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Nov. 22, 1999 (Atlanta) -- How well the placenta functions may play a crucial role in determining the risk of developing high blood pressure and heart disease later in life, according to the results of two studies published in the Nov. 20 issue of The British Medical Journal.
Many studies have shown newborns with low birth weight have higher blood pressure as children and as adults. However, researchers haven't been able to convincingly sort out if the low birth weight causes the high blood pressure, or to what extent other factors such as maternal lifestyle (smoking, nutritional habits) or familial genetics play a role.
"[It] does look from our study to be a genuine causative association whereby babies that are born small tend to have higher blood pressures later in life," the lead author of one of the studies, Neil R. Poulter, MBBS, tells WebMD. "There have been about 36 of these studies published, all of which are pretty consistent in showing that link," Poulter is a professor of preventive cardiovascular medicine at Imperial College School of Medicine in London.
Poulter and colleagues analyzed blood pressure and birth weight data from almost 500 pairs of female twins with an average age of 54. The reported difference in birth weight was calculated between the heavier and lighter twin and arranged into four categories (0 grams, one to 500g, 501 to 1,000g and greater than 1,001g). Weight, height, and tobacco and alcohol consumption were considered as potential important factors.
Before and after adjusting for these other factors, the researchers discovered blood pressures were higher among the lighter than the heavier twins. The researchers also found that the greater the birth weight difference between the twins, the greater the difference in blood pressure.
"Because we did it in twins and because it was the same mother that was eating for both the heavier and the lighter twin, you can't blame the maternal diet -- at least in the context of our study -- for producing those differences in weight and therefore the differences in blood pressure later in adult life," says Poulter. "It has to be something other than the maternal diet [and genetics], and this would presumably mean it is some aspect of [the placenta's function]."
Poulter says he cannot describe the mechanism. "In dizygotic twins -- they have separate placentas -- it may be that one just implanted better than the other. In the monozygotic twins, they share the same placenta, but one artery from the baby to the placenta was bigger than the other; it was just an unequal distribution of the goodies," he says.
The second study, led by Terence Dwyer, a professor at the University of Tasmania, studied the association between blood pressure and birth weight by analyzing data from almost 900 eight-year-old children. The researchers found that blood pressure decreased with birth weight and increased with current body mass.
Like Poulter, Dwyer and colleagues list unequal distribution of nutrients and oxygen as a possible factor that could be associated with birth weight and blood pressure. They write that reduced nutrient and oxygen availability could lead to permanent adverse programming of the cardiovascular system.
Finally, words of caution from Poulter. "We are not saying maternal smoking [isn't responsible] for small babies; we are not saying that diet doesn't affect the size of the baby," he says. "What we are saying is that it isn't those maternal which produce a spurious association between birth weight and blood pressure; it is a real association, independent of what the parents do."
"And this also doesn't say that if you're born small, you've had it, you are bound to get hypertension [high blood pressure], or if you are born fat you are bound not to get hypertension," cautions Poulter. "It in no way suggests that the major environmental factors aren't still very valid."
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