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.
