Margin of Error in Estimating Dates via Early Ultrasound?

[shihtzulover]

I've searched all over the Internet for this information, but I can't find much data from reliable sources.

According to my basal body temperature and monitoring of ovulation signs (EWCM and cervical position), I am either 7w0d today or 7w1d. However, my ultrasound today measured me at 6w5d. Is this within the margin of error?

I just don't see ovulation any later than that being very likely because all of my signs were exactly on for closer to 7w1d. I also got a really clear BFP at 11dpo, which would only be 8dpo with this new due date.

Anyone out there have experience with ultrasounds this early and having the dating being a little off? I hope I'm just being really paranoid, but I'm afraid something is wrong.

Baby's heart rate was 121, which they said was fine. I asked if it was normal for the measurements to be off a little bit, and they seemed kind of hurried but said yes.

Can anyone offer any insight? Does anyone know what the acceptable margin of error is for a scan this early?

 

[Puppmom]

Re: Margin of Error in Estimating Dates via Early Ultrasound

Shihtzu, I knew exactly when I ovulated with both of my boys and my ultrasound measurements were "off" by 2 days once and 3 days once. At my second scan it was 1 day off both times. I'm pretty sure my tech said plus or minus 3 days via early scan then the margin of error goes to a week in the later scans. Both my boys also had heart rates in the low 120s at around the same time.

I was paranoid about everything too so I totally sympathize!

 

[Laila619]

Re: Margin of Error in Estimating Dates via Early Ultrasound

3 days off is nothing. You're fine, don't worry!

I knew *exactly* when I ovulated as I did a trigger shot, and my twins were measuring a little behind at the first ultrasound. They caught up and are measuring right on target now. Moral of the story: try not to worry!

 

[amc80]

Re: Margin of Error in Estimating Dates via Early Ultrasound

Another thing to remember is the measurement is only as accurate as the person doing it. If the tech clicks their mouse just a hair too early, the measurement will be off.

 

[mia1181]

Re: Margin of Error in Estimating Dates via Early Ultrasound

I'm pretty sure the margin of error is +/- a few days. Your measurement is just fine and no cause for worry. You may think you know when you ovulated, but the only way to know exactly when is by ultrasound. CF and CP are not as accurate as BBT and even BBT doesn't always exactly coincide with ovulation. And then there is no way of knowing exactly when you implanted (which can happen anywhere from 6-12 days after ovulation). Plus you could have really high HCG which could make a pregnancy test darker, sooner. And then you add the fact that the ultrasound tech is taking a really tiny measurements and being off by a hair can add or subtract a day.

All that to say, ultrasounds aren't going to be exactly accurate, but they are close and reliable enough to get a pretty accurate due date. With each one you get after this one, the measurements will be a little ahead or a little behind too. But that first dating one is the most accurate in terms of due dates. I think because as baby gets bigger things like baby positioning and genetics (how big your baby is meant to be at birth) get factored into the equation.


Congratulations! Sounds like your baby is doing great so far!

 

[pancake]

Re: Margin of Error in Estimating Dates via Early Ultrasound

Yep, the margin at early scans is somewhere around 3-4 days. It gets bigger as time goes on.

 

[dani13]

Re: Margin of Error in Estimating Dates via Early Ultrasound

Yep, the margin at early scans is somewhere around 3-4 days. It gets bigger as time goes on.

This exactly. Sometimes it's difficult to get a great measurement that early because of the position of the fetus, etc. don't worry, you're fine!!!

 

[monkeyprincess]

Re: Margin of Error in Estimating Dates via Early Ultrasound

A bunch of people have already said the same thing, but I'll chime in to give you just a little more reassurance. My son was conceived via IUI after taking a trigger shot to induce ovulation, so I was very certain of my ovulation date. Yet, my first ultrasound at 6w6d had him measuring 4 days smaller than he should have been and his heartrate was like 109 or something, and I drove myself crazy with worry and assumed something was wrong. I had another scan 2-3 weeks later, and he measured exactly on target. I know it's a really stressful time, but it sounds like everything is going well so far. Best wishes for a healthy pregnancy!

 

[shihtzulover]

Re: Margin of Error in Estimating Dates via Early Ultrasound

Thank you all for all the helpful responses! I truly appreciate all the reassurance. I've always been a worrier, but I'll try my best to calm down.

 

[Guilty Pleasure]

Re: Margin of Error in Estimating Dates via Early Ultrasound

My friend had two different pictures from her first ultrasound and they each had a different age listed at the top of the screen. As someone previously posted, it's only as accurate as the person clicking the mouse to measure the crown to rump length, and we're talking a matter of millimeters! In my friend's case, the doctor was clicking the mouse and happened to measure the length different on the two pictures.

I would not worry about this at all.

 

[partgypsy]

Re: Margin of Error in Estimating Dates via Early Ultrasound

Entirely within the range of normal. I was given some cause for concern with my 2nd, because the ultrasounds showed her in lower in weight (earlier gestation age) than what we all thought she was (more like a week difference toward the end). Throughout that time I was encouraged to eat and warned that that she would be undersized (5 1/2, 6 pounds). Yet when she was born she was 7 pounds, 1 ounce and her length was normal. It made me glad I didn't overly worry about it at the time.