Background: Maternal obesity and advanced age have been associated with an increased risk of structural congenital heart defects in the offspring. Whether these factors may also cause abnormalities in infant cardiac dimension and function is unknown. This study investigates whether maternal body mass index (BMI) and maternal age are associated with changes in left ventricular (LV) dimensions and function in the newborn. Methods: Infants enrolled in the Copenhagen Baby Heart Study (CBHS), who were born at term, and contributed with a transthoracic echocardiography (TTE) within 60 days of birth were included. The exposure variables were prepregnancy maternal BMI (kg/m²) < 18.5; 18.5–24.9 (reference); 25–29.9; 30–34.9 and ≥ 35 and maternal age (years) < 25; 25–29; 30–34 (reference); 35–39 and ≥ 40. Outcomes were LV parameters ascertained by 2D-echocardiography. Associations between each maternal factor and infant LV parameters were analysed with either a linear model adjusted for the child’s weight and length at birth, gestational age, sex, age at TTE, and maternal smoking, or a linear mixed model, further adjusted for random effects of analyst and month of analysis. Analyses investigating impact of maternal BMI were adjusted for maternal age, and vice versa. Results: The study cohort included 24,294 infants. Compared with infants in the BMI reference group, infants born to women with a BMI ≥ 25 kg/m² generally had smaller measures of LV internal diameters in end-diastole, reaching statistical significance for BMI 30–34.9 kg/m² [-0.11 ± 0.04 mm, p = 0.01]. All groups of infants born to women with a BMI ≥ 25 kg/m² had significantly smaller LV internal diameters in end-systole: BMI 25–29.9 kg/m² [-0.04 ± 0.02 mm, p = 0.04], BMI 30–34.9 kg/m² [-0.12 ± 0.03 mm, p = 0.001] and BMI ≥ 35 kg/m² [-0.11 ± 0.05 mm, p = 0.03]. Compared with infants in the age reference group, infants born to women ≥ 40 years had significantly smaller LV internal diameters in end-diastole [-0.15 ± 0.04 mm, p = 0.001] and end-systole [-0.09 ± 0.04 mm, p = 0.009]. Conclusions: Systematic population-based echocardiography of infants showed that a maternal prepregnancy BMI ≥ 25 kg/m² and maternal age ≥ 40 years were associated with smaller systolic and diastolic LV diameters. The long-term effects are unknown. Clinical trial registration: April 2016, Copenhagen Baby Heart, NCT02753348 .