There are a few schools of thought about how to get the most accurate due date, but the most common method, Naegele’s Rule, is made by counting out 280 days from the start of your last menstrual period. Just like any due date calculator, this one isn’t exactly perfect since it makes the assumption that your cycle is 28 days, but it’s a fairly good place to start.
How a Due Date Calculator Works
Assuming your cycle is 28 days, that means your luteal phase, the time after ovulation, where an egg travels down the fallopian tube chancing an encounter a sperm, typically happens around day 12-14 of your cycle. This is your window of opportunity, and is the time frame where fertilization generally occurs. But for those who have longer or shorter luteal phases, conception can happen several days in either direction of this window, moving the due date accordingly.
Is There a More Accurate Way for Calculating the Due Date?
Yes and no. The Mittendorf-Williams technique predicts the average due date to be 288 days from your LMP, and uses 16 factors that may have an influence. Studies suggest it is twice as accurate as Naegel’s Rule, but again, all due dates are a rough estimate of when the baby will be here, rather than a hard and fast rule.
What If I’ve Had IVF or Know The Exact Moment of Conception?
Your doctor should be able to tell you when your babe is expected to make their arrival, but even knowing the exact moment of conception doesn’t guarantee a darn thing.
No matter what method you use for calculating your due date, only about 4% of babies are born on their actual due dates, and designating a birth month is a more accurate (albeit more annoying) way to think about things.