In less than half of all cases, the condition begins shortly after conception.
RSS can be seen in identical twins, although it only affects one of the fetuses because twinning can disrupt the normal gene regulation. The association with twinning may not be identified because in 10 percent of identical twins, one twin dies early in gestation.
Increases in RSS can also be seen with the use of assisted reproduction techniques because the procedures can also disrupt gene regulation.
The syndrome is not inherited and there is no demonstrated cause in about half the cases, Graham said.
Many of the children suffer from bouts of hypoglycemia. Some have a tendency as adults to develop Type 2 diabetes.
"They are programmed [in the fetus] to grow and survive on much less nutrition than anyone else," Graham said. "Their metabolism is set to run on a leaner fuel mixture."
Graham estimates that the syndrome is under-diagnosed and as many as 1 in 10,000 children may have the condition.
The British media reported this week the story of Abbie King, 2, who is so tiny that she wears newborn-sized clothes and specially shoes. Standing, she barely reaches the arm of the sofa.
Two-feet tall and weighing less than 14 pounds, she is not much larger than her dolls. She has never had a high-chair because she can't see over the tray and, for the same reason, she's never had a baby walker. Like Ian, Abbie gets lots of stares and questions.
"When I took her to the supermarket when she was a baby, everyone cooed over her and said she looked like a little doll, but they always asked if she was premature," her mother, Emma Smith, told Britain's Daily Mail newspaper.
"As she's got older, people have said, 'Your baby shouldn't be eating a sandwich,' or, 'It's cruel to make that baby walk.' But when I tell them how old she is, they usually look embarrassed."
Like others with RSS, Ian and Abbie have distinguishing features: triangular face, a prominent forehead, narrow chin, small jaw and a mouth with turned-down corners. They can also have a curving of the fifth finger. Some have asymmetric growth in parts of their body, such as the legs, or learning disabilities.
Heather Earley, who has a healthy son, Alexander, 9, said she knew something was wrong when she gave birth to her tiny baby.
"Alexander was almost 9 pounds on my 110-pound frame, but Ian was only 4 pounds and he looked like an alien. He had a big head and was only 16 inches long," she said. "His head circumference was in the 80th percentile and his body was below the third percentile."