Tea is one of the most commonly consumed beverage worldwide [1]. It contains over 2000 components, including caffeine and polyphenols, which were recognized as important constituents [2]. Tea has been shown to provide beneficial effects for cardiovascular function in non-pregnant adults [3], [4], although the results from different studies evaluating the effects of tea on blood pressure were inconsistent [5], [6], [7], [8]. There is a paucity of evidence regarding the effects of tea consumption on the risk of pregnancy induced hypertension (PIH).

Polyphenols and caffeine are major compounds present in tea, and both have considerable pharmacological significance [9]. Regular caffeine intake has been found to increase blood pressure [10]. Caffeine is able to antagonize endogenous adenosine, leading to vasoconstriction and elevated total peripheral vascular resistance [11]. It can also increase oxidative stress and lead to endothelial dysfunction [12], which might cause PE [13]. A positive association has been reported between caffeine intake and SBP in the first and third trimesters of pregnancy [14]. Homocysteine is an intermediate in methionine metabolism that can be influenced by folate, vitamin B6, vitamin B12, betaine, and polyphenols [15], [16]. Dodds et al.[17] found that a high total concentration of homocysteine in early pregnancy was a risk factor for PE and that an increase in total homocysteine leads to abnormalities of the placental vasculature. Polyphenols act as acceptors of methyl groups during the metabolism of methionine to homocysteine. This suggests that an increase in dietary polyphenols may result in an elevation of the total concentration of plasmic homocysteine [15], [16]. Hodgson et al. [18] suggested that drinking black tea can cause a small acute increase in total homocysteine. Folate can reduce homocysteine levels [19], however, tea can lower folate levels during pregnancy by inhibiting folate metabolism pathways [20], [21].

PIH is one of the most common complications during pregnancy, affecting up to 10% pregnant women worldwide [22]. The clinical presentation of PIH is characterized by hypertension, proteinuria, and/or oedema. PIH includes gestational hypertension (GH), preeclampsia (PE), eclampsia, chronic hypertension (of any cause), and PE superimposed on chronic hypertension [23]. PIH is also a major cause of maternal and neonatal mortality and morbidity [24], [25], and it is also a known risk factor for the future cardiovascular disease in women [26]. A few epidemiological studies have linked tea consumption to PIH, however, the results of these studies were inconsistent. Some studies reported that tea consumption is related to higher risk of PE and PIH [27], [28], while others found that there was no association between the risk of PE and tea consumption [29]. In addition, there were no studies evaluating the association between tea consumption and PIH in the Chinese population.

Considering the uncertain relationship between tea consumption and PIH, it is necessary to confirm whether tea consumption during pregnancy has negative effects on blood pressure. Therefore, this study aimed to investigate the relationship between tea consumption and the risk of PIH using data from a large cohort study conducted in Lanzhou, China.