How accurate is a six week ultrasound?

[etonmymind]

I know that OBs say it's really accurate...I was just wondering what you all thought.  I'm having doubts/questions about my due date of March 1 (which I realize is very much an estimate anyway!).  My pregnancy was a surprise and as a result I had zero clue about my LMP (we had been traveling 9 times zones away for over 6 weeks, so I wasn't regular and had been in a big jet-lag fog for a few weeks before my positive test).  When I went to the ER to have my IUD removed, they did an u/s that dated me at six weeks, and a blood test that measured HCG levels and put me at 6 weeks, 3 days.  We've used the 6 weeks, 3 days to compute my due date.  This makes me 39 weeks tomorrow.  The only other u/s I had were at 17 weeks (gender/mid-term measurements) and 19 weeks (quick follow-up to a question from the prior one).  My uterus is measuring about a week and a half behind, which m/w thinks is a small baby, but this would surprise me b/c my two other boys were very large for their gestation and so I expected another large boy.

I'm kind of going crazy anyway b/c even though I'm only 39 weeks, that's 2 weeks further than I've ever gone before. :)   This is the insanity of a mom who's only had preemies and suddenly has a full-term baby and doesn't know how to deal with being so far along!  So I'm just looking for ways to keep my brain busy.  I've been wondering about the gestational age for the entire pregnancy.  Thoughts?

best,

et

[tessie]

Early ultrasounds are pretty accurate for dating a pregnancy. 

Enjoy the relentless 'any news yet' texts, emails etc. ;)

[Steady101]

They are pretty accurate, but can still be off by up to a week that early depending on when your baby implanted.

[etonmymind]

Originally Posted by tessie 

Enjoy the relentless 'any news yet' texts, emails etc. ;)

OMG, yes.  I hate everyone.  I ignore the phone and the FB messages.  It's not rational OR polite of me, since it means people care...but I can't deal with the fact that everyone expected me to have this kid by now, and he's not out yet!  How I'm going to deal if I go another three weeks, I don't know...vacation close by maybe, to somewhere with no phone?
 

[Marissamom]

with the measurements thing, it could mean a baby who's a little smaller, it could also mean a little less fluid or just be baby's position or how you're carrying. I've heard that early u/s is accurate to within 3 days, but it could be a little more off, but a week at the most. and hcg levels baking it up make me thing that the date is at least pretty close to accurate. 

[jenninjapan]

 I would say that early ultrasound can be wildly inaccurate.  And better to trust the 17 week or 19 week for gestational age.  For this pregnancy I had an ultrasound (transvag) at 7 weeks.  The baby's ges age was 4 weeks.  Then I was told to come back at 9 weeks.  The babys age again was about 6\7 weeks.  It wasnt until the 10.5 mark that baby 'caught' up.    I KNEW the day I ovulated so I was also sure of my dates. 

Before this experience I felt I really needed these early ultrasounds to calm my fears (my first pg ended in miscarriage) but this time round def found out how absolutely useless they are.

Fundal height tends to be pretty accurate b/n weeks 20 and about 34ish.  If you are not sure about dates, I would go by that rather than an ultrasound.

[BabyBearsMummy]

Early ultrasounds tend to be accurate for dating date of conception do to rapid changes in development early in a pregnancy. But given gestation time lines vary widely they do not accurately give a due date. Enjoy the rest of your pregnancy and take advantage of voice mail as you get closer to your due date. The relentless questions about if anything is happening yet can make the time seem to drag out even more.

[LisaG]

Both of mine were right on.  The first was with a pregnancy via IVF so we knew dates for sure.  This time around w/o IVF it's been right on with my best guess of LMP.

[Marissamom]

Originally Posted by BabyBearsMummy 

Early ultrasounds tend to be accurate for dating date of conception do to rapid changes in development early in a pregnancy. But given gestation time lines vary widely they do not accurately give a due date. Enjoy the rest of your pregnancy and take advantage of voice mail as you get closer to your due date. The relentless questions about if anything is happening yet can make the time seem to drag out even more.

um, what do you think all due-dates are based off of? that's why there's a 5 week window that's considered the normal "full term"

[purplepaperclip]

Originally Posted by Marissamom 

um, what do you think all due-dates are based off of? that's why there's a 5 week window that's considered the normal "full term"

Actually, she is correct. Early ultrasound is quite accurate in telling how far out from conception you are. This is not to be confused with "when you will have the baby". Due dates are generally not based off conception date in most women. HCPs mostly use LMP to estimate due dates. But as we all know, by going from your LMP, you are only guessing when conception occurred. Early fetal development follows a pretty strict timeline.

[lamamaloca]

Research seems to indicate that early ultrasounds are accurate to within three or four days. By a 20 week ultrasound, they are only accurate to within a couple of weeks. Some pregnancies just go longer. :)