August 17, 1999
A new mathematical analysis of a previous study shows that calcium supplements do not prevent preeclampsia in women at low risk for the condition. Preeclampsia is a dangerous, sometimes fatal, disorder of pregnancy that often strikes without warning.
The re-analysis, conducted by researchers at the National Institute of Child Health and Human Development (NICHD), appears in the August 18 issue of the Journal of the American Medical Association (JAMA).
"The results of this study will provide useful information to pregnant women and their physicians about the use of calcium supplementation during pregnancy," said Duane Alexander, MD, Director of the NICHD.
The NICHD researchers, Rebecca DerSimonian, Ph.D, and Richard J. Levine, M.D., both of NICHD's Division of Epidemiology, Statistics and Prevention Research, undertook the re- analysis to learn why the initial study differed in its conclusion from the NICHD-sponsored calcium supplementation for the prevention of preeclampsia (CPEP) trial. CPEP found that providing pregnant women at low risk for preeclampsia with calcium supplements does not reduce their chances for preeclampsia. (A summary of the CPEP trial, "Study Finds Calcium Does Not Prevent Potentially Fatal Disorder of Pregnancy," appears on the Mass Media page of the NICHD website, at http://www.nichd.nih.gov/newsreleases/.)
The study that Drs. DerSimonian and Levine re-analyzed was a meta-analysis, a statistical technique used to analyze a number of studies with inconclusive or inconsistent results. This study, "Effect of calcium supplementation on pregnancy-induced hypertension and preeclampsia: a meta-analysis of randomized controlled trials," was originally published in JAMA in 1996.
"Pregnant women need calcium," Dr. DerSimonian said. "But taking calcium supplements so that the total calcium consumed exceeds the recommended level for calcium doesn't appear to offer any benefit for women at low risk for preeclampsia." According to the Food and Nutrition Board of the Institute of Medicine, pregnant women age 19 and over need 1000 milligrams of calcium a day. Pregnant girls from ages 14 to 18 need 1300 milligrams of calcium a day.
Briefly, the NICHD authors contended that the studies included in the meta-analysis varied considerably from one another. Because these variations were not taken into consideration, the meta-analysis incorrectly concluded that calcium supplements appear to prevent preeclampsia.
In their article, Drs. DerSimonian and Levine noted that many of the studies included in the meta-analysis differed in their definition of preeclampsia. In addition, many of the studies failed to include placebo groups. "In the absence of a placebo-control group," the authors wrote, "any effect of calcium supplementation cannot be distinguished from a placebo effect."
Some of the studies included only women at high risk for preeclampsia, whereas others included only women at low risk for the condition. The studies also differed substantially from each other in the amount of dietary calcium the women consumed, with some women having consumed comparatively low amounts of calcium and others having consumed comparatively high amounts.
After these sources of variation were taken into account, a re-analysis of the combined data from all the studies showed that calcium supplements did not offer any beneficial effects in women at low risk of preeclampsia.
The authors noted that the re-analysis implied a possible benefit of calcium supplementation in high-risk women. They cautioned, however, that the small number of women who took part in these studies and inconsistencies in the data from one study made it difficult to determine if this finding is valid.
"Additional carefully controlled randomized trials are needed to establish whether calcium supplementation may indeed reduce the incidence of preeclampsia in healthy women at high-risk, especially in the presence of low dietary calcium intake," they wrote.
Preeclampsia is a life-threatening complication of pregnancy in which a woman may develop dangerously high blood pressure and begin excreting protein in the urine. About 5 percent of first-time mothers and 1 to 2 percent of mothers having subsequent pregnancies develop the condition. Preeclampsia may occur suddenly, without warning--sometimes in as little as a few hours. In cases where preeclampsia does not progress to eclampsia--convulsions and risk of death--the children born to mothers with preeclampsia may be extremely small for their age or may be born prematurely. This may, in turn, place them at risk for a variety of other complications of birth. Although the high blood pressure accompanying preeclampsia can be treated with blood pressure lowering drugs, the only effective way to end the condition is by delivery.