Green LJ, Kennedy SH, Mackillop L, et al. - Since there is insufficient knowledge regarding physiological blood pressure alterations in pregnancy, thus, this secondary analysis of a prospective, longitudinal, observational cohort study (2009 to 2016) was carried out to detail blood pressure alterations across healthy pregnancies from the International Fetal and Newborn Growth Consortium for the 21st Century (INTERGROWTH-21st) Fetal Growth Longitudinal Study (FGLS) to generate international, gestational age-specific, smoothed centiles (third, 10th, 50th, 90th, and 97th) for blood pressure. Enrollees were healthy females at low risk of pregnancy complications. At 12 weeks, the lowest systolic blood pressure was evident: median was 111.5 mmHg, increasing to a median maximum of 119.6 mmHg at 40 weeks’ gestation, a difference of 8.1 mmHg. From 12 weeks, reduction in median diastolic blood pressure was seen: 69.1 mmHg to a minimum of 68.5 mmHg at 19 ⁺⁵ weeks’ gestation, an alteration of −0·6 mmHg. In fewer than 10% of females, there was reduction of systolic blood pressure by >14 mmHg or diastolic blood pressure by >11 mmHg at any gestational age. A rise in systolic blood pressure by >24 mmHg or diastolic blood pressure by >18 mmHg was seen in fewer than 10% of women at any gestational age. To facilitate earlier identification of worsening health in pregnant women, this study offers international, gestational age-specific centiles as well as limits of acceptable change. The notion of a clinically significant midpregnancy decline in blood pressure is challenged by these centiles.