Dating question... how far along am I?

[mollsworth]

Okay, many of you ladies are so well-versed, I would love a lesson!

LMP: May 31

Suspected ovulation: June 17-18 (using saliva monitoring/ferning)

Cycles typically around 30-31 days

EDD: March 10

It was my first month really monitoring anything, so I didn't have time to gain expertise. By my calculations, I am now about 9 weeks. 

At our ultrasound 10 days ago, baby measured 7w6d. So now presumably would be about 9w2d...

Had our first MW appt today (which was FANTASTIC!) and using an EDD of March 10, she put me at 8w4d. Huh?

I asked her about it and she just said 'everyone calculates a little differently'...perplexed! Help? 

[Brisen]

I'm not an expert, but I've always heard that the date they get with an u/s trumps any other date, and that's what my care providers have done.  But counting backwards from March 10, you would be at 9 weeks on Aug 6.  To figure out your due date from your u/s, you could start at when you would be 8 weeks and count forward to 40 weeks.  There is probably an easier way than that, but I always just open up the calendar and count weeks.

I don't see how you can calculate it differently, though.  Unless it's a leap year and someone forgets Feb 29, but that would only be one day off.

[JMJ]

There are several different ways to calculate due dates.  For example, most OB's use your LMP and assume that you are 0 weeks on the first day of your last period and 40 weeks when you give birth.  Some more accurate methods guess O date and assume that you are 2 weeks on your O date.  (This is often done with CM that you are assumed to be 2 weeks on Peak day.)  The Prem rule uses temperature rise, and your due date is first day of temperature rise minus 7 + 9 months.  Depending on how long the months are, you might be expected to be pregnant a little longer or a little shorter, but due to the average unassisted pregnancy (not UC, just not induced in any way) going a little longer, it's more accurate than most other methods.

[Brisen]

But it doesn't sound like the MW is talking about the different ways of figuring out what the due date is -- it sounds like the MW is giving a different week & day for how far along Molls is now based on the same due date, March 10.  The mw says 8w4d, but if you count backwards from her due date, she was 8w6d.  That's the part that I don't see how you can just calculate it differently, unless the MW isn't using 40 weeks = the due date = March 10.

[Cellist]

Could it be that your midwife is trying to give you extra time in case you go over your due date so you don't have pressure to get a c-section for no reason?  That's about all I can imagine she's doing...unless she's figuring a different O date for you or something...

[RachaelsMommy]

I don't understand how it could be calculated differently using the same due date. The 9 weeks 2 days the u/s is giving you sounds spot on with your EDD.

[mollsworth]

Thanks for all the responses everyone! I'm not sure how she did it either, but I had mentioned to her that I was concerned about being forced into a c-section by going over since my family has a history of needing a long time to cook. Maybe it was her sly way of giving me extra time. :) I'll let her stick with that due date, and I'll stick with the u/s date.

Originally Posted by Cellist 

Could it be that your midwife is trying to give you extra time in case you go over your due date so you don't have pressure to get a c-section for no reason?  That's about all I can imagine she's doing...unless she's figuring a different O date for you or something...