Ultrasound due dates

[tri31]

So I am confused, and I'm hoping someone can straighten me out. I was looking through my checkbook and came across the entry for my pregnancy test in July. I counted back 40 weeks from the ultrasound date and discovered that I would have only been 3 dpo. Am I calculating wrong? Have any of you had the ultrasound dates off by weeks?

[Monkey Keeper]

I may be a bit confused, but the following is based on assuming from what you wrote that your u/s estimated due date means that you would have gotten a BFP at 3DPO.

When was the u/s done? That makes a difference, I think.

And you could have gotten a BFP as soon as 9DPO (sometimes earlier), so if the u/s was of a few days then you'd be within the margin of error. For me, I used a fertility monitor and charted, so I knew my dates spot on. At 7w3d, baby was measuring 3 days ahead. In your case, that would make it close.

[tri31]

It was the 20 week scan. I have never gotten a early positive (this is my seventh pregnancy). My first day of my lmp was 6-16-09. They moved my due date after the scan. I know it is just a estimate. Still I can't help but wonder which is the better guess.

[Monkey Keeper]

I just found an article that states that u/s EDDs have a margin of error of 1.2 weeks. If I were you, I'd push for going with your LMP date. That's a decent difference, and I wouldn't want to be pushed into inducing early when the dates are suspect.

[MaerynPearl]

yeah at 20 weeks they can easily be off by over a week.

I went in today at only 9 weeks and measured dead on but... its only 9 weeks.

[honeybee]

Ideally, the due date is just a best guess and it wouldn't matter. But, if you are having a hospital birth, you need to get every day of grace period you possibly can before they start hassling you about inductions. Yes, you can say no, but who wants to go through the stress and hassle?

I would absolutely not let them use a 20 week u/s to move you due date earlier. Take your calendar in to your doc and show him the date you had the bfp. There's no way you would have tested positive on 3 days po. Even an OB should be able to acknowledge that.

[MiaMama]

I requested and read my detailed US report. It was done at 18w.

They measure various parts of the baby and their computer comes up with an estimate. They measure femur length, head circumference, biparietal diameter, and abdominal circumference, I think.

Each of those measurements had a different "gestational age" next to them for my baby. The idea that each baby in utero is exactly the same size (or proportions) is just plain silly, IMO.

The computer estimated that my due date was 4 days different than my date by LMP. My midwives decided not to change the EDD.

*Disclaimer* I think that early ultrasouds are much more accurate for dating because certain features develop in a pretty predictable timeline. Once the baby is formed, however, and they are doing measurements on size alone, the margin for error is much bigger.

[kalamos23]

Are you or your husband tall? My early u/s with DD was on target (well, they said she measured 2 days ahead - I was charting with her and 100% sure with my dates) but with the 20w u/s, her femurs were so long she threw off the dates and was measuring quite a bit ahead. I'm tall and so is DH and I have long legs, so she really just was taking after us.

[tri31]

The ultrasound date moved my date back a week. I figure the baby will come regardless of what we think. If it's early than I can assume the lmp dates are right. If not, no big deal. That's why I didn't fight them on it.

[kalamos23]

I think what people are saying is because your date got moved back a week, then if your LMP dates are correct, you could easily be 41w at 42w. If you don't normally go late, it's not really that huge of a deal (or if you are with a OB/MW that doesn't pressure an induction at 41w), but if you do tend to go later, it might be an issue because you don't have that extra week before they start pressuring you.

This actually happened to my mom, she got pregnant without having an AF in between kids due to nursing, they did an u/s at 20 weeks and my sis was measuring ahead. My mom tended to go late because she ovulated later, and so she got to 42 weeks according to u/s due date and they induced her.

My sis was born clearly not full term, by physical markers, vernix, muscle tone, etc. She's actually had other issues related to coming early as well. Obviously this drastic of a case doesn't happen quite as much anymore since this was 22 years ago and the u/s machines are somewhat better, but *shrugs* some OBs get really antsy when you hit 42 weeks if they let you get that far in the first place.

[RedOakMomma]

I would fight having them date my pregnancy by my 20-week u/s. All of my boys have been tall, and by 20 weeks the estimated due date was VERY off...by more than a week!

If you go overdue, this could be bad for you. Suddenly a week of wiggle room could evaporate, and by then it's probably too late to argue about it because the dr.s are already considering you overdue.

Talk with your OB or dr. Explain your LMP/ovulation date and how the 20 week u/s is off. They should know enough to know that 20-wk u/s give notoriously bad estimates on due dates...it's only the early ones that are accurate at all.

[Pinoikoi]

My u/s tech said that the dates can be off + or - 10 days in either direction..

But I have gotten a positive pregnancy test at 8 days past Ovulation.. and more than once.

[JessieBird]

My 20 week ultrasound with DS was the type that calculated all the various body part measurements and spat out a due date based on the overall development of the baby at that stage. It said Nov 30, while my DD based on ovulation was Dec 8. He had also measured ahead, though not by that much, at my 12 w ultrasound. DS was born Nov 30, exactly what the ultrasound had predicted, though I know there's probably a lot of coincidence in that. DH is tall and DS was on the big side (8lb2oz), fwiw.

My midwife gave me the choice of what I wanted indicated in my file so we left it at Dec 8 and made no mention of the Nov 30 date so that I'd have more time before I was considered over-due and forced to be transferred to medical/hospital care.