pH values in the umbilical artery are the best measurement for evaluating the presence and intensity of fetal acidosis, and even the direction of the pH change in the immediate neonatal period has prognostic value.1 This practice can also aid in reducing the risks of claims against obstetricians.2 The data obtained after a latency period in the subgroup of cords with acidosis at the time of birth are evaluated in this study, to determine whether they are useful for the clinic.

The umbilical cords from 48 term infants were clamped immediately after delivery. The local research ethics committee approved the study. The cord segment obtained was further divided into three parts by attaching two plastic cord clamps. The cord was preserved under no special conditions and at room temperature. Samples of artery and vein blood were drawn 5, 60 and 120 minutes post partum and pH, po₂ and pco₂ levels were measured. The selection was established at an arterial pH level <7.20, 5 minutes after delivery. Results were analysed using a repeated measures analysis of variance after checking normal distribution of the values, followed by comparison of the main effects with Bonferroni adjustment; p<0.05 was considered significant. The size of the samples allows detection of an increase or decrease of arterial pH by four hundredths, with a security of 95% and a potential of 80%.

The average age of the patients was 30 years and the average generational gestational age was 39 weeks. Significant differences were found after 60 minutes in the average values for the pH in the arterial paired samples, with respect to the analysis conducted immediately after birth (7.120 (0.062) at 0 minutes and 7.14 (0.061) at 60 minutes), registering an increase in arterial pH values of 0.02 (95% CI 0.005 to 0.042) (table 1). Venous pH values were not significant (7.219 (0.090) at 0 minutes and 7.220 (0.080) at 60 minutes). Arterial and venous pco₂ values declined over time; however, the arterial and venous po₂ values were not significantly modified.

Although the pH level of the subgroup of newborns with acidosis, the subgroup of greatest medical interest and with the most risk of claims, presents such a rapid increase over time that makes it statistically significant, this increase is so modest that the data obtained could still be valid in the event that an immediate analysis of the umbilical cord is not possible.