Age, Gestational (Corrected Gestational Age [CGA] and more...)

Below posted by *rob challen* AATT orange DOTT net possibly on openehr February 2005 Additional notes:
  • code would want to be folded into the gestational calculator we already have
  • We could accept the entry or derivation of each of
    • LMP and
    • estimated gestational age by alternate method
    • if only one of the above is entered, use it in the calculation of Gestational Age hence Estimated Date of Delivery
    • if both are available, options which to use would include
      • user specifies which (or an average) to use or
      • algorithm decides (e.g. if estimated gestational age as at date of study was > [hardcoded or user-specifiable # of weeks], use LMP) (a respondent on openehr suggested something below

> > 
> >From the paediatric / neonatal point of view 3 dates are recorded routinely
> in the notes.
> Firstly, and most importantly, is actual birth date and time (Dob)
> Secondly 2 dates related to Expected Date of Delivery (EDD):
> - One calculated from Last Menstrual Period (LMP+40 weeks). This is a bit
> unreliable as the information may or may not be available, as the woman may
> or may not have a regular cycle.
> - The other is the EDD as determined by a radiologist during the 12 week
> antenatal scan. Determined by foetal head size amongst other factors. It
> becomes less reliable the further away from 12 weeks the dating scan was
> done.
> EDD by scan is more reliable than EDD by LMP, and is used in preference.
> Once a premature neonate is born, his gestation is calculated. This is
> calculated as [40 - (EDD - Dob) / 7] and usually expressed as e.g. 27+1/40
> (27 weeks and 1 day gestation). 
> Subsequently the number of days since birth is also recorded** (and is what
> people use to refer to as "age" or "chronological age"). Corrected
> Gestational Age (CGA) is then the sum of gestation and chronological age
> Therefore when describing an infant you would say "Baby X was born at
> 27+1/40 gestation, he is 42 days old, and has a Corrected Gestational Age of
> 33+1/40" or in shorthand "27+1/40 gest --> day 42 --> CGA 33+1/40"
> Gestation is an estimate, based on scan or LMP. When an unbooked pregnancy
> delivers prematurely & often no information, even of LMP, is available (it's
> a zoo out there...) If no information is available there are scoring systems
> to estimate gestation such as the Dubowitz or Ballard scales.
> In current practice CGA remains the age used for growth markers and
> developmental milestones until the (CGA) age of 2 years when paediatricians
> arbitrarily decide to start using the normal date of birth to calculate the
> age.
> For neonatal drug dosing the main marker used is current weight, although
> sometimes you also need to know the age (from Dob) as well as the gestation
> at delivery.
> The two most useful numbers to the neonatologist and paediatrician are
> therefore Chronological Age (from D.o.b) and Corrected Gestational Age, all
> the rest can be worked out if you know todays date.
> o DOB = Today - Chronological Age
> o Gestation at birth = CGA - Chronological Age
> o Est date delivery = Today - CGA + 40*7
> o Est date of conception (rarely used in medicine) = Today - CGA + 15***
> Given the uncertain nature of life I think it is eminently sensible to
> record a confidence interval for CGA, as was previously suggested.
> Sorry about the verbal diarrhoea. Hope this is of some help, it obviously
> doesn't address the concept of elderly patients when birthdates are unknown
> etc, or fuzzy age ranges.
> Rob.
> ** P.s. currently (although I think stupidly) Chronological age in days is
> usually recorded as number of middays the baby has seen. Therefore, at
> 12:30pm an infant born at 11:58am is 1 day old, whereas one born at 12:02pm
> is 0 days old. Brilliant eh...
> *** P.p.s All this is calculated from the historical notion that birth
> occurs 40 weeks after last menstrual period. BUT in a regular cycle
> ovulation usually occurs 15 days after LMP. Conception will probably occur
> in the next 24-48 hours, foetal implantation 2-3 days later, and birth on
> average will occur about 37.5 weeks after this. If any bit is early or late
> the timing will all be off. Hence scans being more reliable.

Replies including

> Three of  the above 4 computations  are CGA based,  whereas CGA
> itself remains uncertain (being based on gestation). Do we base
> gestation on something like this ?
> if ( ultrasound done in 12 to 16 weeks ) {
>    gestation = ultrasound_gestation
>    confidence = (+/-) 2 days
> }  else  {
>    gestation = ($today - $LMP) + 14
>    confidence = (+/-) 3 days
> }

Topic revision: 25 Feb 2005, JamesBusser
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