Step 1: AFLP is a late-pregnancy emergency, so the answer should point to the latter part of gestation.
Step 2: The classic timing is the third trimester (it may spill into the early postpartum period), when fat accumulates in hepatocytes and the mother can develop jaundice and fulminant liver failure.
Step 3: The first trimester is far too early, and the intrapartum and immediate postpartum options do not match the recognised onset window as well as the third trimester does.
\[\boxed{\text{3rd trimester}}\]