Professional guidelines on delayed cord clamping are constantly updated. The Enhanced Recovery After Surgery Society guideline recommended the timing of cord clamping in term newborns was at least 1 min . The ACOG recommended at least 30–60 s in preterm and term neonates . To date, the optimal timing of DCC in cesarean section remains uncertain. In our study, we defined DCC as a delay of cord clamping for at least 30 s, which was consistent with the ACOG guideline. We investigated the short-term effects of DCC at different time on neonatal jaundice, the rate of jaundice requiring phototherapy and the early hematological status of newborns.
In this trial, we found that clamping the umbilical cord beyond 30 s increased the transcutaneous bilirubin on the day of birth, while this effect disappeared from the first to the fifth day of birth. The transiently elevated bilirubin from the amount of extra blood volume on the day of birth might be fastly metabolized, which caused no damage to the newborns. Clamping the cord at 30–60 s significantly increased the rate of neonatal polycythemia without the enhanced requirement of neonatal phototherapy and other adverse outcomes.
ECC rather than DCC is often implemented in cesarean section due to the fear of increasing requirement of phototherapy for neonatal jaundice and excessive maternal blood loss. However, our study demonstrated that DCC did not increase the rate of phototherapy, although DCC resulted in elevated transcutaneous bilirubin on the day of birth temporarily and the increasing rate of polycythemia on the third day after birth. This outcome was consistent with previous studies [4, 7]. What’s more, the conclusion of the little relationship between DCC in vaginal delivery and the requirement of phototherapy was approved by most researches [3, 6, 19]. Conversely, our previous study  and the report from Japan  observed that DCC led to a higher risk of neonatal jaundice requiring phototherapy in healthy term newborns. The different findings may be due to the diversities in the study design, the sample size, and the study population. The difference between our previous study and the current study is due to different delivery methods. It was reported that cesarean section was associated with a less placental transfusion compared with vaginal delivery . In addition, no significant difference in maternal postoperative hemorrhage was found between the ECC group and the DCC group [4, 21], which is corroborated our findings (the results not shown). Therefore, delayed cord clamping in healthy term neonates is a safe procedure during cesarean section without apparent harmful effects on the neonates and their mothers.
For term neonates, several randomized controlled trials [3, 6, 22, 23] had reported DCC resulted in improved hemoglobin levels at birth or within the three days of life, which was in agreement with our results. But few studies [3, 22, 23] took the effect of different delivery methods on DCC into account. The pattern of placental transfusion differed between vaginal delivery and cesarean section delivery . Our study demonstrated delayed clamping at 30–60 s (the median time 53.5 s) increased hemoglobin and hematocrit levels on the third day after birth. Increasing the duration of cord clamping from 60 s to 120 s (the median time 76 s) did not result in further increases in hemoglobin and hematocrit levels but led to a decreasing trend, which may be the result of placental blood flow reflux. Clamping the cord at 30–60 s in cesarean section may be a better choice rather than 61–120 s.
However, a randomized controlled trial in 2019 showed that the hematocrit in a capillary at day 2 of life increased by 6% in neonates receiving DCC beyond 60 s during the cesarean section . The conflicting results suggested that hematocrit and hemoglobin levels within several days could not completely reflect the impact of DCC at different time on placental transfusion. Multicenter large sample studies with long-term follow-up for the neonate hemoglobin levels are required for more reliable data.
The short-term beneficial effects of DCC in hemoglobin and hematocrit could make sense in the growth and development of newborns. A prospective study showed that the difference in cognitive function could not be eliminated between healthy formerly iron-deficient anemic children and normal ones after iron treatment for ten years . Several studies confirmed that DCC improved iron status at 2 , 4  and 6  months of age in term neonates. In the 4th month, the level of ferritin in neonates born by elective cesarean section with DCC at 30 s was higher than those born vaginally with ECC and similar to those born vaginally with DCC at 180 s . Reportedly, effective placental transfusion merely occurred in the first 40 s after birth in cesarean section . A similar result was found in our study that DCC at 61–120 s could not further improve hematological status compared with DCC at 30–60 s. DCC at 30–60 s should be an optimal time in cesarean section, which could benefit the neonates in the long term.
Limitations of this study include the small blood sample size of the DCC group on day 3 for the ethical reason that heel peripheral blood collection is an invasive procedure, and focusing on the effects of DCC in cesarean section on short-term hematological status rather than long-term hematological effects. The strength of this study was repeated measurement in bilirubin level and blood indicators, ensuring the accuracy of the effect of DCC on neonates. Additionally, our trial was one of the few researches on the implementation of DCC during cesarean section.