Strictly speaking, a lunar (or synodic - from new moon to new moon) month is actually 29.53 days, which makes 10 lunar months roughly 295 days, a full 15 days longer than the 280 days gestation we've been lead to believe is average. In fact, if left alone, 50-80% of mothers will gestate beyond 40 weeks...
Newer research gives a better estimate:
...To easily calculate this EDD formula, a nullipara would take the LMP, subtract 3 months, then add 15 days. Multiparas start with LMP, subtract 3 months and add 10 days...
And of course there are variations in cycle length & inaccuracies of ultrasound, both of which are discussed. Good stuff & nice to have some facts for clients to discuss with their care providers when needed.