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Fat-busting ultrasound machines are giving overweight mums-to-be the chance to see their unborn babies' faces for the first time.

The technology at Auckland's Middlemore Hospital is a first for New Zealand and was developed to cope with a rise in the number of obese pregnant women.

Middlemore has the busiest maternity ward in the country. The number of obese pregnant patients increased from 15 per cent in 2006 to 35 per cent last year.

Several had body mass indexes higher than 50 - well above the recommended maximum of 25, said consultant obstetrician Yvonne Lake.

Ultrasounds allow staff to identify birth defects and other potential problems.

Layers of fat made it hard to get a clear picture of an unborn baby, she said.

The machines provided much clearer images, and could show them in 3D.

They have allowed staff to identify four cases of congenital heart disease that may not have been found otherwise.

"It's made a huge difference," said Lake.

Middlemore bought two $200,000 machines with funds from the Mayoress Ball and Lion Foundation.