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Dr Helen Paterson, also a senior lecturer at the Dunedin School of Medicine, said research in the board's catchment area last year showed many lead maternity carers allowed women to self-report heights and weights, rather than take a measurement.
Exact measurements were important because the data helped track the growth of the fetus.
Sophisticated modelling tools were available but relied on correct inputs.
Dr Paterson believed leadership from the Ministry of Health was needed.
While lead maternity carers were required to record height and weight, there was no guidelines on how they were collected.
Obesity was associated with an elevated risk of having a stillbirth, needing a Caesarean section, and serious conditions such as pre-eclampsia.
Having correct information could save a baby's life if a clinician intervened preventing a stillbirth, she said.
Dr Paterson said the data "empowered" women and she was keen to work co-operatively with lead maternity carers to improve the situation.
Dr Paterson's views had changed on obesity, which she no longer believed was a "choice". Choices were limited by a person's environment and genes.
The research, jointly carried out by Dr Paterson and Dr Sarah Parker, showed 69% of Southern DHB lead maternity carers used the woman's own estimate of weight, and 60% used the woman's height estimate.
Dr Paterson said the next step was determining the accuracy of estimates, based on other research. Early indications were they were not a reliable guide.