New research estimates the cost of surgery for children with brain and spine defects is about $1 million each - twice the amount of previous estimates for their lifetime.
Researchers say making it mandatory for bakers to add folate to bread could prevent about 70% of these defects and save the health system billions of dollars.
The study found the average cost of surgery for six teenagers with neural tube defects was $944,000 each, with three patients costing more than $1 million each.
The Otago University medical student who carried out the research, Eammon Deveral, says the total health bill including GP fees, physiotherapy and mobility supports, would be much higher.
The Paediatric Society says as half of all pregnancies are unplanned, it is too late for many women to take supplements to prevent birth defects and mandatory fortification of bread is needed.