The first generation of New Zealand babies exposed to methamphetamine in the womb is reaching school age, and researchers are finding they are worse affected than babies in the United States.

Auckland University senior lecturer Dr Trecia Wouldes co-authored a study comparing 120 New Zealand women who used methamphetamine while pregnant to a group of US drug mothers.

New Zealand's methamphetamine is reportedly purer and stronger than the American form, and the New Zealand newborns were found to have worse scores for central nervous system stress and reflexes responses.

There were also more babies born with "hypertonicity" and "hypotonicity" – babies that are either too floppy or too rigid.

Central nervous system stress was indicated by higher than average hiccuping, spitting up, and very tight blinking.

"It's a sign your central nervous system has been affected by something," Wouldes said.

She said studies of older children exposed to methamphetamine in the womb showed changes in brain structure, particularly in areas that governed "executive functions" linked to learning.

At about age four, there started to be "significant differences in IQ and functional ability to learn" caused by inability to concentrate, sit quietly and "attend" to lessons.

Wouldes said she began enrolling babies in her study five years ago and some of the older ones were "definitely" showing signs of developmental difficulty.

The 120 mothers in the study all came from Auckland and Waitakere and were referred by midwives and neo-natal carers.

Of those referred, many had to be ruled out as too young, or for using too many other drugs.

Wouldes said prevalence of meth use during pregnancy was hard to gauge but one indicator was the difficulty in finding non-drug using mothers to use as a control group.

"In young mothers, 17 and 18, without NCEA qualifications, we couldn't find enough mothers who didn't use meth," Wouldes said. "Every time we'd find a mother who didn't have NCEA, she'd be using meth."

Though children of methamphetamine users were showing learning disabilities, Wouldes said it was too early to attribute it purely to the drug. "Our mothers are using more cannabis, smoking more and using meth throughout their pregnancies more than in the US," Wouldes said.

Also, alcohol was still New Zealand's biggest drug problem, she said, with 60 per cent of the methamphetamine mothers drinking through pregnancy, 13 per cent drinking "heavily".

The study also found 87 per cent smoked cigarettes and 64 per cent used marijuana while pregnant.