['To document the ambulatory arterial stiffness index (AASI) during pregnancy compared to three months after delivery in singleton and twin pregnancies.', 'Descriptive study with a follow-up design.', 'University hospital in Eastern Finland.', '43 childbearing women; 32 with singleton and 11 with twin pregnancies.', 'Ambulatory blood pressure measurements were conducted using a digital ambulatory blood pressure system. AASI was calculated as 1 minus the regression slope of diastolic on systolic blood pressures obtained from 24-hour monitoring.', 'Arterial stiffness measured by AASI.', 'In normotensive pregnant women, the 95th percentiles of AASI were 0.40 in singleton and 0.46 in twin pregnancies, respectively, implying arterial normality and high elasticity. There were no differences in AASI values between singleton and twin pregnancies and no changes were observed postpartum. After delivery, but not during pregnancy, AASI correlated negatively with nocturnal systolic and diastolic blood pressure reduction (dipping) (r=-0.536, p=0.001; r=-0.674, p<0.001) and with maternal HDL (r=-0.363, p=0.038) and positively with maternal age (r=0.440, p=0.009), whereas maternal BMI had a significant impact on AASI both during and after pregnancy (r=0.366, p=0.016; r=0.377, p=0.028).'] Is ambulatory arterial stiffness index unchanged in uncomplicated third-trimester singleton and twin pregnancies?