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Cesaran section was associated with a four-fold greater risk for postpartum venous thromboembolism (VTE) compared with vaginal delivery, according to a review and meta-analysis.

Based on the pooled data analysis, roughly three in 1,000 women who deliver by C-section will develop the life-threatening blood clots, reported , of Switzerland's Geneva University Hospitals, and colleagues.

But the authors also noted that low utilization of thromboprophylaxis in the U.S. in this patient population, which may be due to the lack of recognition of the efficacy of preventive therapy by care providers, they wrote in .

"We found that cesarean section is an important independent risk factor for the development of VTE in the postpartum period and that approximately three VTE will occur for every 1,000 cesarean sections (CS) performed, with greater risks for nonscheduled emergency CS," Blondon noted in a press statement.

The risks were largely independent of other factors like maternal age and body mass index (BMI), the authors added.

Pregnancy is associated with an increased risk for VTE, partly due to hormonal changes. About one in three women in Europe and North America deliver by C-section.

C-sections have been associated with a higher risk for VTE than vaginal deliveries in some, but not all studies, and there remains some debate about the impact of surgical delivery on blood clot risk, the researchers wrote.

"A better estimation of the associations of cesarean sections with VTE and the absolute risk of VTE following cesarean section may help inform obstetricians and their patients, as well as the development of future guidelines on thromboprophylaxis during the postpartum period," they added.

Blondon's group searched PubMed, Embase, and conference proceedings from 1980 to late 2015 for reports with information on both delivery method and postpartum VTE, including incidence of VTE following cesarean delivery. Studies on thrombophilia or recurrent VTE were excluded, and the search was limited to prospective studies when assessing the incidence of VTE.

In all, 28 mostly retrospective observational studies comparing VTE risk following C-section and vaginal deliveries (>53,000 VTE events) were identified along with 32 prospective studies reporting risks of VTE following cesarean delivery (218 VTE events).

The researchers noted that most of the reports included in the analysis were of good quality, with representative samples and a low potential for bias when comparing cesarean and vaginal delivery.

Compared with vaginal delivery, the relative risk of VTE following C-section ranged from 1 to 22, with a meta-analytic odds ratio of 3.7 (95% CI, 3.0-4.6).

Adjustment for age and BMI had a marginal influence on the estimated pooled OR. Associations were observed for both elective and emergency C-section, with stronger estimates of associations for emergency C-section.

The pooled incidence was 2.6 VTE per 1,000 cesarean sections (95% CI, 1.7-3.5) and the incidence was greater in studies with a longer and better follow-up in the postpartum period (4.3 per 1,000 cesarean section).

"We can estimate, in the United States, that the annual approximately 1.2 million cesarean deliveries that occur would lead to about 3,000 to 4,000 postpartum VTE every year," the researchers wrote. "However, thromboprophylaxis seems widely underutilized in the United States: it is prescribed in 25% of women following cesarean section (mainly compression stockings with <3% heparin prescriptions)."

The American College of Obstetricians and Gynecologists (ACOG) for all women undergoing C-section, with a consideration for adding heparin in patients with additional blood clot risk factors.

The American College of Chest Physicians following C-section in women with no additional risk factors, and heparin in those with additional risk factors, while the Royal College of Obstetricians and Gynecologists in the U.K. recommends low-molecular-weight heparin for 10 days following all C-sections, except elective procedures that occur in women without additional risk factors.

"Taken together, these recommendations seem in line with the results of a previous decision-analysis model from our group: some (marginal) benefit of the use of low-molecular-weight heparin was suggested if the risk of VTE following cesarean section was >1.5 to 2.2 per 1,000 cesarean sections, but it seems clinically more relevant at a greater risk of VTE," the researchers wrote.

Study limitations included the non-population design and short follow-up of most studies included in the analysis.

They stated that additional observational studies and randomized trials are needed to better understand the VTE risk associated with C-section and the efficacy and safety of thromboprophylaxis.

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