The incidence of hypertensive disorders of pregnancy (HDP) is approximately 3–10 % of pregnancies [1]. HDP includes preeclampsia (PE), gestational hypertension (GH), chronic hypertension (CH), and superimposed PE (sPE), which is associated with an increased risk of adverse pregnancy outcomes [2]. In addition, these outcomes are poorer in pregnant women with PE than in those with GH and CH [3], [4], [5]. The identification of pregnant women with HDP and high risk for imminent delivery is crucial because intensive monitoring and interventions, such as steroids for fetal lung maturity [6], reduce maternal and fetal morbidity.

Excess circulating soluble fms-like tyrosine kinase-1 (sFlt-1) secreted in the placenta contributes to the pathogenesis of PE [7]. Furthermore, circulating sFlt-1 inhibits placental growth factor (PlGF), and circulating PlGF levels decrease in pregnant women with PE [8]. Pregnant women with GH also have similarly elevated levels of sFlt-1 and reduced levels of PlGF [9]. Although the degree of early placental hypoxia is expected to be greater in women with CH than in healthy women due to pre-existing endothelial disease leading to a higher production of sFlt-1 and lower PlGF, there are contrasting reports [10], [11].

These circulating angiogenic factors are helpful in several clinical scenarios involving hypertension associated with pregnancy, such as being indicators of possible PE [12], [13]. Zeisler et al. showed that retesting of sFlt-1 and PlGF 2 or 3 weeks after the initial test improved risk stratification of PE in women with suspected PE [14]. Other studies reported the use of the sFlt-1/PlGF ratio to predict the remaining pregnancy duration in pregnant women with PE [10], [15], [16], [17]. We have shown that the sFlt-1 value could predict the period from diagnosis to delivery in pregnant women diagnosed with PE at < 34 weeks of gestation [18], [19] and that the sFlt-1 value was more significantly associated with the period until delivery than PlGF and the sFlt-1/PlGF ratio [19]. In other words, pregnant women with PE may be at considerable risk for imminent delivery if the sFlt-1/PlGF ratio or the sFlt-1 concentration at diagnosis is high. Conversely, it might be difficult to predict the period from diagnosis to delivery if the sFlt-1/PlGF ratio or the sFlt-1 concentration at diagnosis is low. We hypothesized that periodic measurement of sFlt-1 and PlGF from diagnosis to delivery could contribute to solving this problem.

Thus, we conducted this pilot study to explore whether periodic measurement of sFlt-1 and PlGF concentrations from diagnosis to delivery in pregnant women with PE could contribute to predicting the remaining pregnancy duration. In our previous studies, we never confirmed whether sFlt-1 and PlGF can predict the remaining pregnancy duration in pregnant women with GH and CH; therefore, we also aimed to address this question.