Gestational age-dependent reference values for pH in umbilical cord arterial blood at term

Despite a change of fetal physiologic and biochemical functions with gestational age, stationary umbilical cord acid-base reference values are used as measures of obstetric care quality. The aim of this study was to assess the risk of low Apgar score relative to low pH when using stationary versus gestational age-adjusted reference values.

Umbilical artery pH was determined in 24,390 term singleton vaginal deliveries with a 5-minute Apgar score of 9 or greater. Low pH was defined as less than 7.10 as a stationary cutoff value, whereas pH less than (mean - 2 standard deviations [SDs]) was used when gestational age was considered. The 5-minute Apgar score less than 7 at low pH was studied in 44,978 term deliveries.

A significant negative correlation was found between gestational age and umbilical artery pH. The odds ratio (OR) for pH less than 7.10 was continuously increasing, from 0.6 at 37 weeks to 1.5 at 42 weeks. In contrast, the OR for pH less than (mean - 2 SDs) was steady until 42 weeks, whereafter it increased (OR 1.24; 95% confidence interval 1.05, 1.47). A linear decrease of the association between Apgar score less than 7 and pH less than 7.10 with increasing pregnancy duration was found (P =.097), but for pH less than (mean - 2 SDs) there was no such association.

We found a physiologic linear decline of umbilical artery pH with gestational age at term. Gestational age-adjusted umbilical cord artery pH reference values result in fewer diagnoses of cord acidemia than a stationary cutoff of pH less than 7.10.