Confused about dating scans

[Mmmmcheese]

I am very certain of my dates as was checking temperature and we only had sex once in a 2 week period!! (I was feeling exhausted and we didn't get around to it!). I've had a number of scans due to previous complications and at all of the scans - at 8 weeks, 10 weeks, 12 weeks, 14 weeks and 16 weeks the baby has measured 4 days earlier (older) than my dates.

BUT when they plot the measurements the baby comes out as at the very very lowest centiles for head circumference, femur length etc.

I don't understand this - if she is measuring small then why have they "bumped" her up by 4 days? It is all very confusing.

[Renaissance227]

Does this really matter? Maybe I'm totally stupid but I can't see why this would matter as long as your baby is doing well.

[LaWeasel]

You can conceive not on the day you had sex due to sperm living up to 7 days etc.

It doesn't sound a bit strange, but since they've only adjusted by 4 days, I wouldn't worry about it, or talk to your MW and see if she can explain.

[LaWeasel]

It does sound a bit strange, I meant to say.

[rainbowtoenails]

It will make a big difference when induction talk starts at 41 weeks.4 days makes a big difference then. Afaik, but may be wrong but i thought the date ran from the implantantation date not the fertilization date.

[LaWeasel]

Only if you are desperate not to be induced. 4 days is extraordinarily unlikely to make a difference to the maturity of the baby at birth.

[Crosshair]

Talk to your mw about your concerns?

[Mmmmcheese]

I thought the dates ran from ovulation? And I'm certain of the ovulation date.

The only reason I'm concerned is that it might mean that the baby is classed as measuring small for dates (as she is at the very bottom centiles at the moment).

I will ask MW when i see her next but that's not for 3 weeks.

[Renaissance227]

I thought that the dates went from the first day of your last period or am I completely stupid and talking rubbish?!

[LaWeasel]

Your original (estimated) due date runs from the first day of your last period.

The way I understand it:

When they date at a scan they use various measurements to work it out, very often those dates don't match up you know you had sex because of the difference between the time you had sex, ovulated, conception and implantation all of which might happen very close together or might happen a lot longer apart.

It does seem a little strange that they would decide on a date which made baby small, so I would talk to your MW about it, or if you are going to be having more extra scans speak to your sonographer as they will be able to tell you how/why they made their decision.

[sassylassie]

Mmmmcheese The EDD the sonographer gave me at my dating scan doesn't really add up either. I would have to have conceived before we actually had sex in that cycle for the date to fit! Hmmm - can sperm do time travel...? I can only guess that it just means the foetus is growing faster than average and when you have your dating scan they count as much backwards from when they think your due date is likely to be (based on the baby's size) to get the "13 weeks" figure, which they gave me last week (as 13 weeks would have taken it to a few days before my previous period started!). Whereas the midwife giving the initial date just went off the date of my last period. Would that be right? I have to say I already find it extremely confusing that they date a baby from before it was actually conceived anyway. And my DH just cannot get his head around that at all!