How accurate is 6 week 5 day ultrasound?

Answer 1

When figuring the due date by a sonogram, the doctor will measure the crown-rump length (CRL).

The length of the embryo on the longest axis constitutes the crown-rump length. This is actually considered to be the best parameters to date the embryo. The accuracy of it is plus or minus 3 to 5 days.

A rough rule of thumb is that CRL + 6.5 = Menstrual Age in Weeks.

That said, it doesn't matter what week the sonogram is taken to determine its accuracy, because it is based on the CRL measurement of the fetus. No matter what the length is (because it was done on different weeks of gestation) is will directly relate to the week of gestation and ultimately determine the EDD (estimated date of delivery). The EDD is just that, and educated guestimate (guess/estimate) and is not meant to be written in stone.

When the due date has been set by an accurately measured CRL, it shouldn't be changed by a subsequent scan. If another scan done 6 to 8 weeks later says that you should have a new due date that is further away, they should not normally change the date. Instead, they should interpret this finding as that the baby is not growing at the expected rate.

After more testing has been done to determine whether the baby is growing properly and there is no other cause for concern, then it is feasible that perhaps the original measurement of the CRL wasn't properly measured by the person operating the sonogram. In this case, the new date is the actual date (considering the new measurement had been measured properly).

Keep in mind, however, when trying to figure your conception date that a full term pregnancy is considered to be 38-42 weeks, and that the accuracy of the sonogram being plus or minus 3-5 days. The date you actually give birth is directly related to the CRL and other factors of development of the fetus. Also, an egg doesn't become fertilized and implanted immediately after ejaculation. It can take up to 72 hours post-sex before that happens.

By taking the due date you have been given, count back 40 weeks. The date you get is only a rough estimate of the date of conception. It could be off by about one week. The day you recall having sex should be within that week, either before or after the date you counted back to.

Please note: This really gives you a two week timespan. Example: If the conception date you come up with is May 10 and you know you did not have sex that day, you could have potentially conceived that day (but that is kind of beside the point for the moment). You may have had sex anywhere from May 3 to May 10, or May 10 to May 17 and conceived up to 72 hours after you had sex in between May 3 to May 17.

Answer 2

Estimating due dates by ultra sound in the first 18 weeks are considered very accurate with a margin of error 1.2 weeks. If the ultra sound dates and LMP dates are off more than a week, your doctor will probably do another ultra sound in a couple weeks to compare. Very often a women will conceive earlier or later than they thought due to an irregular cycle.

As long as the repeat ultrasound shows consistent growth and there are no signs of problems, there is no reason to be concerned. Regardless of your due date, it is only an estimate for the baby's actual birthday, as 39 to 41 weeks is still considered within the norm. Only 5% of all babies arrive on their due date.

Answer 3

no not yet