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Pregnant women are more likely to have deep vein thrombosis in their left leg than their right, researchers have found.

In a literature review, 88% of women for whom information on the location of thrombosis was available had it in their left leg, Wee-Shian Chan, MD, of Women's College Hospital in Toronto, and colleagues reported online in the Canadian Medical Association Journal.

"Anatomic distribution of deep vein thrombosis in pregnant women, and perhaps the pathophysiology of the condition, may indeed differ from that reported in the general population," the researchers wrote.

Studies in nonpregnant patients have shown that most deep vein thromboses of the lower limbs originate in the calf veins and progress proximally.

Yet less is known about the anatomic distribution of thromboses in pregnant patients. Observational studies have suggested it may be different in this population, but noninvasive diagnostic studies aren't usually conducted because of risks associated with exposing the fetus to radiation or a contrast agent.

So, to determine the distribution, the researchers conducted a review of six studies totaling 124 pregnant women who'd been diagnosed with deep vein thrombosis.

Overall, involvement of the left leg was reported in 88% of the patients for whom that information was provided.

The vast majority of thromboses -- 71% -- were restricted to the proximal veins, without involvement of the calf veins.

Among these proximal cases, 64% were restricted to the iliac and/or femoral vein.

"In addition to what was previously known -- that left-sided deep vein thrombosis is more common in pregnancy -- we also found that proximal deep vein thrombosis, restricted to the femoral or iliac veins, is also more common."

One possible explanation for the finding, they said, may be that a May-Thurner-like syndrome brought on by compression of the left iliac vein by the gravid uterus -- at the point where it crosses the right iliac artery -- "plays a major role in the increased incidence of iliofemoral deep vein thrombosis in late pregnancy."

However, they added that thrombosis occurs with equal frequency in all trimesters, so this hypothesis "wouldn't apply to deep vein thrombosis in early pregnancy."

Patients with proximal deep vein thrombosis have a higher risk of pulmonary embolism, so the researchers said the findings may change the way pregnant patients are diagnosed and managed.

"Until prospective diagnostic studies are available for pregnant patients, it may be prudent to conduct a routine examination of the iliofemoral venous system when a pregnant patient presents with suspected deep vein thrombosis," they concluded.

Researchers said the study was limited by the individual limitations of the included studies, and by a potential underestimation of the prevalence of the condition.

They pointed out that "deep vein thrombosis of the lower extremity was assessed with compression ultrasonography in more than half of the cases. Compared with venography, compression ultrasonography is relatively insensitive for the diagnosis of deep vein thrombosis of the calf and isolated iliac vein thrombosis. Therefore, we might have underestimated the prevalence of isolated calf vein or iliac vein thrombosis."

In a related editorial, Risto Kaaja, MD, of Turku University in Finland, wrote that based on the findings, perhaps there is "no common natural course of deep vein thrombosis" during pregnancy.

"In each instance, the site of the thrombosis and propagation can vary depending on individual risk factors for the condition," he wrote. Also, the fibrinolytic capacity of the woman may play a role.

"Pregnancy could contribute to a more chronic and thrombophilic state and therefore predispose to proximal deep vein thrombosis," Kaaja concluded, adding that the findings should be confirmed by larger prospective studies.