Tracking daily blood pressure in pregnancy could predict low birth weight sooner

By Vijay Kumar MalesuReviewed by Susha Cheriyedath, M.Sc.Oct 21 2025

Japanese researchers found that monitoring cumulative blood pressure load during the first trimester better predicts low birth weight than simple averages, highlighting how home tracking could transform prenatal care.

Study: Impact of cumulative blood pressure load during early pregnancy on the risk of low birth weight: the BOSHI study. Image Credit: Dragana Gordic / Shutterstock

In a recent study published in the journal Hypertension Research, a group of researchers compared cumulative home blood pressure (HBP) load with mean blood pressure for predicting low birth weight (LBW).

Background

About one in fifteen newborns has LBW, a warning for future hypertension, diabetes, cardiovascular disease, and chronic kidney disease, yet many drivers are modifiable early in pregnancy. Blood pressure dips in the first trimester and later rises, so simple averages can miss risk. HBP monitoring captures readings, while cumulative blood pressure load integrates how high and how long values exceed a threshold, which is linked to organ damage in adults. Asian populations often have lower early-pregnancy pressure yet higher LBW. Because pregnancy-specific targets are uncertain, further research should test whether cumulative load improves stratification.

About the study

This prospective cohort analysis used data from the Babies and Their Parents’ Longitudinal Observation in Suzuki Memorial Hospital on the Intrauterine Period (BOSHI) study in Sendai, Japan. Eligible participants were pregnant women with singleton gestations who recorded at least four HBP measurements between 10 weeks 0 days and 15 weeks 6 days. HBP was measured each morning after micturition and a brief rest. Daily systolic blood pressure (SBP) and diastolic blood pressure (DBP) values were joined linearly to compute area; cumulative blood pressure load was the percentage above a threshold set at the cohort’s median average blood pressure (derived after excluding participants with chronic hypertension) (SBP and DBP analyzed separately), while elevation was defined as > 20% (selected as the first-quartile cutoff excluding 0% and 100%).

The primary outcome was LBW (< 2,500 grams). Risk ratios (RRs) were estimated using Poisson regression, adjusted for maternal age, body mass index (BMI), smoking, primiparity, and history of hypertensive disorders of pregnancy (HDP) as covariates. Sensitivity analyses varied the elevation cutoff (10% and 30%), restricted to women without chronic hypertension and to those with ≥ 8 measurements, and compared cumulative-load with average-blood-pressure models using the Quasi-likelihood under the Independence model Criterion (QIC). Multiple imputation addressed missingness, while analyses were conducted using R and SAS.

Study results

Among 729 women (mean age 31.2 years), about half were primiparous. The median birth weight was 3,060 grams; 47 newborns (6.5%) were LBW. Median cumulative loads were 49% for SBP and 51% for DBP; median average SBP/DBP at home was 103.9/61.8 mmHg. Cumulative load strongly correlated with averages (Spearman’s rho 0.964 for SBP, 0.956 for DBP). Compared with no elevation, isolated elevated cumulative SBP load, despite normal average blood pressure, was associated with higher LBW risk (RR 2.86, 95% CI 1.33–6.17). High average SBP showed a similar pattern (RR 3.57, 95% CI 1.38–9.24). For DBP, isolated cumulative-load elevation (RR 2.22, 95% CI 1.08–4.58) and high average DBP (RR 3.35, 95% CI 1.08–10.34) were also associated with greater risk.

Continuous analyses revealed a nonlinear relationship: LBW risk rose as cumulative SBP load exceeded ~20% and peaked near ~40%, whereas DBP-related risk increased to ~40% then plateaued. Models based on cumulative load had a better fit (lower QIC) than models based on average blood pressure across classifications. When participants were grouped to match primary sample-size distributions, a significant increase appeared only in the high average SBP category. In terms of tertiles, SBP-related risk increased in the second and third tertiles relative to the first, while DBP-related risk increased in the highest tertile. Changing the elevation threshold to 10% or 30% preserved the association, and restricting to women without chronic hypertension or to those with at least eight HBP readings yielded similar effect sizes with improved fit.

Descriptively, no women below a cumulative DBP load of 100% or cumulative SBP load of 90% exceeded the Japanese Society of Hypertension (JSH) limits (115/75 mmHg). Model-comparison metrics were lowest at 20% for SBP and 30% for DBP; this supports the prespecified cutoff for SBP and suggests that a 30% cutoff may provide a slightly better fit for DBP. Clinically, brief clusters of mornings above a threshold may signal risk not fully captured by the mean. This is an actionable insight for pregnant patients using connected cuffs at home and for clinicians who coach lifestyle and follow-up from day-to-day patterns rather than a single clinic value. This pattern emphasizes early, consistent home monitoring for safety.

Conclusions

Elevated cumulative blood pressure load during early pregnancy was associated with LBW even when average HBP remained within guideline-defined normal ranges. Monitoring HBP and counting days above a threshold may refine risk stratification beyond means alone, particularly when cumulative load exceeds ~20% (where risk begins to rise), and could be integrated into digital hypertension programs for pregnancy. The findings support obtaining home readings in the first trimester and exploring mechanisms involving tyrosine kinase-1, soluble endoglin, and placental growth factor. Validation in diverse populations and trials testing threshold-based feedback are needed. Work should improve maternal and neonatal outcomes.