On Tue, Feb 01, 2005 at 08:00:39AM +0930, Sam Heard wrote:
>
> Age is time after birth - we are not going to change that.
>
That's fine ... now for the second ...
> We have agreed that we need to record one more date in the demographic
> model - which is 'approximate date of conception', or 'expected date of
> birth'
We should confine ourselves with available recommendations of
the WHO, which is based on LMP only. All other parameters are
derived from this. Besides, LMP is a fixed recordable event.
>From the XML, I understand, you may be thinking in terms of
some gestational history for the newborn. Calculations can be
done from the mother's obstretic history, as per parameters
defined by the WHO. This is the current recommendations:
o LMP = first day of the Last Menstrual Period
o Prematurity = birth < (LMP + 37 wk) gestation
o Normal gestation = birth > (LMP + 38 wk) < (LMP + 41 wk)
o Postmaturity = birth > (LMP + 42 wk) gestation
Please note the hazy zones between 37-38th week and 41-42nd wk.
We can expand the band of normalcy from 37 to 42, and eliminate
this vagueness without significant real-life issues.
Date of Conception is normally not used because it depends upon
the periodicity of the menstural cycle of the mother and other
factors. The variations are too many to be correctly instituted
as a measure.
If you are thinking of an additional DV_Textual_ordered class
as seen from the XML (quoted below) then:
> <DV_Textural_ordered>
> <text>"Birth"</text>
> <value type="DV_QUANTITY">
> <magnitude>0<magnitude>
> <units>"days"</units>
> </value>
> </DV_Textural_ordered>
>
o The 'magnitude' would obviously have to be taken from the EDD
(Expected Date of Delivery). Since the 'normal range' itself
is a wide band, with the midian point at (LMP + 274 days) the
above representation would have to be re-structured.
o Incidentally, there is also an alternate 'prematurity' defini-
tion based on weight - the original WHO recommendation of 1948
(viz birth weight < 2500 gm). This covers the "small-for-date"
newborns, burn within normal gestation period. This is still
in use in several countries (mainly developing countries).This
should not be ignored.
o Postmaturity is another factor which would need inclusion in
such a textual class. To keep it simple and self explanatory,
I suppose, the following should do:
<DV_Textural_ordered>
<text>"Gestation_period"</text>
<value type="DV_QUANTITY">
<magnitude>0<magnitude>
<units>"weeks"</units>
</value>
</DV_Textural_ordered>
<DV_Textural_ordered>
<text>"Birth_weight"</text>
<value type="DV_QUANTITY">
<magnitude>0<magnitude>
<units>"grams"</units>
</value>
</DV_Textural_ordered>
Keeping the gestational period kept as an absolute number leaves
things like normal range, prematurity, postmaturity etc as a
search/ derived parameter. Any changes in WHO criteria would not
affect the database.
Just a suggestion ...
Dr USM Bish
Bangalore
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