Due date discrepancy

[annabenbow]

After having my first scan last week , I was told I am 13+3 weeks however this is impossible as OH wasn't back from Spain at his time , by our calculations I was around 12+4 at the time of the scan.
Wondering how they date a pregnancy, is it by size , development, ? Does this mean I have a big baby? And also is this a problem for later pregnancy , or does one week not make much difference?

[JennyBlueWren]

Remember that the dates are not from when you conceived and that conception can be after sex.

"Normally" you ovulate and conceive at week 2 (so 11+3 weeks ago) but people can ovulate earlier or later. The sperm can hang around waiting for an egg for about 7 days so if you had sex you might conceive a week later. Eggs don't last as long though (24hrs I think) so conception is within a day of ovulating. But you might ovulate straight after your period or just before your next one.

How long are your cycles normally? Mine are a bit erratic -up to 6 weeks- so our dates were a bit unknown. Went to scan at 12+3 according to dates but found out I was about 9+5!

[AnnoyedByAlfieBear]

My 12 week scan put conception at 4 days before sex. I'd just mc'd so I know it wasn't earlier. It's just one of those things.

[Thegreatunslept]

I wouldn't worry too much about due dates mine was changed at my 20 week scan by 5 days and ds was still 11 days over. It's a rough guide to be honest as people go early and over all the time.

[PenguinsHatchedAnEgg]

This happened to me too.

Measuring ahead isn't a problem, except for the fact that, if you go overdue, you will reach the point of induction suggestions before you really need to. This happened to me. However, I told the midwife throughout that the dates from the scan were wrong (and would mean immaculate conception!) and she was supportive when I said I would be delaying induction until I thought I was closer to 42 weeks. In the end, he came at +13, so all was well.

Remember that they are measuring such tiny bits of your baby at this stage that it can easily be a bit out. Mine was 8 days and there is widely recognised to be at least a 5 day margin of error even by skilled sonographers.

It doesn't mean anything for baby size. A friend of mine has measured massively ahead with all three of hers (IVF, so they know the measurement date is wrong), and has had very average babies each time. Mine were all large-ish, but that applied to the two who didn't measure ahead as well as the one who did.