Table of Contents market sales. This growth was partially offset by an unfavorable impact related to a letter issued in March 2019 by the FDA to healthcare professionals regarding the use of paclitaxel-coated devices in the treatment of peripheral artery disease, which impacted sales of our drug-coated balloon products. Interventional segment operating income was as follows: As discussed in greater detail below, the Interventional segment's operating income in 2020 was primarily driven by a decline in gross profit margin. Operating income in 2019 was driven by improved gross profit margin and operating expense performance. BD’s worldwide revenues by geography were as follows: U.S. revenues in 2020 were relatively flat compared with 2019 as the Life Sciences segment's Integrated Diagnostic Solutions unit's sales related to COVID-19 diagnostic testing largely offset the declines noted above for the Medical segment's Medication Management Solutions and Medication Delivery Solutions units, as well as for the Interventional segment's Surgery and Peripheral Intervention units. U.S. revenues in 2019 reflected growth in all three segments. U.S. revenues in 2019 were favorably impacted by the inclusion of revenues associated with Bard's products in results for the first quarter of fiscal year 2019, as noted above. Revenue growth in 2019 was also attributable to sales in the Medical segment's Medication Management Solutions unit as well as to sales in the Interventional segment's Urology and Critical Care and Surgery units. U.S. revenue growth in 2019 was unfavorably impacted by results in the Medical 30 (Millions of dollars) 2020 2019 2018 Interventional segment operating income $ 724  $ 903  $ 306  Segment operating income as % of Interventional revenues 19.2  % 23.0  % 10.1  % 2020 vs. 2019 2019 vs. 2018 (Millions of dollars) 2020 2019 2018 Total Change Estimated FX Impact FXN Change Total Change Estimated FX Impact FXN Change United States $ 9,716  $ 9,730  $ 8,768  (0.1) % —  (0.1) % 11.0  % —  11.0  % International 7,401  7,560  7,215  (2.1) % (2.2) % 0.1  % 4.8  % (5.0) % 9.8  % Total revenues $ 17,117  $ 17,290  $ 15,983  (1.0) % (1.0) % —  % 8.2  % (2.3) % 10.5  % • Gross profit margin was lower in 2020 as compared with 2019 primarily due to unfavorable product mix and increased levels of manufacturing overhead costs that were recognized in the period, rather than capitalized within inventory, as a result of the COVID-19 pandemic. Gross profit margin was higher in 2019 as compared with 2018 primarily due to the unfavorable prior-year impact of recognizing a fair value step-up adjustment relating to Bard's inventory on the acquisition date and lower manufacturing costs resulting from continuous improvement projects, which enhanced the efficiency of our operations, and synergy initiatives. These favorable impacts to the Interventional segment's gross margin were partially offset by unfavorable product mix and unfavorable foreign currency translation. • Selling and administrative expense as a percentage of revenues in 2020 was lower compared with 2019 primarily due to lower expenses resulting from cost containment measures. Selling and administrative expense as a percentage of revenues in 2019 was relatively flat compared with 2018. • Lower research and development expense as a percentage of revenues in 2020 as compared with 2019 primarily reflected the prior-period impact of a write-down recorded by the Surgery unit. This write-down drove higher research and development expense as a percentage of revenues in 2019 as compared with 2018. • The Interventional segment's lower income in 2020 additionally reflected the expiration in 2019 of a royalty income stream acquired in the Bard transaction. Geographic Revenues