Fix CHIP FCEP immigration and entity handling

[daphnehanse11]

Problem

PolicyEngine-US currently models CHIP FCEP as a pregnant-person eligibility category and applies an immigration-status screen to the pregnant person. That appears inconsistent with the federal FCEP / unborn-child CHIP option.

CMS describes the From-Conception-to-End-of-Pregnancy (FCEP) option as CHIP coverage for targeted low-income children from conception to end of pregnancy, and says states can provide prenatal care and pregnancy-related benefits "regardless of their parent's citizenship or immigration status":

https://www.medicaid.gov/chip/chip-eligibility-enrollment

42 CFR 457.10 also defines "child" for CHIP purposes as an individual under age 19, including the period from conception to birth:

https://www.law.cornell.edu/cfr/text/42/457.10

This is distinct from #8521, which concerns Alabama county/geographic limits for FCEP. This issue is about the entity and immigration treatment.

Evidence in the code

policyengine_us/variables/gov/hhs/chip/is_chip_fcep_eligible_person.py:

• labels the variable "Pregnant person eligible for CHIP through FCEP"
• documents FCEP as the "Family Coverage Extension Program"
• requires is_pregnant
• checks the pregnant person's immigration_status
• excludes UNDOCUMENTED people with immigration_eligible = ~undocumented

istatus = person("immigration_status", period)
undocumented = istatus == istatus.possible_values.UNDOCUMENTED
immigration_eligible = ~undocumented

return (
    is_pregnant
    & state_has_fcep
    & immigration_eligible
    & ~medicaid_eligible
    & income_eligible
)

policyengine_us/variables/gov/hhs/chip/chip_category.py also represents the category as PREGNANT_FCEP, and generic is_chip_eligible then feeds into ACA eligibility blocking through policyengine_us/variables/gov/aca/eligibility/pays_aca_premium.py.

Reproduction

Same Alabama 2026 pregnant household, around 194% FPL, not Medicaid eligible. The only difference is immigration status.

from policyengine_us import Simulation

for status in ["CITIZEN", "UNDOCUMENTED"]:
    situation = {
        "people": {
            "p": {
                "age": {"2026": 25},
                "employment_income": {"2026": 42_000},
                "is_tax_unit_head": {"2026": True},
                "is_pregnant": {"2026": True},
                "immigration_status": {"2026": status},
            }
        },
        "tax_units": {"tu": {"members": ["p"]}},
        "families": {"f": {"members": ["p"]}},
        "spm_units": {"spm": {"members": ["p"]}},
        "marital_units": {"mu": {"members": ["p"]}},
        "households": {"h": {"members": ["p"], "state_name": {"2026": "AL"}}},
    }
    sim = Simulation(situation=situation)
    print(status)
    print("medicaid_income_level", sim.calculate("medicaid_income_level", 2026))
    print("is_medicaid_eligible", sim.calculate("is_medicaid_eligible", 2026))
    print("is_chip_fcep_eligible_person", sim.calculate("is_chip_fcep_eligible_person", 2026))
    print("chip_category", sim.calculate("chip_category", 2026))
    print("is_chip_eligible", sim.calculate("is_chip_eligible", 2026))

Current output:

CITIZEN
medicaid_income_level [1.9408503]
is_medicaid_eligible [False]
is_chip_fcep_eligible_person [ True]
chip_category ['PREGNANT_FCEP']
is_chip_eligible [ True]

UNDOCUMENTED
medicaid_income_level [1.9408503]
is_medicaid_eligible [False]
is_chip_fcep_eligible_person [False]
chip_category ['NONE']
is_chip_eligible [False]

Expected behavior

For states with an active FCEP income limit, FCEP eligibility should not be denied solely because the pregnant parent is undocumented. The non-financial test should follow CMS's FCEP framing:

• pregnant woman is uninsured
• pregnant woman is ineligible for other Medicaid/CHIP coverage
• pregnant woman is a state resident
• household income is at or below the state's FCEP standard
• any additional state-specific FCEP criteria

But the parent's immigration status should not be a disqualifier for the FCEP unborn-child option.

Possible implementation approach

• Rename/clean up FCEP labels and documentation from "Family Coverage Extension Program" to "From-Conception-to-End-of-Pregnancy".
• Remove the pregnant person's immigration-status exclusion from the FCEP formula.
• Consider whether the FCEP variable/category should be renamed away from *_person / PREGNANT_FCEP, or at least document that it is a proxy for unborn-child CHIP coverage because the model has no unborn-child entity.
• Re-check the ACA interaction: current is_chip_eligible can make FCEP block ACA marketplace eligibility for the pregnant person as though she herself has full CHIP coverage. If FCEP is only unborn-child / pregnancy-related coverage, this may need separate handling from ordinary child CHIP.
• Add regression tests for at least one FCEP state showing that undocumented-parent and citizen-parent cases with otherwise identical facts get the same FCEP result.