Dr. MacCallum and coworkers argue that we incorrectly reported our venous thromboembolism incidence rates for the first and second postpartum weeks as 3.6% and 1.5%, respectively, because of incorrect denominator calculations. The denominator calculations were based on the total number of live births in Olmsted County (n = 50 080) and "period at risk" definitions of 9 and 3 months for pregnancy and the postpartum period, respectively. A 1-week period of risk is 1/52 of a year (0.019), such that the woman-years at risk during the first postpartum week were 0.019 times the number of live births (0.019 x 50 080 = 951 woman-years). Thirty-four women had documented incident venous thromboembolism during this week, providing an incidence of 0.036 (34 of 951), or 3.6 per 100 woman-years (i.e., 3.6%). The incidence rate for the second postpartum week was calculated similarly. However, although the incidence rates are correct as reported, we incorrectly compared these rates with the proportion of patients developing venous thromboembolism after total hip replacement. According to data reported from the California Patient Discharge Data Set, among 56 720 patients undergoing elective total hip replacement between 1 January 1992 and 30 September 1996, 1358 developed symptomatic venous thromboembolism within 3 months (0.25 year) after surgery (1). Thus, there were 14 180 person-years at risk (56 720 x 0.25), and the venous thromboembolism incidence was 0.096 (1358 of 14 180), or 9.6 per 100 person-years. While this rate is similar to our incidence rates for the first and second postpartum weeks, at least some of the patients who had hip replacement probably received prophylaxis such that the incidence in the absence of prophylaxis could have been higher.

Drs. Rajput and Rana suggest that cesarean section is a risk factor for venous thromboembolism. Our present study was not designed to address this question. However, in a previous population-based case–control study specifically addressing risk factors for pregnancy-associated venous thromboembolism (2), we could not identify cesarean section as a postpartum risk factor (univariate odds ratio, 1.17 [95% CI, 0.39 to 3.47]).

Dr. Stein and coworkers cite their study reporting an analysis of administrative data from NHDS and indicate that rates of pregnancy-associated DVT increased between 1979 and 1999 (3). Pregnancy-associated PE was too infrequent to analyze for trends. In contrast, we found a decrease in the overall incidence of pregnancy-associated venous thromboembolism, mostly due to a decrease in postpartum PE; the incidence of pregnancy-associated DVT declined slightly and nonsignificantly. These differences may be explained by different study designs. The time frame of our study was longer (1966–1995). Moreover, we only included women with a first lifetime event, while the NHDS database does not separate incident from recurrent events. Finally, almost 70% of women coded by the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) as having pregnancy-associated DVT may be miscoded (4). Nevertheless, we certainly agree that there is a need for continued vigilance regarding pregnancy-associated venous thromboembolism.