LMP or US????

Question

How do the techs date the ultrasound that you have when first pregnant? I know they measure crown-rump, but is the date they give you dated from your last menstrual period or by transvaginal ultrasound? For instance I had my period 10 weeks ago, would the babies size be a 10 week old or an 8 week old? Just curious. I know that dating of preg goes by your lmp, I didn't know if it stayed like that with ultrasound. Thanks for any help!

Also, there is only one day I could have gotten pregnant. It's the only time I had intercourse, so I know when it would have happened. Would the date go by that possibly?

Answer 1

To start with you go by your LMP date. Then you have a dating scan at about 8 weeks. This will give you a better indication (in your case, should be roughly the same as you LMP date as you know when you conceived).

The most accurate date is the one you will get from your 20 week scan as the baby has started developing more and what they can see and measure on the scan is more accurate. This scan will give you a date based on how the baby is growing and developing.

Remeber 'anything between 36 -42 weeks in the womb is considered full term' it depends on the baby when they want to come out.. (I read this on some website today). . And only 3% of babies come on thier due date... (I read that somewhere today too!) Good luck with it all..

Answer 2

An ultrasound can be used to determine your due date if your is last menstrual period is unreliable. Ultrasound due dates are found by obtaining ultrasound images and taking measurements of the baby. The underlying assumption of an ultrasound due date is that there is a statistical correlation between certain ultrasound measurements of the baby and it's gestation. The term "gestation" means how far along is the pregnancy. If we obtain ultrasound measurements in thousands of pregnancies where we know the exact gestation, we can find the average measurement for any given gestation. For example, it is known that for a measurement known as the crown-rump length (CRL), the average is 31 millimeters at 70 days (10 weeks) gestation.
Unfortunately , people tend to give a lot more credibility to ultrasound due dates than is warranted. That is, many people assume that if an ultrasound due date is different from their due date based on a last menstrual period, that the ultrasound date is more accurate. However, this isn't necessarily true. Remember that the basic assumption behind an ultrasound due date is that a given measurement corresponds to a given gestation.


Basically when they do an ultrasound the computer is programmed to use the CRL measurement of the baby to determine gestation and therefore due date. Hopefully this coresponds to your LMP but can usually be off by a few days either side because of differences in the growths of individual babies.

I had a dating ultrasound last monday that took three measurements, one said 6wks6days, the 2nd was 7wks 1day and the 3rd said 7wks 3 days so the technician worked out the average and said I was 7wks 1 day pregnant with a due date of 13th july. My doctor however is still using my LMP as my basis for gestation and puts me at 8wks5days now with a due date of 9th july.
As you can see, ultrasound can be unreliable and your LMP date if it is close to the US date is still most reliable.

hope that wasn't too confusing and long winded!

Answer 3

Usually a 'dating' ultrasound is done at around 18-20 weeks and that is the EDC. Most times it changes from the original date.