The problems are gestational hypertension — or high blood pressure associated with pregnancy — and preeclampsia, a medical disorder of pregnant women characterized by high blood pressure and protein in the urine.
If untreated, both conditions can lead to potentially life-threatening medical problems for the mother and complications of preterm birth for the baby, including low birth weight and size, respiratory distress, long-term neurological deficits and even death.
“Pregnant women from communities with low dietary calcium who received at least 1.5 grams of calcium by mouth [daily] during the second half of pregnancy had a lower risk of hypertension and preeclampsia, and of severe complications including death, than women who received placebo treatment,” said lead review author Dr. G.J. Hofmeyr.
The results were less dramatic for women with opportunities for better prenatal nutrition: “Women from communities with adequate dietary calcium had a small (10 percent) reduction in the risk of hypertension, but no significant reduction in the risk of preeclampsia or other adverse outcomes,” said Hofmeyr, head of the obstetrics and gynecology department, East London Hospital Complex in South Africa.
The review appears in the current issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.
The authors looked at 12 randomized controlled studies, one involving more than 4,000 expectant moms in North America. The most recent and largest of the studies — conducted by the World Health Organization — involved more than 8,000 pregnant women internationally.
Preterm birth, or birth before 37 weeks, often stems from high blood pressure and is the leading cause of newborn deaths, particularly in low-income countries. A newly released report from the Institute of Medicine found that 12.5 percent of U.S. births are preterm and cost society an estimated $26 billion a year.
The Cochrane review found that calcium supplementation during pregnancy is a safe and relatively inexpensive way to reduce the risk of high blood pressure in women more likely to develop it, as well as in women from communities where consumption of dietary calcium is low.
“Calcium supplementation may be of some benefit in reducing the morbidity associated with preeclampsia, and it does no harm,” said Dr. John T. Repke, chairman of obstetrics and gynecology at Penn State University College of Medicine.
“There is really almost nothing to lose from trying this approach, and based on this Cochrane Review, potentially something to be gained,” added Repke, who has been conducting research on preeclampsia and related disorders for nearly 30 years.
In January 2002, the American College of Obstetricians and Gynecologists issued the following statement: “Daily calcium supplementation has not been shown to prevent preeclampsia and, therefore, is not recommended.”
But the review may change the minds — and clinical recommendations — of some physicians because it is based on data not available in 2002. “The review is likely to convince health providers of the need to ensure that pregnant women have adequate calcium intake,” Hofmeyr said.
Hypertensive disease occurs in approximately 12 percent to 22 percent of pregnancies and is directly responsible for 17.6 percent of maternal deaths in the United States alone, according to ACOG. An estimated 40,000 women around the world die each year due to gestational hypertension and its complications.
Preeclampsia affects an estimated 5 percent to 6 percent of first-time moms, and 2 percent to 3 percent of them develop severe forms of the disease.
Because preeclampsia is frequently a “silent” disease that may occur without any symptoms, women are encouraged to have their blood pressure checked regularly during pregnancy, even when they feel well, said Hofmeyr. When symptoms do occur, they may include swelling, headache, blurred vision and upper abdominal pain. Severe disease can result in seizures, and the disease is then called “eclampsia.”
The exact cause of preeclampsia has yet to be identified. The only cure for the disease is delivery.
The review did not specify how much calcium in pregnancy is enough, but the authors said 1.5 grams (or 1,500 milligrams) per day was effective. They said more research was needed to find out if a lower dose would be effective in preventing gestational hypertension and preeclampsia.
Daily calcium supplements are thought to be safe by most doctors, and many women are already taking calcium as an added preventive safeguard against osteoporosis. One possible side effect associated with daily use of calcium supplements is stones of the kidney and urinary tract, but this remains controversial.
Most pregnant women in the United States are advised by their doctors to take prenatal vitamins daily, but the most common brands contain 500 milligrams of calcium or less, according to Repke.
Hofmeyr GJ, Atallah AN, Duley L. Calcium supplementation during pregnancy for preventing hypertensive disorders and related problems (Review). The Cochrane Database of Systematic Reviews 2006, Issue 3.
The Cochrane Collaboration is an international nonprofit, independent organization that produces and disseminates systematic reviews of health care interventions and promotes the search for evidence in the form of clinical trials and other studies of interventions. Visit http://www.cochrane.org for more information.