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Contents: Review

Effect of Delayed Cord Clamping on Umbilical Blood Gas Values in Term Newborns

A Systematic Review

Nudelman, Matthew J. R. MD, MAS; Belogolovsky, Esther; Jegatheesan, Priya MD; Govindaswami, Balaji MBBS, MPH; Song, Dongli MD, PhD

Author Information
doi: 10.1097/AOG.0000000000003663

Delaying cord clamping for at least 30–60 seconds after delivery is recommended by national and international organizations.1–6 In term newborns, delayed cord clamping has been shown to increase blood volume,7 hematocrit,8,9 hemoglobin,8–10 ferritin,8,9 and iron stores.8,9 It decreases anemia,8,9 and improves long-term neurodevelopmental outcomes.11,12 Delayed cord clamping likely does not increase maternal postpartum hemorrhage, nor interventions for neonatal hyperbilirubinemia or polycythemia.9,10,13

Cord blood gas analysis is used to assess acid–base status of newborns and to diagnose and treat those who are acidemic.14,15 It has significant medicolegal implications.16,17 Current cord blood gas reference ranges were defined when early cord clamping at less than 30 seconds was routinely practiced.18–22 As delayed cord clamping has become more prevalent, understanding its effect on newborn acid–base status and cord blood gas is important. Our objective is to compare the effect of early cord clamping and delayed cord clamping on arterial and venous cord blood gas values in term singleton newborns delivered vaginally.


This systematic review was prepared using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) guidelines.23 Details of the protocol for this systematic review were registered on PROSPERO (ID Number: CRD42019135779) and can be accessed at

Randomized controlled trials (RCTs) comparing delayed cord clamping at or after 30 seconds to early cord clamping in term newborns that measured cord blood gas parameters were eligible. Observational studies were included if cord blood gas parameters evaluated serial samples from the same umbilical cord collected at less than 30 seconds and again at or after 30 seconds after delivery. Outcome measures assessed were arterial and venous cord blood gas parameters including pH, PCO2, PO2, HCO3, lactate, and base deficit. A comprehensive literature review was performed on April 20, 2019. EBSCOhost-OneSearch was also used to simultaneously search, identify, and compile records from MEDLINE, CINAHL, and CENTRAL databases using the same search terms. EMBASE and were searched separately. Forward and backward reference searches were also performed on each study ultimately included in this review. Broad search terms were used to search the titles, abstracts, and keywords: cord AND clamp* AND gas AND umbilicus*.


Two reviewers (M.J.R.N. and E.B.) independently performed a database search and reviewed each paper. Discrepancies between the two reviewers were discussed and further evaluated by an adjudicator (D.S.). During initial record screening, M.J.R.N. and E.B. classified each article as either “include, exclude, or unsure” and then evaluated full texts of screened records for eligibility. Two reviewers (M.J.R.N. and E.B.) independently extracted data using an Excel-based data-extraction form. If a study reported more than one study group, we included only the results and methodologic characteristics most pertinent to this research topic.

We performed a domain-based risk of bias assessment of individual studies using the following domains: randomization sequence generation, allocation concealment, blinding, incomplete data, confounding, successful blood draws, and sample validation. Each domain was rated as having either “high,” “low,” or “unknown” risk of bias.24 We used the GRADE (Grading of Recommendations, Assessment, Development and Evaluation) approach to evaluate the quality of evidence and rated it as high, moderate, low, or very low.

The preferable summary measure was the difference in means between early cord clamping and delayed cord clamping cord blood gas parameters. When a difference in means in cord blood gas parameters between delayed cord clamping and early cord clamping groups was unavailable, we extracted other available descriptive statistics such as mean, SD, median, and range for each group. The 95% CIs for the mean and the difference in means were calculated using the following equations: , Difference in means (95% CI)=, when sufficient information was available and gross assumptions about a study's original data were not required. Cohen's kappa coefficient (k) was used to measure inter-rater agreement between M.J.R.N. and E.B. All statistical analysis was performed using Stata 14.


The reviewers screened 148 unique records and identified 23 eligible studies. Subsequently, two RCTs and three observational studies records were included in the review (Fig. 1). The details relevant to the pertinent study arm from each study were summarized (Table 1 and Appendix 1 [Appendix 1 is available online at]). Inter-reviewer agreement was 97% (k=0.89 [95% CI 0.81–0.91], P<.001) for the initial record screening and 100% during the eligibility assessment. Risk of bias within each study was low (Table 2). The domain-based risk of bias assessment is documented in Appendix 2, available online at

Fig. 1.:
Record selection flow diagram. Of the 22 studies excluded during the eligibility assessment, six were excluded from this review for methodologic reasons but are still cited because of their relevant contributions to this field of research.25–30 *Two full-text articles assessed for eligibility were identified by backward reference searching.Nudelman. Delayed Cord Clamping and Cord Blood Gas. Obstet Gynecol 2020.
Table 1.:
Summary of Study Characteristics
Table 2.:
Risk of Bias Assessment Within Individual Studies

We gave “high” quality of evidence rating for RCTs and the observational studies because they used within-individual serial sampling. The overall quality of evidence was downgraded to “moderate” based on indirectness related to intervention heterogeneity in early cord clamping timing (0 seconds, less than 10 seconds, less than 30 seconds) and delayed cord clamping timing (45 seconds–180 seconds), which may have affected the summary estimate across the studies. The domain-based quality of evidence assessment is documented in Appendix 3, available online at

Each study used different descriptive statistics, and only Valero et al31 reported a difference in means. For both De Paco et al studies,32,33 we calculated 95% CI for each cord blood gas parameter using the reported M, SD, and n and an alpha of 0.05. We summarized the arterial and venous cord blood gas results of the included individual studies in Tables 3 and 4, respectively. Appendices 4 and 5, available online at, provide additional findings for PO2, PCO2, and lactate.

Table 3.:
Summary of Study Results That Compared Umbilical Artery Blood Gas Parameters After Performing Early Compared With Delayed Cord Clamping
Table 4.:
Summary of Study Results That Compared Umbilical Venous Blood Gas Parameters After Performing Early Compared With Delayed Cord Clamping

The studies using within-individual serial umbilical arterial samples showed that delaying cord clamping 45–90 seconds was associated with mean decreases in pH (0.02–0.03), HCO3 (0.3–0.8 mmol/L), and mean increases in base deficit (0.3–1.3 mmol/L) and lactate (0.2–0.6 mmol/L) compared with early cord clamping at less than 30 seconds.31,34

Within-individual serial samples showed that delaying cord clamping 45–90 seconds was associated with mean increase in arterial PCO2 (0.8–3.2 mm Hg) compared with early cord clamping at less than 30 seconds.31,34

One RCT and one observational study showed that delaying cord clamping 45–120 seconds was associated with a mean increase in arterial PO2 (2.4–6.0 mm Hg) compared with less than 10-second early cord clamping.32,34

The study using within-individual serial umbilical venous samples showed that 90 seconds of delayed cord clamping was associated with mean decrease in pH of 0.01; delaying cord clamping 45–90 seconds was associated with mean decrease in HCO3 (0.1–0.2 mmol/L), increases in base deficit (0.1–0.3 mmol/L) and lactate (0.1–0.3 mmol/L) compared with 0-second early cord clamping.34

One study using within-individual serial umbilical venous samples showed that delaying cord clamping for 90 seconds was associated with mean increase in PCO2 by 0.9 mm Hg compared with immediate cord clamping.34

None of the studies showed any difference in the venous PO2 between delaying cord clamping 45–180 seconds and delaying it for less than 30 seconds.

We did not perform a meta-analysis owing to the methodologic heterogeneity between the studies. Additionally, consistent summary estimates (ie, difference in means and SDs) were unavailable and could not be estimated without making unsupportable assumptions about the study's original data.


This review identified two RCTs and three observational within-individual studies that evaluated the effect of delayed cord clamping compared with early cord clamping on cord blood gas values in 452 vaginally delivered, healthy, term singletons. Values obtained after delayed clamping were more acidemic, but only slightly, and were still within normal reference ranges.18–20,22,36

One within-individual study observed that delayed cord clamping was associated with a minimal decrease in pH, HCO3, and increase in PCO2, lactate, and BD in cord venous blood gas.31,34,35 However, the second within-individual study and the RCT studies showed that normal acid–base balance was maintained in cord venous blood, suggesting that effective placental gas exchange continues during delayed cord clamping.32,33

Cord blood sampling from a segment of double-clamped cord is widely used in clinical practice.37 Another cord blood sampling method is collecting blood from an unclamped, pulsating cord connecting the newborn and placenta. Andersson et al25 compared cord blood gas collected using the two sampling techniques and found that cord blood gas values were comparable. Collecting cord blood from an unclamped, pulsating cord in the setting of delayed cord clamping has been advocated by Xodo et al38 because this allows cord blood to be obtained immediately after birth, without being affected by delayed cord clamping. However, this method requires a designated person for sample collection at the time of delivery which may be not be feasible in some clinical situations.

Larger studies, similar to those originally used to describe cord blood gas reference ranges,18–20,22,36 are needed to more accurately describe the effect of delayed cord clamping on blood acid–base balance in term, singleton vaginal deliveries. Our review was limited to healthy, term newborns. Because delayed cord clamping is increasingly performed in preterm newborns, twins, and those born by cesarean delivery, its effect needs to be more widely studied given the direct implications of cord blood gas has on the diagnosis and treatment plan in medically complicated newborns.

There are discrepancies across the five studies. The observational studies showed delayed cord clamping up to 120 seconds had effects on both arterial and venous cord blood gas values, however, the magnitude of this effect is clinically insignificant in healthy, term, vaginally delivered newborns.


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