Created at 3:56 a.m. Jan, 10, 2026
Author: nmajo97
Related Note: 1463944377495 2
Rationale for change

Issue: The current card wording (“What day is characterized by chorion formation?”) is misleading because it conflates histologic membrane formation with twin-splitting compartment commitment.

Clarification:

Embryologically, the amnion forms before the chorion (amnion ~day 8 from epiblast; chorion ~days 9–12 from trophoblast + extraembryonic mesoderm).

In twin development, “day 4 = chorion” refers to commitment of chorionicity, meaning that after day 4, monozygotic twins must share a chorion due to shared trophoblast fate.

Recommendation: I reworded the card to explicitly reference mono­zygotic twin chorionicity commitment, not literal membrane formation, and add an explanatory note to prevent conceptual confusion.

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