Dating: U/S vs. LMP

[deschutes]

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I'm on my Ob/Gyn rotation right now, and am having trouble with the rules governing dating (...pun intended 😛 )
Did a quick search and couldn't find any threads on the subject.

I know to stick with the EDC by U/S if it was done <12 weeks.
But what if the ultrasound is done >12 weeks?

I vaguely remember the number 10 being involved. So... if the EDC by U/S is within 10 days of the EDC by LMP, then stick with the LMP dates? Or do I have it backwards? 😕

Any help you can offer would be great!

 

[csmith1]

We were told this on my rotation (end disclaimer)...

1st trimester U/S can be off by 1 week.
2nd trimester by 2 weeks.
3rd trimester by 3 weeks.

Only change dates if LMP and U/S disagree by more than the time allowed in each of the trimester.

i.e.-- second trimester U/S disagrees with EDC gathered from LMP by 16 days, if all is okay with the pregnancy, you can change the date based on the U/S.

 

[deschutes]

1 week, 2 weeks and 3 weeks? Haha! I was told 5 days, 10 days and etc. I did not think fetuses only grew Monday to Friday...! 😉

Also, re: applying Naegele's Rule (sp?) what happens when the woman's cycle is not 28 days -- do you +/- n days depending on whether her cycle is 28 days +/- n?

 

[Bookworm]

1 week, 2 weeks and 3 weeks? Haha! I was told 5 days, 10 days and etc. I did not think fetuses only grew Monday to Friday...! 😉

Also, re: applying Naegele's Rule (sp?) what happens when the woman's cycle is not 28 days -- do you +/- n days depending on whether her cycle is 28 days +/- n?

do any of you have a source for the ultrasound adjustment for edd? I have been trying to look it up for the last 1/2 hour with no luck. Where should I look?

 

[Diane L. Evans]

Source Creasy Resnik High Risk MFM

1st ultrasound is most accurate. If +/- 4 days of LMP then use LMP.
Ist trimester off by 4 days
2nd trimester off approx 8 days

look at fundal height.

2/3 trimester U/s can be off in IUGR or DM cases

 

[Bookworm]

Source Creasy Resnik High Risk MFM

1st ultrasound is most accurate. If +/- 4 days of LMP then use LMP.
Ist trimester off by 4 days
2nd trimester off approx 8 days

look at fundal height.

2/3 trimester U/s can be off in IUGR or DM cases

Thank you for this info...So how do you correct LMP if U/S doesn't match? If you have a 2nd trimester U/S and it is of by say 10 days, then would you then go by the U/S due date? What is the accurate adjustment forumula?

 

[Diane L. Evans]

if 2nd trimester u/s not equal to LMP then use 2nd trimester u/s. if 3rd trimester off by 3+ weeks from LMP then rpt u/s in 3 weeks and avg those to get due date if 3rd trimester is dating and you plan to induce for GDM IUGR etc then an amniocentesis is needed to document FLM (fetal lung maturity)