Association between the cumulative duration of abnormal cardiotocography patterns and fetal acidemia

To assess the association between the cumulative duration of abnormal cardiotocography (CTG) findings, classified using the Japan Society of Obstetrics and Gynecology (JSOG) five-tier cardiotocography (CTG) classification system, and fetal acidemia, with analyses performed separately for the first and second stages of labor.

We conducted a single-center retrospective cohort study at the National Center for Child Health and Development between 1 May 2015 and 1 May 1 2022, including women with planned vaginal delivery at ≥37 0/7 weeks of gestation. Cases with an umbilical artery blood pH <7.15 were categorized as the acidemia group; those with pH ≥7.15 served as controls. For each participant, all CTG tracings were reviewed and classified using the JSOG five-tier system (LEVELS 1-5). The cumulative duration of LEVELS 3-5 abnormalities and fetal tachycardia was calculated separately for the first and second stages of labor. Associations with acidemia were evaluated using multivariable logistic regression, adjusting for maternal body mass index, hypertensive disorders of pregnancy, gestational diabetes mellitus, fetal growth restriction, and chorioamnionitis.

A total of 648 deliveries were analyzed (141 acidemia, 507 non-acidemia). During the first stage of labor, LEVELS 3 and 4 abnormalities showed a duration-related association with acidemia, whereas fetal tachycardia was significantly associated when lasting ≥30 min. During the second stage of labor, LEVEL 3 abnormalities showed no significant associations; however, LEVELS 4 and 5 abnormalities and fetal tachycardia lasting >60 min remained significantly associated with acidemia, demonstrating progressively higher odds with longer durations.

Incorporating the cumulative duration of CTG abnormalities into the JSOG five-tier classification may improve the assessment of fetal acidemia risk. Our time-based analysis demonstrated that LEVEL-specific CTG abnormalities and fetal tachycardia showed duration-related associations with acidemia, particularly during the first stage of labor. LEVEL 3 abnormalities and fetal tachycardia during the first stage of labor were more associated, whereas LEVELS 4 and 5 abnormalities during the second stage were more associated with acidemia. These findings may facilitate more informed intrapartum decision-making by identifying fetuses at risk earlier in labor, potentially reducing unnecessary emergency cesarean deliveries and preventing severe acidemia.