In 2017, the American College of Cardiology (ACC) and American Heart Association (AHA) lowered the blood pressure (BP) threshold to define hypertension for the general population by creating a new category called “stage 1 hypertension”, defined as systolic BP (sBP) of 130–139 mmHg or diastolic BP (dBP) of 80–89 mmHg. [1] Thus, BP is classified into four groups; normal BP (sBP < 120 and dBP < 80), elevated BP (sBP 120–129 and dBP < 80 mmHg), stage 1 hypertension (sBP 130–139 mmHg or dBP 80–89 mmHg), and stage 2 hypertension (sBP ≥ 140 mmHg or dBP ≥ 90 mmHg). [1] These changes have been made based on the evidence that each progressive level of BP elevation is linearly associated with a higher risk of developing cardiovascular diseases. [2] However, the applicability of these new criteria for pregnant women has been unclear due to insufficient evidence. Therefore, the current definition of hypertension during pregnancy remains as sBP ≥ 140 mmHg or dBP ≥ 90 mmHg. [3].

Recently, it has been discussed whether the threshold of hypertension during pregnancy should be lowered as well as in the general population. Several studies have indicated that stage 1 hypertension during pregnancy is associated with an increased risk of developing hypertensive disorders of pregnancy (HDP), including preeclampsia. [4], [5], [6], [7] However, the association between stage 1 hypertension during pregnancy and preterm birth (PTB) or small for gestational age (SGA) have been inconsistently reported. [7], [8], [9], [10], [11] One reason for this inconsistency may be the variable periods of BP measurement used in these studies. It is known that BP tends to decrease during mid-pregnancy and increase toward the later pregnancy stages due to hemodynamic changes. [12] However, studies investigating stage 1 hypertension in both the first and second trimesters separately, or examining the influence of BP changes on adverse pregnancy outcomes, have been limited. [4], [13] Therefore, our study aimed to investigate the association between stage 1 hypertension in the first and second trimesters with the risk of adverse pregnancy outcomes, including PTB, and SGA. We also examined the impact of the BP trajectory between the first and second trimesters on the risks of PTB and SGA.