An anti-dioxin campaigner says the latest study of contamination from New Plymouth's Dow chemical factory has missed many birth defects it was supposed to review.
The Health Ministry study compares defects recorded by New Plymouth's head maternity nurse from 1964 to 1971 with results from other hospitals.
It found that when dioxin contamination from the city's chemical plant was at its worst, New Plymouth had significantly more birth defects than the national average.
It also had significantly more deformed babies than all other hospitals studied except the specialist National Women's Hospital.
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The ministry says it cannot be certain dioxin is to blame.