Why do doctors take zero notice of 'calender' dates compared with scan dates?

[Sunflower100]

According to all the date estimations I am due a week before the date from my dating scan. I have a 22 day cycle which is regular as clockwork and pretty sure that we didn't even have nookie after the date I think we conceived (which all ties in beautifully on the Mumsnet calculator). Why when I mention this does the doctor laugh? I have only ever mentioned it in a kind of 'thought it might be relevant' type of a way to them. Im likely to go overdue (did last time) so I thought it might be relevant.

[llareggub]

My scan date and calendar dates are 2 weeks apart. I'm not regular, but remember that ovulation dates can vary and conception doesn't necessarily happen "on the night" so to speak.

[Tee2072]

Because you might not have ovulated exactly when you think you did. Dating scans, while not perfect, are more exact than LMP dates.

And he probably laughs because he has probably heard 'but we only did it one time, I couldn't possibly be (as pregnant, pregnant at all)' a million times!!

[ceb80]

I had the opposite! My dating scan and LMP dates are a week apart but midwife is ignoring scan and going by LMP as she finds it more reliable! (am secretly hoping for scan date as its earlier )

[callmeovercautious]

ime they are right and I was wrong. I put DD as being due a week later. She arrived a few hours over their date.

[spicemonster]

Because they think they know better than you. I conceived my DS through IUI so I knew exactly when he was conceived because I had a scan in the morning and IUI in the afternoon. Despite that, they still insisted my dates were a week out.

[GColdtimer]

can someone explain something to me .

The 40 weeks are measured from the last period date, which is on average 2 weeks before conception? So the clock starts ticking before you are actually pregnant? I have never got that.

[Gentle]

twofalls, I like to think of that as a bonus! I'm not sure what the medical answer is though.

As for scan dates vs calendar dates, anyone see The Great Sperm Race on C4 the other night? There was a good explanation of how the final bit in the fallopian tubes pans out, how the sperm get ready to fertilise the egg, how the egg gets ready, and how they can all miss each other. Helped me to understand the whole "window of conception" thing better.

If you're interested I'm sure that checking it out on 4OD will be more interesting for you than my stilted attempts at trying to give you a synopsis!

[MichaelaS]

the pregnancy dates are because you can only measure dates otherwise with a scan. not everyone wants one, and until fairly recently they were not common unless there were suspected complications (e.g. previous miscarriages, suspected twins).

Women have always known when their LMP was, so the dates are done by that.

Now that scans are more common, they just adjust the dates and go with the old convention.

regarding why the scan dates are taken as more accurate - well I think firstly there is the difficulty in knowing when you ovulated, and then I wonder whether perhaps some babies just grow faster than others? it's all so random when they come out anyway, I don't think a week either way makes much difference.

[theyoungvisiter]

twofalls - i think the dates are measured from LMP because in the days before scans, that was the only date you were really sure of.

You can of course conceive quite a long time after you have sex (didn't you pay attention in sex ed classes about the unreliability of the rhythm method? ) which is why they tend to take scan dates as more reliable. Having said that, they are only as good as the operator.

My first scan was fine and I trusted the date, although I was induced early in the end so I don't know exactly when I would have given birth. My second scan the baby was really curled and the sonographer spent ages trying to get it to straighten up enough to get a measurement. She eventually sighed and said "well that'll have to do!" Not surprisingly it put me a week later than I thought. I went into labour 1.5 weeks "early" according to their dates, 2 days early according to mine. So I think in that instance my dates were probably more accurate as they didn't spend long enough doing the scan right.

[Sidge]

My obstetric consultant thinks women should be given a due month, not a due date.

The historical dating of a woman's pregnancy ie 40 weeks is fairly arbitrary - they knew that it took approximately 265 days to grow a baby and as most women conceive about 2 weeks after their last period 40 weeks was designated the 'standard' pregnancy.

But modern scanning allows a more accurate prediction of a baby's growth and development so will usually override a LMP date.

For the OP, a 22 day cycle is pretty short so it may be that your baby was conceived earlier in your cycle than day 14; remember that eggs hang around for about 48 hours and sperm can hang around for up to 4 days so there can be a fairly long window of opportunity!

[princessmel]

I think this too. When dd was born if I had gone by my LMP date dd would have been 2 weeks and 3 days late and by the scan date she was 3 days late. Mw went by the scan dates.

But she had mecconium in her water and was 8lb 5. I think my LMP dates were the right ones.