## 2 Material and Methods
### Electronic Fetal Monitoring  
To ensure fetal well-being, a Corometrix 256 CX fetal activity supervisor was used for monitoring. This involved the use of two sensors: an ultrasonic transducer for capturing the electrocardiographic (ECG) fetal activity and a Tocotonometer transducer for monitoring uterine activity. These sensors were securely attached to the mother using binding bands, allowing obstetricians to continuously analyze the signals from the later stages of pregnancy until delivery.  
The analysis and interpretation of the EFM data from the last 30 min prior to delivery were performed by an expert obstetrician in the delivery section. The obstetrician, who was unaware of the neonatal outcome, assessed non-ACOG (NICHD) parameters. These parameters included total reperfusion time and deceleration area, which were measured during the 30 min preceding labor. Figure 3 shows the parameters required for their calculation.  
Figure 2: Flowchart of cases and controls recruitment.  
The electronic fetal monitoring (EFM) signal was divided into two segments: deceleration (y) and interdeceleration (x). The interdeceleration period, referred to as x, represented the duration between decelerations. The duration of decelerations, denoted as y, represented the time during which decelerations occurred. The depth of decelerations was indicated as z. With these parameters (x, y, and z), the following measurements were computed:  
* Total reperfusion time: This parameter was calculated by adding up the duration, measured in minutes, during which the fetus maintained a baseline state without any deceleration within the last 30 min (\(\sum_{i=1}^{n_{x}}x_{i}\)), where \(n_{x}\) is the total number of interdeceleration periods.
* Deceleration time: This parameter was determined by summing the duration, measured in minutes, of the period during which the fetus displayed decelerations within the last 30 min (\(\sum_{i=1}^{n_{y}}y_{i}\)), where \(n_{y}\) is the total number of deceleration periods.
* Total deceleration area: This parameter was computed by summing the areas of all the decelerations. Each deceleration's area was determined by multiplying the duration of the deceleration in seconds by its maximum depth of fall from the baseline, given in beats per minute, and dividing the result by two (\(\sum_{i=1}^{n_{y}}y_{i}z_{i}/2\)).  
Additionally, various parameters related to the first and last decelerations were analyzed. These parameters encompassed amplitude, duration, drop, slope, area, fetal heart rate (FHR), excess of deceleration, deceleration instability, and reduced deceleration. The exploratory analysis revealed that the parameters associated with the last deceleration were more significant in predicting acidemia compared to those of the initial period. This finding aligns with the notion that information closer to delivery tends to be more informative. Consequently, only the parameters related to the last deceleration were considered. Furthermore, the study introduced the concept of parameter evolution by defining the difference between the last and first periods. A visual representation of the described parameters can be found in Figure 4.  
Figure 3: The analysis of intrapartum electronic fetal monitoring is performed at a rate of 1 cm/min. The monitoring panel shows the fetal signal (above), which includes parameters such as decelerations (y), reperfusion time (x), and deceleration depth (z). The mother’s uterine contractions, measured in mm Hg (below), are not used for the analysis.