Objective Defining the normal range for the anterior/posterior myometrial wall thickness ratio in a cohort of women without adenomyosis or any other uterine wall anomaly on ultrasound examination. Methods Anterior and posterior miometrial wall thickness was measured in 555 women (mean age 34.6 years old, range: 20-50 years) without any ultrasound findings of adenomiosis or other uterine pathology. Measurements were performed in the longitudinal plane of a stored 3D volume. Two observers made all measurements. The myometrial wall thickness ratio was estimated and distribution by centiles obtained. Correlation of myometrial thickness ratio with patient's age and parity was also estimated, using the Pearson's correlation coefficient. Intra- and inter-observer reproducibility were estimated using the intra-class correlation coefficient (ICC). Results The mean ratio of the myometrial walls thickness (understood as anterior thickness/posterior thickness ratio) was 0.99 (95% CI: 0.96-1.01). The distribution of the ratio by centiles were as follows: 5%: 0.64, 10%: 0.70, 25%: 0.82, 50%: 0.96, 75%: 1.12, 90%: 1.30 and 95%: 1.45). The myometrial wall thickness ratio was not related to patient's age (Pearson's coefficient: 0.039, p = 0.371), neither to patient's parity (Pearson's coefficient: 0.004, p = 0.923). The ICC was 0.94 and 0.88 for observers 1 and 2, respectively. Inter-observer reproducibility was high (ICC: 0.83). Conclusions Myometrial thickness ratio in women with normal uterus at ultrasound examination is about 1. However, centile distribution shows that values as low as 0.64 or as high as 1.45 could be considered as normal.