There's no conclusive evidence that nitrate in drinking water causes adverse birth outcomes such as preterm birth or birth defects.
That's according to my analysis of a decade's research. But we can't be complacent.
Aotearoa New Zealand needs to keep close watch on the emerging research, and on the maximum levels of nitrate we allow in our drinking water.
Around the world, studies show statistical associations between nitrate in drinking water and bowel cancer, and between nitrate in drinking water and adverse birth outcomes.
Farming communities are a key focus, since nitrate from fertiliser and animal waste can leach into water supplies.
The Government asked the Liggins Institute to assess the evidence on birth outcomes because we specialise in research to keep mothers and babies healthy, and because of hypothetical reasons to be concerned.
Animal studies have shown nitrate crossing the placenta, affecting a baby's development. In humans, statistical associations have been shown between nitrate in drinking water and congenital abnormalities, preterm birth, low birth weight and small-for-gestational-age infants.
The potential mechanism for harm is nitrate triggering the conversion of haemoglobin, the blood's oxygen-carrying molecule, into methaemoglobin, which reduces the ability of the blood to transport oxygen to cells.
The research review focused mainly on five studies from the US, Canada, France and Denmark, relating to a total of 5 million people.
Some US research was suggestive of a connection between levels of nitrate in drinking water and an increased risk of giving birth to a baby with a limb deficiency. (A limb deficiency is when a baby's leg, arm, hand, finger, foot or toe is crooked, uneven, partially formed or even missing because part or all of a child's limb does not completely form during pregnancy.)
In US research published in 2013, expectant women exposed to more than 17.09 milligrams per litre of nitrate in drinking water had nearly twice the odds of giving birth to a child with limb deficiencies compared to women exposed to less than 3.14 milligrams per litre.
However, association is not the same as causation.
Effects potentially associated with nitrate could be due to other factors such as mothers' diets or their exposures to nitrate-related "nitrosatable" drugs such as decongestants and antihistamines.
Nitrate levels can fluctuate and water can contain other compounds that are not measured so it is harder still to make a conclusive link. And research methods differ, making direct comparison difficult.
In summary, the evidence in relation to adverse birth outcomes is uncertain, inconsistent and inconclusive.
Let's be clear: we generally encounter very low levels of nitrate in drinking water.
Aotearoa New Zealand deems 50 milligrams of nitrate per litre the maximum acceptable level, the same as the World Health Organisation and similar to a US federal maximum.
A 2020 study estimated that more than 60 per cent of the population were exposed to less than 2 milligrams per litre, and only 0.1 per cent to more than 25 milligrams, based on the data available for monitored supplies.
Women drinking water from a registered supply have no cause for worry about adverse birth outcomes based on the evidence available. However, women living on a farm using an unregistered water supply might consider getting it tested. A recent study found the average nitrate level in unregistered supplies is 12.22 milligrams per litre.
It's worth noting that only a tiny proportion of birth defects are caused by exposure during pregnancy to certain chemicals and only 10 per cent of our total nitrate intake comes from water; most is from food.
And although any nitrate effects remain uncertain, there are known and preventable risk factors for birth defects, which affect about 2400 babies each year.
Genetics, a lack of the right nutrients in pregnancy, smoking, alcohol and catching rubella during pregnancy can all be causes.
The lack of conclusive evidence on nitrates does not mean we should not be cautious.
We should step up nitrate monitoring, which is so limited that it would be very difficult to discover any statistical association here. We also need to keep reviewing the maximum allowable level of nitrate, given concerns about the potential effects of chronic exposure at lower levels.
In the US, all public water systems are required to be monitored at least annually. Here, in 2019, nitrate monitoring was only required for water supplies servicing 1.1 per cent of the population.
For science to prove a causal connection would require a randomised trial, ethically impossible as it would be wrong for pregnant women to ingest nitrate for research.
The only option is for scientists to continue studies covering large numbers of women, featuring more precise evidence of the levels of nitrate they're drinking, and of other potential contaminants in the water. And this country needs to keep a close watch on those studies.
• Dr Luling Lin is a Postdoctoral Fellow at the Liggins Institute.