Lactation Duration and the Risk of Subtypes of Stroke Among Parous Postmenopausal Women From the China Kadoorie Biobank

IMPORTANCE: Previous studies have reported favorable associations between lactation and cardiovascular diseases. Various stroke subtypes are caused by different pathological processes; however, to date, the associations of lactation duration with different stroke subtypes are less well established. OBJECTIVE: To examine the associations of lactation duration with stroke and its subtypes in parous postmenopausal women. DESIGN, SETTING, AND PARTICIPANTS: This population-based prospective cohort study included parous postmenopausal women aged 45 to 79 years in the China Kadoorie Biobank (CKB) study at baseline (2004-2008). Lactation duration was counted as lifetime, mean per child, and for the first child. New-onset stroke and its subtypes (ischemic stroke, intracerebral hemorrhage [ICH], and subarachnoid hemorrhage [SAH]) were assessed via disease registries and national health insurance claim databases during follow-up (2008-2015). Data were analyzed from June to December 2021. EXPOSURES: Lactation duration (lifetime, mean per child, and for the first child). MAIN OUTCOMES AND MEASURES: The main outcomes were total stroke, ischemic stroke, ICH, and SAH. Multivariable Cox regression was applied to calculate the adjusted hazard ratios (aHRs) with 95% CIs for stroke and subtypes. RESULTS: Of 129 511 parous postmenopausal women (median [IQR] age, 58.3 [54.0-64.6] years) without prior stroke at baseline, 15 721 developed stroke, with median (IQR) lifetime lactation duration of 42.0 (24.0-70.0) months among 13 427 women who had ischemic stroke, 54.0 (36.0-84.0) months among 2567 women who had ICH, and 36.0 (24.0-64.5) months among 284 women with SAH. Compared with parous postmenopausal women who had never lactated, those with lifetime lactation duration of at least 7 months had lower risks of ischemic stroke (aHRs varying from 0.52 [95% CI, 0.50-0.55] to 0.64 [95% CI, 0.59-0.69]) and ICH (aHRs, 0.56 [95% CI, 0.49-0.63] to 0.78 [95% CI, 0.64-0.96]). However, for SAH, such associations were found only in participants with lifetime lactation duration longer than 24 months (aHR, 0.61 [95% CI, 0.47-0.79]). Additionally, women with mean lactation duration per child or lactation duration for the first child of 7 months or longer were less likely to develop stroke (aHRs varying from 0.53 [95% CI, 0.52-0.54] to 0.65 [95% CI, 0.63-0.67]) and its subtypes (aHRs varying from 0.51 [95% CI, 0.30-0.87] to 0.75 [95% CI, 0.69-0.81]). CONCLUSIONS AND RELEVANCE: In this cohort study, lactation was significantly associated with a lower risk of stroke, especially ischemic stroke, emphasizing the importance of promoting breastfeeding as a targeted prevention strategy of stroke.