There were 55,636 parous women who were eligible for our analysis, of whom 80.6% had breastfed their first child, 16.8% had breastfed their first child for ≥12 months, and 14.6% had exclusively breastfed their first child for ≥6 months. Longer lactation duration for the first child was associated with higher parity (), as well as lower BMI during follow-up, less family history of hypertension, less frequent smoking, and oral contractive use, higher DASH diet score, more frequent vigorous exercise, and more analgesic use.
Age-standardizeda Characteristics of 55,636 Parous Women in the Nurses’ Health Study II During Study Follow-up, by Duration of Lactation for First Child, United States, 1991–2005
Both total duration and exclusive duration of lactation for the first child were correlated with subsequent parity (Mantel-Haenszel chi-square test for trend, P
< 0.0001), and lactation durations among siblings were highly correlated (Pearson’s r
= 0.46–0.81) (P
< 0.0001). Thus, parity may be a “collider,” and adjustment for parity may produce false associations between lactation duration and outcome (31
). For our primary analysis, we therefore analyzed the association between lactation duration for the first child (not all children) and incident hypertension.
We ascertained 8,861 cases of incident hypertension during 660,880 person-years of follow-up. We found a dose-response association between never or curtailed breastfeeding and incident hypertension risk. In IPW-adjusted models, mothers who had never breastfed had a 1.27-fold risk of incident hypertension (95% confidence interval (CI): 1.18, 1.36) (across breastfeeding duration categories, Ptrend < 0.001) compared with mothers who lactated for ≥12 months after their first birth (; ). To test whether BMI mediated this association, we further adjusted for BMI during follow-up, which modestly attenuated the association (for never vs. ≥12 months of lactation for the first child, BMI-adjusted hazard ratio (HR) = 1.22, 95% CI: 1.13, 1.31) (across breastfeeding duration categories, Ptrend < 0.001). When we measured the association between duration of exclusive lactation and incident hypertension, we found that women who did not lactate had an IPW-adjusted 1.29-fold risk of incident hypertension (95% CI: 1.20, 1.40) (across breastfeeding duration categories, Ptrend < 0.001) compared with having exclusively lactated for ≥6 months after the first birth (). Again, this association was somewhat attenuated in models adjusting for BMI during follow-up (for never vs. ≥6 months of exclusive lactation for the first child, BMI-adjusted HR = 1.22, 95% CI: 1.13, 1.31) (across breastfeeding duration categories, Ptrend < 0.001). All associations were materially unchanged with adjustment for lifetime parity.
Figure 2. Association between duration of total breastfeeding for the first child and incident hypertension among 55,636 parous women in the Nurses’ Health Study II, United States, 1991–2005. Adjusted for age and inverse probability weight, derived (more ...)
Association Between Duration of Breastfeeding for the First Child and Incident Hypertension Among 55,636 Parous Women in the Nurses’ Health Study II, United States, 1991–2005a
We then considered the degree of unobserved confounding that would explain our results. Assuming an unobserved binary confounding factor perfectly correlated with lactation in the longest duration category, we found that unmeasured confounders with effect sizes of 0.85 for ≥12 months’ total and 0.83 for ≥6 months’ exclusive lactation for the first child would be needed to explain the observed difference in incident hypertension among women in these groups versus those who had never breastfed. These effect sizes are similar in magnitude to the associations between 7 days/week of vigorous physical activity versus <1 day/week of vigorous activity (HR = 0.87, 95% CI: 0.81, 0.93) or a DASH diet score in the highest versus lowest quintile (HR = 0.82, 95% CI: 0.77, 0.87) in prior studies (3
To determine whether measuring health behaviors at the time of the first birth affected our results, we limited our population to 8,318 women with a first birth after 1989. In IPW-adjusted models, we found a hazard ratio = 1.31 (95% CI: 1.00, 1.71) for women who never lactated and 1.40 (95% CI: 1.07, 1.82) for women who lactated for >0–3 months compared with women who lactated for ≥12 months. Adjustment for BMI during follow-up modestly attenuated these associations (never vs. ≥12 months, HR = 1.22, 95% CI: 0.93, 1.60; >0–3 vs. ≥12 months, HR = 1.22, 95% CI: 0.93, 1.59).
We then estimated the population-attributable risk due to suboptimal lactation, defined as <6 months’ exclusive lactation and/or <12 months’ total lactation, and incident hypertension. If this association is causal, we estimate that 12% (95% CI: 4.9, 19) of incident hypertension in our population could be attributed to suboptimal lactation.
In a secondary analysis, we measured the association between mean lactation duration per child and incident hypertension in the full study cohort (). With adjustment for lifestyle covariates and hypertension risk factors, we found a 1.22-fold risk (95% CI: 1.13, 1.32) (across breastfeeding duration categories, Ptrend
< 0.001), comparing women who had never breastfed versus those who lactated ≥12 months per child. Curtailed lactation was also associated with increased risk: compared with ≥12 months per child, women with >0
–3 or >3–<6 months per child had a 1.21-fold (95% CI: 1.12, 1.30) and a 1.19-fold (95% CI: 1.11, 1.28) increased risk of incident hypertension in covariate-adjusted models. We similarly found an increased risk of incident hypertension with never or curtailed exclusive lactation (). With adjustment for covariates, we found a 1.16-fold risk (95% CI: 1.05, 1.27) (across breastfeeding duration categories, Ptrend
< 0.001) for women who had never breastfed compared with a mean exclusive duration of 6 months or more. These results were materially unchanged with adjustment for current BMI and parity.
Association Between Mean Duration of Total and Exclusive Lactation per Child and Incident Hypertension Among 59,852 Parous Women in the Nurses’ Health Study II, United States, 1991–2005a
We found that adjustment for ever-history of preeclampsia slightly attenuated associations between lactation duration and incident hypertension. Our results were materially unchanged with adjustment for other pregnancy complications. We found no evidence for an interaction between ever-history of gestational hypertension or preeclampsia, lactation duration, and incident hypertension (for log likelihood ratio test, all P > 0.15).