Whanganui midwives are working to educate people about the dangers of wrapping babies in polar fleece blankets.
"We know polar fleece blankets are made from the likes of recycled bags and bottles," midwife Angela Adam said.
Mrs Adam, who is Whanganui's maternity community quality co-ordinator, said polar fleece blankets were made from plastic-based fibre which meant the materials did not breathe.
This meant babies could overheat or sweat, then get too cold because the sweat would not evaporate. Both of these could contribute to sudden, unexpected death in infancy.
"As midwives in the maternity unit, we don't support the use of polar fleece blankets," Mrs Adam said.
Charge midwife Lucy Pettit said the maternity ward had boxes of wool-based blankets people had donated, which they used as the fabric could breathe.
"I don't think the general public realise exactly what [polar fleece blankets] are made of and how that can cause your baby to overheat," Mrs Pettit said.
Mrs Adam said babies who were wrapped in polar fleece had no circulation of air around their bodies.
While there was no confirmation that polar fleece blankets contributed to sudden unexpected death in infancy, "it's actually the overheating that can contribute to SUDI or cot death, but polar fleece blankets contribute to overheating."
She said babies looked like "little beetroots" when they had been wrapped up in polar fleece.
At Children's Day on Sunday, the midwives set up a stall to educate Whanganui people about the blankets. Mrs Adam had a blanket stitched out of plastic supermarket bags which she used to demonstrate what polar fleeces were like.
She said a polar fleece blanket could be bunched up with some water poured inside, and they would not leak, because of the non-breathable material.
Mrs Pettit said she would be surprised if there wasn't a polar fleece blanket in every household that had a baby. Raising awareness about their dangers was all part of encouraging safe sleeping, she said.