Option to use Expected Due Date for weight percentiles (Gestational/Adjusted Age)

[pietrouk]

When tracking premature infants, using their actual birth date incorrectly positions them on the WHO weight growth charts. This makes them appear significantly underweight compared to full-term peers. Clinical practice routinely plots premature infants using their "adjusted age" (calculated from their expected due date) until they reach 2 years of age.

Issue #369 previously discussed this under a much larger scope (a full "Adjusted Age" mode toggling timelines, developmental milestones, and age calculations across the entire application). Because of that broad scope, it possibly became complex and stalled out?

I would like to propose a much simpler, isolated implementation: an optional field in the Child model for Expected Due Date, which is only used to shift the horizontal axis of the growth charts.

• Child Profile Setup: Add an optional field Expected Due Date.
• Growth Chart Logic: If Expected Due Date is populated, the growth percentile charts (Weight, Height, Head Circumference) calculate the child's age relative to that date rather than the Birth Date. If not yet present, ideally populate the WHO charts from W23, see here for an example

Thank you!

[pietrouk]

Following up on my own post: I had a look through the code, and the isolated version of this turns out to be small for the two charts that already have WHO overlays (Weight and Height).

The percentile graphs already position the WHO curves relative to the child's birth date. In reports/graphs/weight_change.py and
height_change.py, each reference point is plotted at birthday + age_in_days. So switching to corrected age is just a matter of feeding those functions a different anchor date, with no changes to timelines, age display, or anything else in the app.

Concretely, I think it's four small touch points:

1. core/models.py: add an optional due_date to the Child model (DateField, blank=True, null=True).
2. A migration for that field.
3. core/forms.py: add due_date to ChildForm.fields with a DateInput() widget.
4. reports/views.py: in the Weight and Height report get_context_data, pass child.due_date or child.birth_date as the birthday anchor. The boy/girl variants inherit this method, so one line each covers all of them.

That's the whole behaviour change. If due_date is left blank, everything stays exactly as it is today for term births.

Happy to open a PR if that'd be welcome.
Thanks again!

[cdubz]

Open to PRs! Just slow to respond a lot of the time 😁

[pietrouk]

Done!