"If there are no obvious things, which is the usual case, you will see babies who are basically all over the place - and that must be a very frightening experience for them."
Rowley said often the mother could also have been using other drugs at the same time, which could lead to "withdrawal" symptoms that needed close monitoring and, in the worst cases, medical treatment.
Symptoms included a high-pitched cry, difficulties being consoled, sneezing, tremors, hyperactive reflexes, vomiting, diarrhoea, dehydration and unstable temperatures.
"Babies worst affected by P to the point of needing treatment would be given morphine on the assumption that other drugs are exacerbating the signs of withdrawal."
Morphine would be given in regular doses that are slowly reduced until the symptoms subsided.
He said with the appropriate support the mothers and babies could be helped through the withdrawal period.
"These symptoms can usually be managed with good midwifery advice and support in the first few weeks. Of more concern is what happens after that - the next months and years."
Currently he said there was little good research around the short- and long-term outcomes for these babies.
Rowley estimated the hospital was alerted to 25 to 50 babies - "one every couple of weeks" - who had been exposed to P in pregnancy.
However, he said this was likely the "tip of the iceberg" as many mothers failed to disclose their drug use.
Nationwide the number of babies exposed to methamphetamine was not readily available and even figures around its overall use was dated.
The Ministry of Health's New Zealand Drug Harm Index 2016, which drew on 2012/2013 Health Survey data, reported amphetamines had the third highest number of dependent users -1400 (4.7 per cent) of 29,900 drug users around the country.
They had the fourth highest number of casual users - 24,300 (6.7 per cent of all drug users) of 358,100 .
The most common drug in both categories were cannabinoids, with 26,000 (86 per cent) dependent users and 253,300 (71 per cent) casual users.
However, amphetamines were deemed to be the most harmful drugs overall and were accountable for 32 deaths, 43 per cent of all drug deaths in 2011.
Massey University's Illicit Drug Monitoring system indicated use of meth was growing.
It showed the percentage of frequent meth users who reported having injected meth in the past six months increased from 28 per cent in 2006, to 36 per cent in 2009 and to 53 per cent in 2014.
Researcher Liz Gordon, who published some of her latest findings in a Grandparents Raising Grandchildren report released this week, described P as a "terrible scourge" on society.
She said the number one reason the grandparents took over care of their grandchildren was because of a drug addiction.
"For most of those it was P, a high percentage if not virtually all," she said. "The key thing about it is it's so addictive. It's a poison."
Rowley said inherently these mothers wanted to do good for their children - but were in many ways victims of their social circumstance.
"I once asked somebody who had given it up, how she achieved it. She said: 'Quite frankly, I just stopped seeing all my friends and acquaintances'.
"It's the peer group that's doing it and often in this situation there's a partner who's responsible for sustaining the lifestyle."
Director of the New Zealand arm of a longitudinal study into the outcome of these "P babies", Trecia Wouldes, said methamphetamine was one of the more problematic drugs in society.
Called the Ideal (Infant Development Environment and Lifestyle) study, it had tracked the neuro-development of 107 children in Auckland whose mothers had used meth during pregnancy.
But her latest batch of funding had run out and she was applying for grants in the hope that someone would underwrite the project and ensure its future.
So far the study had established these children did lag behind their peers in the first few years of life.
"Boys, particularly Maori boys, who were exposed to P were lagging in motor skills and cognitive development."
However, she said there was also evidence to indicate the environment also played a significant role in how these children fared in later years.
"If [a child] goes into a nurturing environment it's going to have a better outcome than a baby that goes into a less optimal environment."
Wouldes was keen to see the study go further to see how the children fared as they entered adolescence.
"Our kids are starting to turn 11. It's important to be able to determine if they got a poor outcome because they are exposed prenatally to drugs, or if there's a combination of the drug and environment."
She said New Zealand had a drug abuse problem and women were using drugs during pregnancy so it was important to understand what this meant.
"How do we intervene and stop the intergenerational process of children born to drug users becoming drug users?"
The numbers
1400 dependent users
29,900 casual users
53% of frequent users say they injected the drug during part of 2014
32 deaths attributed to amphetamines in 2011
