Background Breast hypoplasia is one reason for insufficient milk supply. Case reports use wide intra-mammary width and certain breast appearances as markers of breast hypoplasia. However, the reliability of these variables has not been determined. Research aims To test the (i) interrater and intrarater reliability of intra-mammary width measurement and interrater reliability of categorizing women’s breasts into breast types, and (ii) feasibility and acceptability of study procedures for the participants. Methods This was a prospective, longitudinal, non-experimental design with survey and observational components of a convenience sample of early postpartum women ( N = 31). Interrater and intrarater reliability were measured using intraclass correlation coefficient for agreement for intra-mammary width measurements. Interrater reliability was measured using weighted kappa for agreement for categorizing breast type. Feasibility and acceptability of study procedures were collected 1 month later. Results Excellent intrarater and interrater reliability for the intra-mammary width measurement (ICC = 0.99, 95% CI [0.99, 0.997] and ICC = 0.88, 95% CI [0.74, 0.94], respectively) and fair interrater reliability for breast type categorization (k = 0.35, 95% CI [−0.05, 0.75]) with high level of agreement between raters (97%) were achieved. Proportions of participants agreeing to breast photography and acceptability of study procedures were 68% (21/31) and 100% (28/28) respectively. Conclusion The excellent reliability for the intra-mammary width measurement means it provides a useful measure for future research. Since the “fair” reliability of categorizing breast type was due to lack of breast shape range in our sample, future research could assess the breasts of women with different medical profiles.