Returns CMS monthly enrollment for a Medicare Advantage plan or contract: the member count, the trend over time, and, at contract level, the MA-only vs Part-D split, plus org/parent and plan type. Filter by contract_id (required) + optional plan_id (plan grain vs contract total), and optional year ('2024') or month ('2024-03'); with neither, returns the latest month plus a trailing 13-month trend. Monthly coverage back to 2006. Every row carries the year-over-year change (yoy_change / yoy_pct) vs the same month one year earlier; plan rows carry the plan's SNP class (snp_type) when applicable.

RETURNS a detail payload, not a bare count: value.summary (latest_month, latest_enrollment, yoy_change / yoy_pct, ma_only_count / part_d_count, trend min/max and change over the window), value.months (the per-month trend rows), value.row_counts (matched / returned / truncated / limit) and value.query (the filters actually applied). limit bounds the trend rows returned (default 13 months, max 2000); a truncated list always carries a caveat.

Enrollment is the SPINE every other MA metric joins to (per-member, share-weighted). SUPPRESSION is honest: CMS redacts cells under 11 enrollees, which come back as count=null with a [lower, upper] bound and suppression_reason='redacted_small_cell', never a zero, and the suppression caveat fires only when a returned month is actually redacted. A contract id that does not exist returns found=false with a not_found_reason, never a suppression story.

Pair with get_pbp_benefit or get_stars to weight or normalize by members. SOURCE: CMS Monthly Enrollment by Plan / by Contract.