Katie also began her physical development earlier than expected. At the age of seven she was a full head and shoulders over her classmates, and she too had developed conspicuous breasts. These were not the only harbingers of sexual maturity: one month after her eighth birthday, Katie had her first period.
These two children were both diagnosed by specialists as having a rare condition known as premature sexual maturity. Although precocious sexual development can occur in either sex, it is more common in girls.
Puberty normally starts between the ages of eight and 14 years, with menstruation following later, between the ages of 10 and 16. But Dr Tony Price, a paediatric endocrinologist at the Royal Manchester Children's Hospital, sees girls who develop sexual characteristics far earlier.
"We know that the onset of puberty is associated with diet and body weight, but we still don't know what actually triggers the process at a particular time," he says. "It all begins with a hormone called gonadotropin releasing hormone (GRH), which is secreted by the hypothalamus in the brain. This stimulates the pituitary gland into releasing hormones that activate the ovaries. But why this sometimes happens in very young children is still a mystery."
Jessica's height and physical development were not the only changes: puberty is normally accompanied by a growing sexual awareness, particularly of the opposite sex. Her mother, Anne, admits that she found her daughter's flirtatiousness hard to cope with.
"There were times that I could hardly bear to look at her," she recalls. "I had to keep reminding myself that she was just an innocent child under the influence of a condition she was not ready to handle. It meant I had to keep an eye on any men who came to the house - even close friends and family. I had to watch Jessica even more closely."
Katie, too, faced some painful difficulties. Her mother, Rose, says: "Katie seemed only a baby. She still believed in Father Christmas and the tooth fairy. I'd watch her lying on the floor playing with her doll's house, and I'd have to go upstairs and cry because this child, my baby, was wearing a sanitary pad."
Katie faced embarrassment at school. Primary schools are not generally prepared for girls who have periods: lavatory doors do not have locks, and there are no disposal bins. Katie regularly had to be excused from swimming classes. She felt very alone with her problem, unable to tell even her best friend.
A further problem for girls like Jessica and Katie is that when bones mature too early, they also fuse and stop growing too early. While these girls may be too tall for their age as youngsters, they may never achieve their potential full height, and are often shorter than average when adults.
Fortunately, in girls as young as this, doctors have found a way of switching puberty off - using GRH, the same hormone that sets it in motion. The discovery was a spin-off from fertility treatment. Manufactured gonadotropin releasing hormone was given to infertile women to stimulate output of hormones from the pituitary gland, in the hope that this would stimulate the ovaries to release eggs. Instead, this form of GRH had the opposite effect: it blocked hormone production, so the ovaries were never activated.
In girls who experience early puberty, gonadotropin releasing hormone is already secreted by the brain to stimulate the release of hormones from the pituitary gland, which in turn activates the ovaries. But doctors have found that giving an "overload" of GRH stimulates the feedback mechanism to "switch off" hormone production entirely.
The hormone is given by long-acting injection or by implant under the skin every 3-4 weeks. Treatment is continued until the girl reaches an age when puberty would be more appropriate.
However, there are drawbacks. The injection is painful. Hormone treatment can only arrest development - it cannot turn the clock back. And although no serious side-effects have been reported, doctors are uncertain whether it could have a long-term effect on fertility.
"When you treat children with a product that has only been developed in the last dozen years, you can't be sure there are no long-term effects," says Dr Price. "We can't really say what will happen in 20 years." The primary reason for treating sexual precocity is psychosexual, he adds; medical opinion is divided as to whether GRH has an effect on growth.
Jessica's mother felt she had no choice but to go ahead with hormone injections. At four years old, neither the little girl nor her family would have been able to cope with the changes puberty brings: allowing a four-year-old to menstruate would have been bizarre. The treatment was given by a paediatric specialist at a hospital near their home in the south of England. It began a few weeks before her fifth birthday.
After five years, doctors stopped giving Jessica hormone injections. The treatment arrested puberty and stopped her periods; but now she is 10 her physical development is free to continue and she has a mature look that belies her age and sexual innocence. "Everyone has a hard time remembering just how young she is, especially men," says her mother.
Because Katie was older, her case was considered more borderline and her mother decided to let her daughter's early puberty continue. Anne emphasised to Katie that what was happening was normal, but had just occurred a little earlier than usual. "If she was already suffering from embarrassment because of her height and bodily changes, I didn't want her to feel abnormal too," she says. "Having monthly injections would only give her the signal that what was happening to her was a bad thing, instead of a normal female event that happens to all girls sooner or later."
Without treatment, Katie's development continued to progress. At 10 she has the very attractive body of an average 16-year-old. Despite fears about her height, she is continuing to grow and has reached 5ft 1in. Rose worries about her sexual development, though - particularly when men look at her with a glint in their eyes. They too find it hard to believe she is still only a 10-year-old who plays with dolls.
Jessica has shared her secret with a few friends at school. Katie, meanwhile, still copes alone. For her, it will remain a secret for some time to come.
The Child Growth Foundation, a national charity, offers support and advice to families of children with growth problems: 2 Mayfield Avenue, Chiswick, London W4 1PW Tel: 0181-995 0257.Reuse content