Measuring placental transfusion for term births: weighing babies with cord intact

Authors


Dr D Farrar, Bradford Institute for Health Research, Duckworth Lane, Bradford Royal Infirmary, Bradford, BD9 6RJ, UK. Email diane.farrar@bradfordhospitals.nhs.uk

Abstract

Please cite this paper as: Farrar D, Airey R, Law G, Tuffnell D, Cattle B, Duley L. Measuring placental transfusion for term births: weighing babies with cord intact. BJOG 2011;118:70–75.

Objective  To estimate the volume and duration of placental transfusion at term.

Design  Prospective observational study.

Setting  Maternity unit in Bradford, UK.

Population  Twenty-six term births.

Methods  Babies were weighed with umbilical cord intact using digital scales that record an average weight every 2 seconds. Placental transfusion was calculated from the change in weight between birth and either cord clamping or when weighing stopped. Start and end weights were estimated using both a B-spline and inspection of graphs. Weight was converted to volume, 1 ml of blood weighing 1.05 g.

Main outcome measures  Volume and duration of placental transfusion.

Results  Twenty-six babies were weighed. Start weights were difficult to determine because of artefacts in the data as the baby was placed on the scales and wrapped. The mean difference in weight was 116 g [95% confidence interval (CI), 72–160 g] using the B-spline and 87 g (95% CI, 64–110 g) using inspection. Converting this to the mean volume of placental transfusion gave 110 ml (95% CI, 69–152 ml) and 83 ml (95% CI, 61–106 ml), respectively. Placental transfusion was usually complete by 2 minutes, but sometimes continued for up to 5 minutes. Based on the B-spline, placental transfusion contributed 32 ml (95% CI, 30–33 ml) per kilogram of birth weight to blood volume, but 24 ml (95% CI, 19–32 ml) based on inspection. This equates to 40% (95% CI, 37–42%) and 30% (24–40%), respectively, of total potential blood volume.

Conclusion  Inspection of the graphs probably underestimates placental transfusion. For term infants, placental transfusion contributes between one-third and one-quarter of total potential blood volume at birth.

Introduction

At birth, blood flow in the umbilical arteries and veins usually continues for a few minutes. The additional blood volume transferred to the infant during this time is known as placental transfusion. Immediate clamping of the umbilical cord has traditionally been recommended as part of active management of the third stage of labour, together with a prophylactic uterotonic drug and controlled cord traction,1 to reduce postpartum haemorrhage. Use of a prophylactic uterotonic drug clearly reduces the risk of major haemorrhage.2 The timing of cord clamping does not appear to have a major impact on the risk of haemorrhage, although a modest effect remains possible. Deferring cord clamping will allow a larger placental transfusion. Having been a neglected topic for decades, there is now growing interest in assessing the effects of placental transfusion for both term and preterm infants.3,4

Estimates of the volume and duration of placental transfusion are largely derived from studies conducted 50 years ago, most of which used indirect methods.5–8 For term infants having a normal vaginal birth, estimates range from 60 to 240 ml, with an average of 100 ml often quoted.9 This is equivalent to 20–30% of blood volume and red cell mass at birth. The speed and duration of placental transfusion may also be influenced by gravity and the uterotonic drug. Raising or lowering the baby 20 cm or more from the level of the placenta will affect placental transfusion.10 If an intravenous ergot alkaloid is given, placental transfusion is quicker.11,12 Ergot alkaloids are no longer recommended as a prophylactic uterotonic,1 however, because of adverse effects.2,13 In the UK, intramuscular oxytocin is now recommended.1 It has been suggested that placental transfusion may not occur at caesarean births,14 although others disagree.15

Immediate cord clamping remains common practice.16–18 Large randomised trials to assess the effects of deferring cord clamping to allow placental transfusion for term births have been called for.4,19 In order to plan the interventions to evaluate in such a trial, and to guide clinical practice, we conducted this study to measure the volume and duration of placental transfusion for vaginal and caesarean births at term.

Methods

Women planning to give birth at the Bradford Royal Infirmary, West Yorkshire, UK, with a singleton pregnancy and live baby at term were eligible for inclusion. Information about the study was given to women at 26–28 weeks of gestation when they came for oral glucose tolerance test, which is offered to all women at this unit. They were invited to participate either during the first stage of labour or during preparation for caesarean section. Women who agreed to participate provided written consent. They were free to withdraw at any time.

Procedure for weighing

Babies were weighed using digital scales (Mettler Toledo excellence XS precision balance Model: XS8001L, Im Langacher, CH-8606 Greifensee, Switzerland), which automatically calculate an average weight every 2 seconds, with data stored in a linked computer. The scales were zeroed to allow for the weight of a plastic sheet and two towels, used to wrap the baby during weighing.

At birth, babies were placed on the scales as quickly as possible, with the cord intact. The attending clinician was asked not to touch the baby, the cord or the scales until weighing was complete. If the scales were knocked or the baby or cord touched, this was recorded by the research midwife. For vaginal births, the scale pan was either at the level of the bed or the woman’s abdomen. For caesarean births, it was either at the level of the bed or the woman’s thighs. Weighing continued for up to 5 minutes. The cord was clamped earlier and weighing stopped if requested either by the woman or the clinician. Delivery of the placenta was according to normal clinical practice. Once delivered, the placenta was placed in a funnel to drain any residual placental blood. All other aspects of care were at the discretion of the attending clinician.

Data collection

Parity and gestation at birth were recorded. For women having a vaginal birth, data were collected on whether labour was induced or augmented, the use of analgesia, the mode of delivery and the maternal position during the second and third stage. For women having a caesarean birth, data were collected on the indication for caesarean section and the type of anaesthesia. For all women, data were collected on the timing of the uterotonic drug, time of cord clamping, maternal blood loss during the third stage, length of the third stage and use of controlled cord traction. For the baby, information was collected on the time of birth (recorded as delivery of the buttocks for cephalic births, and head for breech births), temperature after cord clamping, need for resuscitation at birth and whether admitted to the neonatal unit. In addition, a log was kept for each weighing, which included events such as the scales being knocked or the cord touched. All data were anonymous, and were checked for completeness and accuracy.

Statistical analysis

The characteristics of the women and events during labour were described for women who had a vaginal birth and those who had a caesarean birth. As 1 ml of blood weighs 1.05 g,20 placental transfusion was calculated by the conversion of weight to volume.

The scales were switched on as the baby was born; the start time for weighing was when the baby was placed on the scales. In our protocol, we planned to calculate the change in weight, between the start time and either cord clamping or when weighing stopped, by manual inspection of a graph of weight over time (an example of which is shown in Figure 1). This proved to be more difficult than anticipated, however, because of artefacts (sharp spikes in the graphs, see example in Figure 1) as the baby was placed on the scales and wrapped in the towels. It was often several seconds before the graph became interpretable. To help overcome this problem of determining the start weight, we used a B-spline to smooth the data.21

Figure 1.

  Weight change from birth to cord clamping.

For the B-spline, first the raw weight data were manually cleaned, with clear artefacts removed. These artefacts occurred in the first few seconds after the baby was placed on the scales, and when the scales were knocked or the baby touched. The weight measurements remained open to considerable ‘noise’ and a smoothing process was used to remove this. In order to examine the dynamics of the change in weight, a Loess smoothed regression was fitted to the first derivative against time. Each measurement is not independent, but will depend smoothly (in a mathematical sense) on the preceding weights: step changes in weight are not anticipated. We therefore modelled the weights as a trajectory, which is a continuous function of weight, collected every 2 seconds in time. A B-spline was fitted to the weight against time plots individually for each baby. A spline allows a number of smooth functions to be fitted to the data, starting at different points, known as knots. The number of knots was allowed to be determined in an automated way, and then manually checked for consistency.

The start and end weights were estimated from the B-spline at the first time a measurement was made and at the last measurement time. The exact weight at birth was not extrapolated. The mean weight, across all babies, was calculated and t-tests were used to determine the significance of the difference in start and end weights.

For the inspection of graphs, two authors (DF and LD) independently assessed the start and end weights. Differences were then compared and resolved by discussion. The mean weight, across all babies, was calculated and t-tests were used to determine the significance of the difference in start and end weights.

Pre-specified subgroups were based on the mode of birth (vaginal or caesarean), position of the baby (at the level of the introitus or on the mother’s abdomen for vaginal births; at the level of the bed or on the mother’s anterior thighs for caesarean births) and, for vaginal births, on the timing of oxytocin administration (with anterior shoulder or after cord clamping). All statistical analyses were conducted in SPSS,22 R23 or Stata.24

Results

From July to December 2008, 78 eligible women were approached, 52 of whom gave consent. Of these, 26 women did not have their baby weighed. The reasons were: software problem (n = 2), short cord (n = 5), birth out of working hours (n = 14) and clinician felt not appropriate (n = 5). For 26 women, the baby was weighed, 13 at vaginal birth and 13 at caesarean. The baseline characteristics and information about labour and birth are given in Table 1. For women having a vaginal birth, the median length of the first stage of labour was 6 hours 50 minutes (range, 2 hours 45 minutes to 14 hours 0 minutes) and, for the second stage of labour, was 40 minutes (range, 7 minutes to 2 hours 24 minutes). Indications for caesarean section were previous caesarean (n = 7), unstable lie (n = 2), breech (n = 2) and failed induction (n = 2). Intramuscular oxytocin was the prophylactic uterotonic drug for all vaginal births and intravenous oxytocin for all caesarean births. All women had controlled cord traction.

Table 1.   Characteristics of the women and events during labour and at birth
 Vaginal birth
(n = 13)
Caesarean birth
(n = 13)
Total
(n = 26)
Primigravid5 (38%)5 (38%)10 (38%)
Gestation at birth (weeks and days; mean, SD)393 (11)393 (13)393 (12)
Use of analgesia or anaesthesia
Entonox77
Pethidine22
Epidural22
Spinal1111
General anaesthetic22
Induction of labour2 (15%)2 (15%)4
Third stage of labour
Oxytocin before cord clamping8 (62%)13 (100%)21 (81%)
Oxytocin after cord clamping5 (38%)5 (19%)
Estimated blood loss (ml; mean, SD)204 (69)531 (205)367 (224)
Manual removal of placenta
Residual placental volume (ml; mean, SD)23 (11)21 (19)21 (11)
Baby at birth
Temperature after cord clamping (oC; mean, SD)36.8 (0.4)36.7 (0.4)36.7 (0.4)
Need for resuscitation
Admitted to special care baby unit

From birth to the baby being on the scales took a mean of 16 seconds (standard deviation [SD], 15 seconds) for vaginal births and 38 seconds (SD, 67 seconds) for caesarean births. An example of weight change over time is shown in Figure 1. Using the B-spline, the mean difference in weight for all births was 116 g (95% CI, 72–160 g) (Table 2). Converting this to the volume of placental transfusion (1 ml of blood weighs 1.05 g) gives 110 ml (95% CI, 69–152 ml). There was no statistically significant difference between vaginal and caesarean births. Based on this approach, placental transfusion contributed 32 ml (95% CI, 30–33 ml) per kilogram of birth weight to the neonatal blood volume. Assuming that the blood volume at birth following immediate cord clamping is 80 ml per kilogram birth weight,25 this equates to 40% (37–42%) of the total potential blood volume at birth.

Table 2.   Weight and weight change at birth using a B-spline and inspection of the graphs
 nStart mean weight (g)End mean weight (g)Mean difference in weight (g) (95% CI)t (df)*P
  1. *t-test and degrees of freedom.

  2. **Intramuscular oxytocin.

B-spline
Total2632953411116 (72–160)5.44 (25)<0.001
Mode of birth
caesarean section1334663597131 (64–198)0.69 (24)0.5
Vaginal1331243225101 (36–167)
Position of baby
Above bed143235333296 (38–154)−0.99 (22)0.3
On bed1233643504139 (64–214)
Uterotonic drug**
Yes2134083530122 (69–174)−0.60 (8)0.6
No52820291393 (-17–204)
Inspection
Total263339342687 (64–111)7.7 (25)<0.0001
Mode of birth
caesarean section133534361379 (57–100)0.8 (24)0.5
Vaginal133144324096 (51–141)
Position of baby
Above bed143265334480 (52–108)0.7 (24)0.5
On bed123425352197 (53–140)
Uterotonic drug**
Yes213457354891 (66–118)0.8 (24)0.4
No52844291369 (6–145)

Using inspection of the graphs, the mean difference in weight for all births was 87 g (95% CI, 64–111 g) (Table 2). Converting this to the volume of placental transfusion gives 83 ml (95% CI, 61–106 ml). Based on this approach, placental transfusion contributed 24 ml (95% CI, 19–32 ml) per kilogram of birth weight to the neonatal blood volume; this equates to 30% (24–40%) of the total potential blood volume at birth.

The time at which net placental flow appeared to cease for most infants was at 2 minutes (data not shown). Nevertheless, for some infants, flow continued for up to 5 minutes.

Although the numbers were small, the volume of placental transfusion did not appear to be influenced by whether intramuscular oxytocin had been given before or after cord clamping at vaginal births, or by whether the baby was level with the bed or raised to the level of the mother’s abdomen (vaginal births) or anterior thigh (caesarean births) (Table 2).

Discussion

The direct measurement of placental transfusion by weighing babies at birth with the cord intact suggests that the mean volume is between 83 and 110 ml, equivalent to 24–32 ml per kilogram of birth weight. Typically, placental transfusion represents between one-third and one-quarter of the potential blood volume at birth. There is considerable variation between individual babies, with a few infants receiving relatively small volumes of placental transfusion, whilst others received quite large volumes. For most infants, placental transfusion appeared to be complete by 2 minutes, but, for some, it continued for up to 5 minutes.

A practical problem in our study was the difficulty in determining the start weight because of the ‘artefact’ associated with placing the baby on the scales and wrapping towels around to keep the baby warm. We had originally planned to use direct inspection of the graphs to measure the weight change, but this was more difficult than we had anticipated because of these artefacts in the first few seconds that the baby was on the scales. We attempted to overcome this problem by smoothing the data using a B-spline. This approach has the advantage that it uses the early weight measurements to estimate the probable start weight, but these early weight measurements are ignored on inspection of the graph, which takes the start weight as the weight when the artefacts cease. We present results from both methods to indicate the difficulty in interpreting these data. Although there are differences, the data are consistent in demonstrating that placental transfusion potentially contributes a significant proportion of the blood volume at birth.

Direct inspection of the graphs is likely to underestimate the true placental transfusion as, when using this method, the start weight is often 20 seconds or more after the time of birth and, by the time weighing begins, some transfusion would already have occurred. The B-spline may overestimate placental transfusion if the smoothing is incomplete. It therefore seems plausible that the ‘true’ value is somewhere between the two estimates. Similarly, we probably underestimated the duration of placental transfusion by up to 30 seconds, as the start time for weighing was when the baby was placed on the scales.

Although placental weight and pathology at birth may affect the volume and duration of placental transfusion, they cannot be altered. Whatever are the placental effects, they are fixed for an individual birth. Our study aimed to measure net placental transfusion to assess the effect on the blood transferred to the baby.

Previous studies have suggested that, although lifting or lowering the baby by 40 cm will influence placental transfusion, raising or lowering the baby by 10–20 cm from the level of the placental bed does not substantially influence the volume or duration of placental transfusion.10,26 Although our study was small, the data presented here support this: raising the baby to the level of the mother’s abdomen or anterior thighs (both of which are common practice) whilst the cord is intact does not appear to have a major impact on placental transfusion.

The use of intravenous ergot alkaloids shortens the duration of placental transfusion at vaginal births.12 These drugs are no longer recommended for clinical practice, as oxytocin has a similar effectiveness with fewer adverse effects.1 Our study suggests that the use of intramuscular oxytocin does not have the same impact on the duration of placental transfusion. Our data for these subgroups should be interpreted with caution, however, as the numbers are small.

Some previous studies have suggested that placental transfusion is reduced for caesarean births,14,15,27 possibly as a result of uterine atony related to the uterine incision or anaesthetic drugs, and/or timing of the uterotonic drug. In our study, the volume and duration of placental transfusion were similar for caesarean and vaginal births. This may be because the use of spinal anaesthesia (as in this study) does not influence uterine tone, whereas many of the early studies would have been conducted with general anaesthesia.

The duration of placental transfusion may be as important as the volume for the newborn infant, as this is a period of transition from the fetal to the neonatal circulation. Before birth, just 8% of blood volume goes to the lungs. As the lungs expand, the infant needs to redirect blood flow to fill the expanding respiratory circulation. If it does not have access to placental transfusion, the infant may need to redirect blood from other essential organs, such as the skin, liver and kidneys. Access to placental transfusion may be the optimal mechanism for establishing the respiratory circulation. Once the neonatal circulation is established, the additional blood volume is rapidly absorbed, and the red cell mass is broken down and iron is efficiently stored. This additional red cell mass seems to lead to a marginal increase in jaundice and, possibly, an associated increase in phototherapy. The long-term effects of placental transfusion are unclear, as no trials have reported long-term follow-up.19

Our study suggests that, for term birth, placental transfusion contributes between one-third and one-quarter of the neonatal blood volume. Immediate cord clamping is recommended in the UK as part of the active management of the third stage of labour.1 Clamping the cord before 2–3 minutes is likely to restrict placental transfusion. The short-term and long-term effects of this simple intervention remain unclear.19 Further evaluation of the effects of alternative policies for the timing of cord clamping at term births has been identified as a priority for future randomised trials1,4 The data presented here would be helpful for determining the appropriate interventions to compare in such trials. As the physiology of placental transfusion for preterm birth may be different from that for term birth, further research is also required to explore how placental transfusion varies with gestational age.

Disclosure of interest

All authors declare that they have no conflicts of interest and therefore have nothing to declare.

Contribution to authorship

LD conceived the idea. The protocol was developed by DF and LD, with comments from the other authors. DF and RA weighed the babies and collected the data. GRL carried out the B-spline analysis. DF and LD drafted the paper, with comments from the other authors.

Details of ethics approval

The study was approved by the Bradford Ethics Committee, with approval being granted on 30 July 2008, REC reference 08/H1302/65.

Funding

The study was supported by the Bradford Teaching Hospitals NHS Foundation Trust.

Acknowledgements

Our thanks to the women who took part in this study and to all the staff who contributed. Thanks also to Jim Thornton for comments on the protocol.

Ancillary