Early scan - how do they date the pregnancy?

[plummymummy]

I went for an early scan today. I estimated that the pg was 8+3 weeks but sonographer said it is dating 7+3. She said it is more important to go by ovulation than LMP. The question I have is how do they do their calculation? Do they count ovulation date as Day 1 or do they still count that as 14 days into the pregnancy? I am trying to work out my due date.

the dates come from the measurements of the baby from the scan, I believe

Yes but do they expect their date to coincide with your lmp or ovulation date?

That should read LMP

Hiya - i would think that if they are giving you a dating scan, they would still base the forecast of 7+3 in the standard way they measure how far along you are- i.e. you are 7+3 based on your LMP. Can you call them and double check?
I also had a viability/ dating scan and was also told that this early - they are not that accurate. The 12 week scan is much more accurate
Congrats on your pregnancy!

LMP is not as accurate as the phase of your cycle before ovulation can vary. After ovulation is usually around 14 days.

Does that make sense?

They date your pregnancy from LMP, so when they say 7+3, this assumes you are 7+3 from start of LMP

HOWEVER this method assumes that every woman ovulates on day 14 of their 28 day cycle, and this is far from the truth!

So at 7+3 from the scan you would be 5+3 from ovulation, which may (or may not, depending on your cycle) be 7+3 from LMP.

If you know you are a late ovulator (or know from when baby might have been conceived that you could be) then the discrepancy between LMP method & scan date would make sense.

The scan method is generally more accurate for dating pregnancy (because of variaton in ovulation as sonographer stated) but they generally don't change your due date until there is more than a week's difference.

this site lwts you work out due date from ovulation date (date of conception)

hope that helps?

I did not remember my last period date when I went for my early pg scan. They measured the pole and gave me the date from that, once I had my official scan at 12 weeks, they dated it exactly the same.

Thank you. It is a bit confusing as I have a shortish cycle (24 days). Oh well, I should be patient and wait for the nuchal