Dating scan: please explain this to me

[StealthPolarBear]

I went for my dating scan today. She measured the baby at 52 mm (I think) and gave me my form with the baby's size right in the middle of the scale, which I understand, it's a dating scan so they're assuming the measurements are right and using them to calculate my dates, right?
So how come I've just read that this week my baby is anywhere between 25mm and 50mm? How can such a huge gap be right? How on earth can they date me to the day when there could be such a variation? I thought the whole point was that at this stage in a 'normal' pregnancy, the baby would be a standard size?

[brettgirl2]

How far on are you?

[SlightlyMadScotland]

Because there probably is abig variation between the beginning of the week and the end.

[jollyjoanne]

25mm is really quite small its only 1 inch. And all babies develop slightly differently they would have told you if they thought there was a problem with the babies development I am sure as they look at things other than just size ie where the babies organs are and the proportions of the head in comparison to the body etc.

[SlightlyMadScotland]

I don't think she is doubtuing her dates.

Ithink what she is saying is that her pregnancy book says the embryo is between 25mm and 50mm. And that 50mm is essentially double 25mm (obviously!)

So how do you date based on measurements when proportionately there is such a big difference in the "normal" measurements.

[StealthPolarBear]

SMS, yes, that's it, thanks. Surely the baby can't grow that much in a week, and if it can, how on earth can they be so accurate?
I'm 11+5, so perfect for a dating scan I think.

[StealthPolarBear]

and the dates were done on length - she said "the baby is 52mm which is right in the middle of the bar as that's how we date your pregnancy"

[SlightlyMadScotland]

I think teh embryo does double between 11wk and 12wk according to a couple of websites I have just looked at. At 11wks embryo would be 35mm, by 12wks it is 60mm according to this random website.

Thats why I think that in your pg book it says 25mm -50mm; because at the beginning of the week it is about 25mm; by the end of the week it is 50mm.

Thing is - until the baby starts to feed properly from the placenta around the 12-14wk mark it's growth is pretty standard as it is about the food stores that are in the egg.

[StealthPolarBear]

wow, that's interesting, thanks!
So they're both right

[SlightlyMadScotland]

Think so.

That's my interpretation anyway....

[jollyjoanne]

There are definitely other things they consider when dating for example until about week 11 your babies intestines are still outside of the body. So presuming that there is not a blob outside of the babies body near to the umbilical cord this also provides another indicator of age.