Should weight be a barrier to getting pregnant?

That's the conversation a visiting professor is encouraging among New Zealand fertility specialists.

Richard Legro, professor of obstetrics, gynaecology and public health sciences at Penn State University College of Medicine, who is in New Zealand to speak at the University of Auckland, told the New Zealand Herald he did not believe there should be a weight limit for people to receive publicly funded fertility treatment.

In New Zealand, a woman has to have a body mass index (BMI) of between 19 and 32 to be eligible for publicly funded fertility treatments such as in vitro fertilisation (IVF).

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Women must also be under 40 and non-smokers to qualify.

Clinics can privately treat people with a BMI of more than 32 but still tended to encourage obese women to lose weight before treatment.

Legro said he believed the effect of weight was over-estimated when it came to pregnancy and obesity should be treated as a disease rather than an individual choice.

"Generally for patients with a disease we do our best to control it and go ahead with treatment. We don't arbitrarily deny somebody treatment on the basis they have a disease."

Data published so far suggested weight loss "might actually be harmful in terms of getting pregnant", he said.

A recent study in Holland found obese women who went straight into fertility treatment had a slightly higher success rate compared with obese women who were put on a six-month programme to lose weight before they were given fertility treatment.

Legro said he believed the pressure to lose weight put unnecessary stress on the body and made pregnancy harder.

"We're a funny organism where actually weight is viewed as a good thing for the organism and the more the better. So whereas reproduction will shut off when someone's thin, there are no upper limits to weight - it doesn't shut off," he said.

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"Whatever weight you reach your brain accepts that weight as being the normal weight ... It will always try to restore the most weight your body has ever had.

"What I would argue is that we view it as a threat to our survival to lose weight even though by public health standards we're obese."

Fertility Associates chairwoman Dr Mary Birdsall agreed recent research pointed to little benefit for overweight woman who lost weight but said there was plenty of evidence showing there was a higher number of miscarriages and an increased risk of high blood pressure, diabetes and pre-term delivery in obese women compared with those with a healthy BMI.

She did not believe the weight limit should be removed but admitted there were concerns ethnicities who tended to have a higher BMI, even when healthy, were missing out.

Birdsall said she was disappointed with the results of the study because she strongly believed losing weight would be beneficial.

The clinic treated many women with a BMI of more than 32 and she still encouraged them to lose weight before treatment because it was better for both mother and baby in the long term.

But the results of last year's study meant she could no longer "put my hand on my heart and say, 'you're going to have a better outcome'".

University of Auckland postgraduate professor of obstetrics and gynaecology Cindy Farquhar, who organised Legro's visit to New Zealand, said the restrictions around publicly funded fertility treatment were introduced in 2000 to create an equitable funding system.

Even overweight women who paid for treatment themselves were encouraged to lose weight first, she said.

"We would usually keep pushing that. We are really quite committed to the concept of a healthy pregnancy," she said. "It's a good public health message.

"If you start off life being overweight as a baby you are likely to be overweight throughout your childhood."

Richard Legro is giving a public lecture at the University of Auckland at 5.30pm next Tuesday, August 1, in lecture theatre 505-011.