Table 4. Results of studies assessing an association between insufficient milk supply and postmenopausal breast cancer; stratified by reference group (a. studies comparing women who experienced insufficient milk to parous women who breastfed successfully, b. breastfed unsuccessfully for other reasons, c. never breastfed).
| Study | N cases / N controls | Age | Definition of Insufficient Milk Supply | Crude OR (95% CI) | Adjusted OR (95% CI): Adjusted for… | Notes About Comparison Group |
| a. | ||||||
| Shema 2007 | 123/321 | 30–75 | Unsuccessful attempt at breastfeeding or quitting before 1 month because of insufficient milk | 6.7 (3.2–14.6) | Breastfed for 1–12 months and 12+ months combined | |
| Newcomb 1999 | 685/638 | 50–79 | Experience of insufficient milk within the first 3 months of the first 3 pregnancies | 1.1 (0.8–1.3) | 1.2 (0.8–1.4); Study site, age, parity, FHBC*, age at menopause, BMI*, education, duration of lactation | It is unclear how long the comparison group breastfed for |
| Freudenheim 1997 | 140/180 | 40–85 | Reported as the reason for cessation of breastfeeding within 1.5 months of any birth | 0.7 (0.4–1.2) | 0.6 (0.3–1.2); Age, parity, education, FHBC, history of BBD*, age at menarche, at first birth | Lactated and did not experience insufficient milk after any birth |
| Newcomb 1994 | 2603/3145 | <75 | Reported experience of insufficient milk within the first 3 months of first or second birth | 0.9 (0.8–1.0) | 0.9 (0.8–1.0); Age, parity, age at menarche, fist birth, first lactation, menopause, history of BBD, FHBC, BMI | It is unclear how long the comparison group breastfed for |
| Yang 1993 | 404/389 | <75 | Breastfed unsuccessfully (<1 month) and reported the reason as insufficient milk | 1.1 (0.7–1.8) | 1.1 (0.7–1.8); Age, parity | Breastfed for ≥2 months |
| Byers 1985 | 172/486 | 40–84 | Reported as the reason for cessation of breastfeeding after the first birth | 1.1 (0.9–2.1) | 1.62; Age | Successfully lactated for any length of time and then stopped for reasons other than insufficient milk supply |
| b. | ||||||
| Yang 1993 | 70/63 | <75 | Breastfed unsuccessfully (<1 month) and reported the reason as insufficient milk | 1.1 (0.5–2.2) | Breastfed unsuccessfully (<1 month) and reported any other reason | |
| c. | ||||||
| Shema 2007 | 93/83 | 30–75 | Reported unsuccessful breastfeeding attempt or reported quitting breastfeeding before 1 month because of insufficient milk | 1.9 (0.8–4.3) | Lifetime duration of breastfeeding = 0 months | |
| Newcomb 1999 | 884/861 | 50–79 | Reported experience of insufficient milk within the first 3 months of the first 3 pregnancies | 1.1 (0.9–1.4) | Did not nurse for reasons other than insufficient milk | |
| Yang 1993 | 195/213 | <75 | Breastfed unsuccessfully (<1 month) and reported the reason as insufficient milk | 1.3 (0.8–2.2) | Never lactated | |
*
FHBC = family history of breast cancer BMI = body mass index (kg/m2) BBD = benign breast disease.