We would like to point out a critical error by Friederich and associates [1] and to report on what we believe is the largest population study of the incidence of deep venous thrombosis and pulmonary embolism in the setting of delivered pregnancies (4 weeks postpartum). Our data suggest that the incidence is not only low but is similar to the incidence in closely age-matched nonpregnant women in another study [2].

The annual 1.8% frequency of deep venous thrombosis in nonpregnant women 20 to 40 years of age cited by Friederich and colleagues is inaccurate. As listed in the original report [2], the correct frequency is 0.018%.

We reviewed records of delivered pregnancies and postpartum periods during which deep venous thrombosis and pulmonary embolism occurred from January 1985 through January 1996 in the northern California region of Kaiser Permanente (a health maintenance organization serving 2.5 million members). A total of 280 793 deliveries occurred; among these, 251 charts were identified by International Classification of Diseases codes with concomitant diagnoses of delivered pregnancy/postpartum and thromboembolic disease. Patients were included in our study if deep venous thrombosis or pulmonary embolism was documented by venography. Doppler ultrasonography, ventilation-perfusion scanning, or pulmonary angiography. Eighty-one patients met these criteria (67 with deep venous thrombosis and 14 with pulmonary embolism); thus, the incidence of documented deep venous thrombosis or pulmonary embolism in this population was 0.029%.

As cited in Friederich and colleagues' study, previous studies have reported frequencies of these conditions of 1.3% to 7% during pregnancy and 6.1% to 23% during the postpartum period [1]. Our incidence suggests a much lower frequency, as does the 0.055% frequency reported in the second largest population study of these two conditions in women giving birth; like ours, this study required objective documentation of thromboembolism [3]. Most noteworthy, in nonpregnant women 20 to 40 years of age, the annual frequency of thromboembolism has been observed to be 0.018% [2]. By using the Fisher exact test, we found no statistically significant difference between this incidence and the incidence of deep venous thrombosis or pulmonary embolism in our study population (P > 0.2).