Step 1: In red degeneration a fibroid undergoes haemorrhagic infarction because its enlarging mass can no longer be perfused by its limited vessels, giving a deep red, fishy smelling cut surface.
Step 2: The growth spurt that triggers this is most marked during the middle and later part of pregnancy, so red degeneration is typically a mid pregnancy event.
Step 3: The woman complains of sudden pain and tenderness localised to the swelling, sometimes with a slight temperature and white cell rise, and the standard approach is supportive care since it resolves on its own.
Step 4: The very early weeks lack sufficient fibroid enlargement, the puerperium involves shrinking rather than expanding tissue, and nulliparity merely predisposes to fibroids without dictating the timing of this change.
\[\boxed{\text{Mid pregnancy}}\]