BACKGROUND AND AIMS OF THE STUDY: Prosthetic heart valve placement in young women does not normally prevent successful pregnancy, though either thrombosis in mechanical prostheses or degeneration in biological prostheses may cause risks for both mother and fetus. Although pericardial prostheses avoid the risks of anticoagulation therapy, it has been suggested that pregnancy accelerates structural degeneration in bioprostheses. The study aim was to assess the influence of pregnancy on five-year outcome after pericardial bioprosthesis replacement. METHODS: A total of 85 women was studied prospectively over five years from prosthesis implantation, using annual clinical and echocardiographic investigations. Women were allocated to two groups based on pregnancy occurring during follow up: 48 patients (56%; group A) became pregnant, and 37 (44%; group B) did not. RESULTS: At five-year follow up, the clinical events occurred in 29 patients (34.1%): 11 (22.9%) women in group A (mean (+/-SE) linearized rate 3.6+/-0.24%/patient-year (pt-yr)), and 18 (45.6%) in group B (linearized rate 8.6+/-0.30%/pt-yr). Probability of freedom from clinical events was higher (p = 0.0096) in group A (77.9+/-6.1 versus 51.3+/-8.2%). Structural bioprosthesis degeneration, thrombosis or infective endocarditis occurred in 13 patients (27.1%) in group A (linearized rate 2.75+/-0.20%/pt-yr) and 11 (29.7%) in group B (linearized rate 2.5+/-0.18%/pt-yr). Probability of structural bioprosthesis degeneration (e.g. stenosis, calcification or rupture) did not differ (p = 0.6440) between groups (85.4+/-5.1 versus 89.5+/-5.1%), even after age-adjusted analysis (p = 0.3112). CONCLUSION: Structural changes found at five years after bioprosthesis placement may be attributed to the natural course of the bioprosthesis, and independent of any occurrence of pregnancy.