Ultrasound dating

[dejagerw]

I just went for my 1st ultrasound. By my calculations (I chart) I'm 19 weeks 4 days, but the ultrasound had me at around 21 weeks. Does anyone know how accurate they are at this stage in pregnancy?

[lizzylou]

The accuracy of ultrasound for dating a pregnancy depends on at what point during the pregnancy the ultrasound is taken. Pregnancy dating is most accurate during the first eighteen weeks of pregnancy. Measurement of the sac at five to seven weeks is not accurate. Measuring the crown-rump length gives an accuracy of plus or minus three days at seven weeks; this test can be used from the seventh to the fourteenth week. Between fourteen and twenty-six weeks, measurements of the biparietal diameter, the femur length and abdominal circumference are generally used. The accuracy is plus or minus seven to ten days. The accuracy further declines with advancing gestation age.

source: http://www.ob-ultrasound.net/joewoo3.html

I had one at 10 weeks... it was about 5 days off. I think charting is much more reliable and I wouldn't accept a date based on u/s, especially not a later date as I went to 42 weeks with my son.

[WifeofAnt]

I hope they didn't change your due date! My ultrasounds were very accurate, off by 1 day at 10 weeks and 2 days at 18 weeks. However they aren't always so spot-on.

"As the pregnancy progresses, the accuracy of ultrasound for predicting due dates decreases. Between 18 and 28 weeks' gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date."

[AngelKissedKids]

Originally Posted by lizzylou

Measurement of the sac at five to seven weeks is not accurate.

source: http://www.ob-ultrasound.net/joewoo3.html

I looked quickly through your source link and couldn't find info specific to a 5 week ultrasound - if you have a sec, could you be more specific where to look? I had an ultrasound that dated me at 5 weeks 5 days and gave me a due date that is 10 days later than the one determined by my LMP (not that that is accurate either since I'm not charting). Just concerned since my family doc is now going by the 5 week ultrasound due date. My previous baby was a preemie so knowing an accurate due date is very important should my current babe also arrive early.

Thanks so much for your help!

[kalamos23]

You could have ovulated 10 days later than you expected - by LMP, I am due April 18th, but I didn't ovulate until CD24, so I'm actually due April 28th. I would definitely go by the early u/s - the development changes so much in teh early days that it is a lot more accurate and easy to pin down an EDD within a day or so vs, later on.

What info are you looking for specific to a 5 week ultrasound? I love this site for figuring out 1st tri ultrasounds and what to see/expect.
http://www.brooksidepress.org/Produc...nd_scannin.htm

[babygrey]

Originally Posted by crystalcm

I looked quickly through your source link and couldn't find info specific to a 5 week ultrasound - if you have a sec, could you be more specific where to look? I had an ultrasound that dated me at 5 weeks 5 days and gave me a due date that is 10 days later than the one determined by my LMP (not that that is accurate either since I'm not charting). Just concerned since my family doc is now going by the 5 week ultrasound due date. My previous baby was a preemie so knowing an accurate due date is very important should my current babe also arrive early.

Thanks so much for your help!

I didn't look at the source, but, as understand and as I was told by my RE- it isn't so much specific to 5 weeks, but US dating is less accurate if you are dating based on the sac not on the CRL (fetal pole length).

FE, the EDD based on my sac measurement @ ~5weeks (before there was a fetal pole big enough to measure) was six days different than my EDD based on CRLs one and two weeks later (have had two since measuring CRLs and they match up)

At nearly 6 weeks, you may or may not have had a fetal pole- I would check. If the pregnancy was dated on the CRL, IMO, I'd be much more comfortable than if it was on the sac alone.

[AngelKissedKids]

Originally Posted by babygrey

At nearly 6 weeks, you may or may not have had a fetal pole- I would check. If the pregnancy was dated on the CRL, IMO, I'd be much more comfortable than if it was on the sac alone.

That makes a lot of sense, thanks for explaining it to me. I'll have to ask my doc what exactly was measured on the ultrasound. I know I got to see a good flickering heartbeat on the screen and it was 93 beats per minute, so hopefully there was enough of a fetal pole to measure.

I have another ultrasound booked for 12w3d for IPS testing and I'm hoping it will help reaffirm my due date.

Thanks so much!

[stormgirl]

totally OT, but the title of this makes me giggle. Like speed dating crossed with arranged marriage...