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.