Longitudinal evaluation of computerized cardiotocographic parameters throughout pregnancy in normal fetuses: a prospective cohort study

Introduction The longitudinal cardiotocographic (CTG) changes throughout pregnancy in normal fetuses have never been fully described. We aimed at characterizing the evolution of CTG parameters in healthy fetuses, from 24 to 41 weeks of gestation. Material and methods A prospective cohort study was conducted in singleton fetuses without structural abnormalities on second‐trimester ultrasound. At least one CTG was performed in each of the following intervals: 24–26 weeks+6d, 27–29 weeks+6d, 30–32 weeks+6d, 33–35 weeks+6d, 36–38 weeks+6d and ≥39 weeks; tracings were analyzed by the OMNIVIEW‐SISPORTO 3.6 system. Cases of preterm delivery, fetal death, birthweight under the 10th percentile, low five‐minute Apgar, umbilical artery acidemia or neonatal intensive care unit admission were subsequently excluded. Results A total of 1049 eligible tracings were obtained from 145 fetuses. There was a significant increase over time in average long‐term variability (LTV), average short‐term variability (STV), number of accelerations and uterine contractions. Conversely, fetal heart rate (FHR) baseline and number of decelerations decreased. A high inter‐fetal variability was observed, but there was considerable intra‐fetal consistency. Fetuses showing a marked decrease in FHR baseline and those with a marked increase in average LTV had a significantly lower birthweight. Cesarean section rate was significantly higher in cases with a decrease in average STV throughout gestation. Conclusions This prospective longitudinal study shows an evolution in computerized CTG parameters during pregnancy, indicating the need to adapt interpretation criteria based on gestational age. The high inter‐fetal variability and considerable intra‐fetal consistency suggests the possible value of using each fetus as its own reference in serial assessments.