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.
The ministry says it cannot be certain dioxin is to blame.
Campaigner Andrew Gibbs, who contributed to the study, says babies born with webbed fingers and toes, tiny skulls or cancer were left out of the review of defects.
The rate of New Plymouth babies with spina bifida was twice Northland's and three times the national rate. Both conditions can be caused by exposure to dioxin.
The ministry's chief adviser of child and youth health, Pat Tuohy, says most rises did not reach statistical significance, so they could be explained by chance.
Picking those three years out could have been a high year just on random chance, or it could have been a high year because other things were happening.
Clearly, there is a concern that dioxins may have been one of those other things, but unfortunately the study wasn't able to prove one way or another whether any or all of these cases were associated with dioxin.
Dr Tuohy says the study did not investigate whether dioxin caused the defects so he can not say for certain what happened.