confused by 12weeks ultrasound

[Mumzieto2x]

today we got to see our baba at our 12 week scan, just a little confused on how they worked out i was 13+2days when my last period was the 6thjuly, but being 13+2 would have brought my last period being the 1st of july, anyway we was so happy to see baby he/she was moving like mad :) xx

By no means an expert but I think they take things like the femur measurement into account.

Hopefully someone who know what they are talking about will be along soon

The 12 week scan is a dating scan, they only use your dates to guess when the 12 week scan needs to be. The measurement at the scan decides the due date, so nothing to do with your last period. They might not change your due date though depending on loads of different things

Mine has always been ahead. Baby is now measuring nearly two weeks bigger than date of 12 weeks scan so I have no idea at all. Midwife said she has a fat bum though?!

The date of last period thing originates from back before scans and technology, it was a way of doctors roughly estimating pregnancy due dates, so not an exact science.

I'm full of useless information ;)

You might not ovulate on the 14th day after your last period that that way of estimating dates relies on

I wouldn't worry about it - according to my dating scan I conceived before I even ovulated

There is margin for area, just take it all with a pinch of salt

I've also been put a week ahead by my dating scan but I know when I ovulated and their dates say I conceived a week before ovulation. I'm going to stick with my own dates, which I know to be correct though the docs won't listen to me.

I also conceived nowhere near ovulation according to my scan! I was so confused as I had been tracking my cycle for months, I had a bleed so had an early scan which put me almost 2 weeks behind the dates I had worked out! Anyway the 12 weeks scan brought it a bit closer but was still 5 days out. At my 20 weeks scan the measurement was exactly spot on with my 12 week scan so I've conceded that i must have ovulated at a slightly different time than I thought. Also I suppose implantation times can vary somewhat. x

Mine really pissed me off actually.

I had a small bleed when I was 11 weeks or so I thought by LMP. I had a scan and they said that the baby measured 12 weeks 4 days.

Had the 12 week scan the next week, they agreed that it measured bigger, 13 weeks 3 days, but that they had to go by LMP, new hospital policy.

I know it doesn't really matter, BUT I am having another planned section.

Ds had other ideas about his section date and I went into labour at 37....so I want my dates not to be too far out. Consultant is nice though, said it was mental and will book me for 38 weeks or a few days before to be on the safe side.

This is an interesting thread as im also sure ive been dated week ahead of what the baby actually is. I had an early scan at 9 weeks which daid baby was exactly 9 weeks, no days. Then 3 weeks later exactly i had my '12' week scan which dated me at 12 + 6, so i gained an extra 6 days. But im also sure when i ovulated the month we concieved and i agree with first scan at 9 weeks. Im very tall and my dp is slightly taller, both dcs are well above average hight, so in guessing this one will just be bigger to :-)

There is a degree of inccuracy with both scans and period dates. You may ovulate earlier or later depending on length of cycle. You probably have a short cycle or did that month.
Most hospitals take the lmp date first but if there is 7 days difference or more they go for the scan date. Neither is 100% accurate.

As for later growth stages baby does not measure ahead if the dating scan/lmp date has been reasonably accurate. We do not alter the dates later on. It annoys me when use that terminology. the baby can be either large or small for dates. They are on a spectrum and the range gets bigger the further away from dating scan they get. So if baby is measuring "36 weeks" and dating scan put you at 35 weeks. Baby is still 35 weeks - just bigger than average.

My baby was only measuring 6 days ahead but they changed my dates, despite the fact that their date doesn't fit with my LMP, when I know I ovulated and early scan at eight weeks. That's why I'm going with my own dates, not theirs.

They measure the length of the baby and use that to decide on a date. I saw the midwife measure and every time it came out with a different measurement, she just took one in the middle.

With DS they put my dates a week ahead, but that was because he was a big baby.