LONDON (AP) -- New research adds to a growing body of evidence that adult health is set, to some degree, by conditions in the womb and suggests that the programming may start earlier in pregnancy than previously believed.

A study published this week in the Journal of Epidemiology and Community Health found that fetuses that had shorter thigh bones 24 weeks into pregnancy had higher blood pressure at the age of six than those with longer thigh bones.

Understanding how life in the womb influences later health has become a hot area of medical research. Most of the study has focused on the effect of birth weight on the subsequent development of health and illnesses such as heart disease, high blood pressure, diabetes and osteoporosis, but the latest study is one of the first to find evidence earlier in human life.

Scientists believe that when a fetus is undernourished, it adapts its development, diverting resources to areas it really needs at the time, such as the brain, at the expense of organs it will need later in life. That may permanently change the baby's structure, functioning and metabolism, experts believe.

"There's a lot of work about the size at birth," said Dr. David Barker, an epidemiologist who pioneered the field of fetal programming research but was not involved in the latest study. "Birth weight is a crude measurement. It tells you very little because babies can reach the same birth weight by many different paths of growth.

"Now, because of technology advances, people are able to study children who have had serial measurements of size in (the uterus) that follow their growth, and these observations take us back into early pregnancy," said Barker, director of the epidemiology unit at the University of Southampton in England. "It looks as though blood pressure may be set fairly early."

The study, led by Dr. Kevin Blake at the University of Western Australia, involved ultrasounds done at 18, 24, 28, 24 and 38 weeks of pregnancy on 707 women with normal pregnancies. During each scan, doctors measured the circumference of the head and abdomen and the length of the thigh bones of the babies.

Blood pressure was successfully measured in about 300 of the resulting children at age six.

The researchers found that for every 2.6 millimeter (one-tenth of an inch) deviation from the typical thigh bone length at key stages in the womb, systolic blood pressure - the higher of the two numbers - at age six was changed by about 2 points. Shorter thighs meant higher blood pressure.

That effect was first seen at 24 weeks into pregnancy.

Neither head nor abdomen circumference were linked to later blood pressure.

"We don't know whether a very small change in blood pressure at age six has any value in predictability of disease, but it's still a move forward in understanding what kind of factors we can look at before birth and use as predictors for potential signs of disease later," said Kent Thornburg, a fetal physiologist who is director of the Heart Research Center at Oregon Health Sciences University and was unconnected with the research.

The thigh bone is one of the easiest to measure, and skeletal growth is a good measure of the rate a fetus is growing, said Mark Hanson, director of the Center for the Fetal Origins of Adult Disease at England's Southampton University.

"Skeletal growth is not just determined by how tall the parents are. There's a complex interaction between the fetal genetic drive to grow - inherited from the parents - and the environment in the womb in early gestation," said Hanson, a fetal physiologist who was not involved in the study.

"This study is focusing our attention on early gestation. It's making it clear that it really is fetal growth we're talking about here, not just some funny thing linked to birth weight itself. And it's pointing a finger very clearly that in the fetal growth process there's an interaction between the (genes) and the environment," he said.

Rat studies have previously indicated that the womb environment, influenced by the mother's nutrition, is important for later disease.

One study showed that pregnant rats given a low-protein diet for just the first four days of pregnancy - before the embryo even implants in the womb and before the placenta is formed - produced offspring with high blood pressure.

"There are extensive experiments in animals which show that fetal programming is a universal biological phenomenon," Hanson said.

The risk of disease in adulthood builds up over a lifetime and experts don't know exactly to what degree fetal programming influences the eventual likelihood of disease, but they believe it is considerable.

"The sort of calculations that have been done would suggest that it's certainly a bigger effect than smoking. It's certainly an effect at least of the magnitude of obesity and lack of exercise," Hanson said.

Copyright 2002 The Associated Press. All rights reserved.