[Fetal monitoring during the active second stage of labor]

To determine the best way of fetal monitoring during the active second stage of labor.

Articles were searched using PubMed and Cochrane library.

Active phase of labor begins with the onset of maternal pushing. It is characterised by frequent and prolonged uterine contractions as well as maternal bearing down efforts. Altogether those mechanical forces lead to an intrauterine pressure increase up to 250 mm Hg and to a marked reduction in placental perfusion. A prolonged period of expulsion will lead to an impairment of fetal oxygenation as well as a rise in carbon dioxide level. Melchior's FHR classification is specific of the active second stage of labor and described five fetal heart rate patterns: from type 0 to type 4. A reduction in pH and a rise in lactate and P(CO2) values occurred as one progresses from type 0 to type 4 (NP4). During the active phase of labor every method has a significant rate of signal loss. Loss of sensor contact occurred in up to 64% of time with oximetry, 35 to 48% of recordings obtained via external Doppler sensors have more than 20% of signal loss. Even 8 to 11% of recordings obtained via scalp sensors have more than 20% of signal loss. Fetal scalp blood sampling does not allow a continuous recording of fetal well-being and is difficult to perform during this stage. No method has a 100% sensitivity to detect metabolic acidosis. Normal FHR of 1.3% are coupled with an acidosis. Even ST analysis exhibited a small but real false negative rate (NP4).

Active second phase of labor is at high risk of fetal acidosis, and required a close follow-up of the FHR. The length of maternal bearing down efforts should be matched to the fetal heart rhythm Melchior's classification pattern. Optimal length of bearing down efforts could be 30 min for type 0, 20 min for type 1 and 10 min for type 2, 3 or 4 (NP4).