by LUCY LAING

Last updated at 08:11 27 November 2007

As a first-time mother, Sara Herman thought it was only to be expected that she had an enormous bump after five months.

What she and doctors failed to realise was that the petite mother-to-be, who is only just over 5ft, was expecting a giant.

When Jack eventually arrived at 37 weeks he weighed an astonishing 14lb 8oz.

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And now, after health problems kept him in hospital for four months, he is enjoying life at home in Manchester with 37-year-old Mrs Herman and her husband Keith.

"At five months I looked like I was going to give birth at any moment and the size of my bump meant it was difficult for me to get around," said Mrs Herman, a product sourcer.

"But because it was the first time I'd been pregnant, I just thought this was normal."

Mrs Herman became pregnant in September last year, shortly after marrying Keith, who runs a sales promotion business.

A scan at 14 weeks showed the baby was growing fast and doctors even thought that the due date was wrong as the baby was so large.

"When they said they thought Jack was going to be big, I imagined a 9lb or 10lb baby at the most," she said. "And that's what the doctors thought too."

When she was five months pregnant, the couple went on holiday to New York, where strangers asked her if her baby was due any day.

She said: "Jack was actually about 7lb then, so he was actually the size of an average new-born baby.

"I was finding it very difficult to get around the department stores, and had to lie down in the afternoons as I was so exhausted from all the weight I was carrying."

At 37 weeks, Mrs Herman was admitted to Wythenshawe Hospital in Manchester as she had not felt the baby move much for the last two days.

"He was actually so big that there wasn't much room for him to move around, which is why I hadn't felt much movement," she said.

"I was taken straight down for a caesarean. The doctors actually gasped when they brought him out."

Tests showed that Jack had grown so big because of a condition called hyperinsulism which causes the body to produce too much insulin, leading to weight gain.

His heart muscle had thickened since it had to work extra hard because of his size.

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Mrs Herman said: "He may have looked big and healthy, but he was actually life-threateningly ill. They had to put him on a drip.

"I went in to see him after a few hours and I couldn't believe his size. He was in the unit with premature babies, who looked tiny compared to him. He actually filled the whole incubator. After a few days, the doctors had to get him a bigger one."

Jack was transferred to Pendlebury Children's Hospital in Manchester, which is setting up a support group for children with hyperinsulism.

At the age of 14 weeks he had 95 per cent of his pancreas removed to treat his condition. The operation was a success and he was allowed home with medication.

His heart condition has also improved.

He weighs nearly 22lb, compared with the average weight at six months of 16lb, and wears clothes for a child aged 12 to 18 months.

Mrs Herman added: "I have to manage his weight very carefully because of his condition. Surprisingly, he doesn't have a good appetite but he's starting to eat rice and mashed banana.

"He's such a happy baby, always laughing and giggling. People can't believe that he's only six months old when I take him out in his pram."

• Britain's heaviest baby was Guy Warwick Carr, delivered in Cumbria in 1992 weighing 15lb 8oz.

The biggest ever born was a boy produced in 1879 by Anna Bates of Canada, who like her husband was more than 7ft tall. The infant weighed 23lb 12oz and died 11 hours after birth.