Gestational age - based on LMP, CRL or conception date?

[Davina84]

Hello!

A bit of a weird question, but maybe someone has asked their obstetrician about it (or is one!).

For the first-trimester combined test, we are asked as always the LMP date and typical period length, which gives a rough estimation of EDD (ovulation is ~ 2wks after LMP, conception is deemed to have happened then, and from that they deduce EDD).

At the scan they revised the EDD from 12w3d to 13w1d, based on the CRL measurements.

But the fact is that we know exactly when the LMP happened , AND we know exactly when conception happened. Due to travel and other constraints there was ONE day for "intimate activities", and necessarily conception must happen on the day, or a couple days after max... but not before!
It was exactly 2 weeks after LMP date, which means the actual gestational age should pretty much match the textbook estimate of 12w3d.

2 questions:

• doesn't the CRL measurement just mean the fetus is bigger than average? Which might be important (?)
• The serum test for DS and other conditions is based on expected concentration of hormones which are supposed to have come up / down as pregnancy progressed. If the gestational age was indeed 12w and not 13w, does that not impact how to interpret the results?

I'd be delighted if anyone had input!

Hi! I had something similar and interestingly went into labour exactly on the date my own due date calc was based upon date of conception, rather than the adjusted due date given after 12wk scan measurement! So I think is safe to say you know the age of the baby more accurately. But it will probably remain on the system as 13wks. (They use the size of the baby as the more accurate due date predictor as apparently it does best predict age over date of last period).

The screening hormones for DS don't vary so much over that one week difference of either 12 or 13 weeks. It's the nuchal measurement which is important to capture as close to poss as 12wks as it then gives the best indication of any issues. So I wouldn't worry about that. And yes it would then appear the baby is just very slightly larger than average but more detailed measurements are all done at 20 week scan.

Thanks for sharing your experience!

Not to belabor the point, but it could be useful for others: the median for these serum measurements moves quite a bit week on week.

Between 87 and 92 days of gestational age, the median Beta-HCG decreases by 10% and the PAPP increases by 25%.

Changing the reference my PAPP MoM from 0.62 to 0.8, and my beta-HCG MoM from 2.48 to about 2.25.

Not huge, but probably enough to move the probability quite a bit.

Hi I’m also confused by this as at my 12 week scan I should have been exactly 12 weeks and the sonographer put me a few days ahead.
based on my last period which was 15th July I should be due 20/04 however they’ve put my due date as the 17/04 which makes no sense at all?
Going off the date provided my period would have had to be 12/07 which I know it wasn’t as I document everything and it was definitely the 15/07.
Can anyone explain this as the sonographer didn’t and I am baffled, ha!