Due Date Calculation Clarification?

[Lizzie9984]

Ok, can someone help clarify something for me real quick? I calculated my EDD with the LMP formula, meaning with that, I'm 42 weeks and 4 days. BUT, that formula counts from the start of the last period, right? So there's a week there for the period length that shouldn't even be counted....Which, if I were to discount that week and count from the END of the period through now, would put me approximately at 41 weeks and 4 days maximum, right?

Just wanted to know if I got the gist of that right, I want to try to get the gestational time as close as possible to accurate, which is hard to do based on ovulation, as my cycles vary widely between 23 and 35 days in length....

And yes, I'm trying to make myself feel better and avoid worrying about taking extra time to cook the babe...

[Undercover Hippie]

Long answer...

Yes, the traditional formula (Naegele's rule) used in the US takes the first day of your last period and adds 40 weeks to it (or subtracts 3 months then adds a week, which ends up being 40 weeks). The 40 week calculation assumes that most people ovulate 14 days after their period starts and they count that 2 weeks (including your period) as part of the pregnancy even though you're not really pregnant yet.

Following that same logic, if a woman knows she didn't ovulate on day 14, she can take her day of ovulation and add 38 weeks to it and get her EDD.

Then there is another rule called Nichol's rule that is based more on statistical evidence of how long real pregnancies last. It goes like this:

1st time moms with 28 day cycles: LMP + 12 months - 2 mos, 14 days = EDD
2nd time moms with 28 day cycles: LMP = 12 months - 2 mos, 18 days = EDD
Cycles longer than 28 days, add the extra days to the EDD.
Cycles shorter than 28 days, subtract the extra days from the EDD.

Whatever formula you use, if you have irregular cycles you're absolutely right to think your EDD might be wrong and you might not be as overdue as you've been thinking... you probably don't ovulate on day 14. But when you don't know when you O'ed or how long your cycle should have been it gets tricky to guess the EDD. Early (first trimester) u/s are pretty userful here if you happened to have one. Or if you started testing for PG pretty early, or got a -- test before your first ++ test, that can help you figure your ovulation date b/c you generally can't get a positive until at least 10-11 days after you ovulated. You can count back and guesstimate when you O'ed.

Clear as mud?