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What is A case of Gestational Trophoblastic Neoplasia belongs to high risk group if disease develops after: and explain in detail?
High risk gestational trophoblastic disease.
FIGO in 2000 devised a staging system for categorisation of gestational trophoblastic neoplasia into good prognostic (low risk) and bad prognostic (high risk) disease.
Good prognosis (Low risk)

Short duration (< 4 months)
Serum β-hCG < 40,000 mIU/ml
Metastasis limited to lung and vagina
No significant prior chemotherapy 
No preceding term pregnancy 

Bad prognosis (Low risk)

Long duration (> 4 months)
Serum 6-hCG > 40,000 mIU/ml
Metastasis to brain or liver
Unsuccessful prior chemotherapy

Gestational trophoblastic neoplasia following term pregnancy.
Also if we have a look at the WHO prognostic scoring system: GTN following a term pregnancy comes under high risk.
Remember:

High risk GIN occurs after term pregnancy .
GTN occurs most commonly after H mole.