Researchers say doctors and midwives are inaccurately recording estimated heights and weights of some pregnant women, which could lead to risks in the pregnancy being missed.
The researchers say there's evidence that women are being asked to report their own height and weight, rather than actually being measured - and that means people are guessing.
A study, published in the New Zealand Medical Journal, found 62 percent of women weighed more than what had been reported.
Measurement differences include women who were up to 13cm shorter, or some who were up to 15kg heavier, than was recorded.
Of the 248 women who took part in the research, only 6 percent had the correct Body Mass Index.
One of the research team is consultant obstetrician and gynaecologist, and senior lecturer at the University of Otago, Helen Paterson.
She says measurements are used to calculate growth charts, which help identify if a baby isn't developing enough, which can lead to stillbirth.
The measurements were recorded on forms referring the women for antenatal screenings for Down Syndrome and other conditions.
Dr Paterson says women should have better access to measuring equipment, for example in pharmacies.
But that's not enough for lobby group, Action to Improve Maternity.
Its spokesperson, Jenn Hooper, says it's an obligation of the GP, or lead midwife making the referral, to take the measurements themselves.
Radio New Zealand spoke to a number of midwives, all of whom said they measure women themselves but some did say that's not the case with everyone.
Professor Lesley McCowan is a fellow at the Royal Australian and New Zealand College of Obstetricians and Gynaecologists.
She says the research figures are alarming but not surprising, and it's part of the college's duty to make sure its students understand the importance of taking the measurements themselves.
Neither the College of Midwives nor the Midwifery Council were available for comment.
- 23% had a weight recorded that was within 0.5kg of the actual weight.
- 62% had under-reported their weight (so they were heavier)
- 15% had an over-reported weight.
- 30% had a correctly reported height
- 26% had an under-reported height (so they were taller than they thought)
- 44% had an over-reported height
- 6% had a correctly reported Body Mass Index
- 69% had an under-reported BMI
- 17% had an over-reported BMI