Feto‐maternal haemorrhage does not account for the differences in cord blood volume obtained from Black and Asian versus Caucasoid donors

Umbilical cord blood is an important source of haematopoetic stem cells, particularly where no human leucocyte antigen (HLA)-matched bone marrow or peripheral blood stem cell donor is available, which occurs more frequently in patients from ethnic minorities. Successful cord blood transplant outcomes correlate with cell dose and better HLA matches (Wagner et al, 2002; Gluckman et al, 2004). Black and Asian mothers’ cord blood donations have lower median volumes (with a median difference of 5 ml), total nucleated cell counts (TNC), CD34 counts and TNC concentrations than Caucasoid donations (Ellis et al, 2007). Many obstetric and neonatal factors are thought to influence the volume of cord blood collected (Shlebak et al, 1998; Donaldson et al, 1999; Ballen et al, 2001; Jones et al, 2003). Feto-maternal haemorrhage (FMH) however, has never been assessed as a possible explanation for the volume differences amongst different ethnic groups, nor in any previous study looking at maternal or neonatal factors. It was estimated that some degree of FMH occurs in 20–50% of normal deliveries (Klein & Anstee, 2005); the present study aimed to determine whether FMH was commoner in Black and Asian than Caucasoid mothers and to study obstetric and neonatal factors that could affect either FMH size or cord blood volume. We studied 460 RhD negative mothers at Queen Charlotte’s Hospital between 1 January 2005 and 31 December 2006, who had a routine Kleihauer test at delivery to ensure sufficient anti-D was given. Kleihauer results were obtained from the Transfusion Laboratory Telepath System and clinical data was obtained from the Obstetric Unit’s database and Hospital Administration System. Elective caesarean sections were excluded from length of labour analysis. Mothers with more than one fetus or gestation of <34 weeks were excluded, as these mothers are excluded from cord blood donation in the UK. Kleihauer results distinguished between FMHs of 0 ml, >0 but <4 ml and ‡4 ml which were confirmed by flow cytometry measurement of fetal D positive cells. Statistical differences between groups were assessed using Analysis of Variance, the Kruskal Wallis test or t-test for continuous data and the Chisquare test for categorical data. Analyses were carried out using the Statistical Package for the Social Sciences (spss) software, and P values <0Æ05 were considered significant. The sample size needed to detect a 150% difference in the frequency of FMH between Caucasoid and other ethnic groups was estimated to be 36 in each group, with 80% power at the 5% confidence level. In fact, of 460 mothers, 24 (5Æ2%) and 27 (5Æ9%) were Black and Asian respectively, and of infants, 27 (5Æ9%) were Black and 27 (5Æ9%) were Asian. FMH occurred in just 58 (12Æ6%) of mothers and there was no significant difference in the frequency between Black, Asian or Mixed versus Caucasoid mothers, or infants (Table I). Although not significant, Black and Asian mothers and infants tended towards lower rather than higher frequencies of FMH compared with Caucasoids and therefore from this study, FMH would not explain the differences in cord volumes collected. Considering the variables reported to affect cord volume, there is conflicting data in the literature. Our study found that mother’s age, birth weight and length of whole labour were significantly different among maternal ethnic groups. Black mothers were on average 5 years younger than Caucasoid mothers, while the length of labour and infants’ birth weight were much lower in Blacks and Asians. Previous studies found that birth weight correlated with cord blood volume, probably because of associated placental size (Shlebak et al, 1998; Donaldson et al, 1999; Ballen et al, 2001; Jones et al, 2003). Smaller birth weights of Black and Asian infants could therefore explain the smaller cord volumes obtained. Longer labour has been found in some studies to be associated with larger cord blood donations, possibly due to higher frequencies of complications such as fetal hypoxia, which can trigger compensatory mechanisms to increase fetal blood flow and volume (Jones et al, 2003). In our study, as Black and Asian mothers had shorter labours, this could also result in lower cord blood volumes. Whether maternal age affects cord blood volume has been controversial: some studies showed no significant effect (Donaldson et al, 1999; Ballen et al, 2001); others found that the yield decreased with maternal age. However, as Black mothers in our study were significantly younger (on average by 5 years) than Caucasoids, this would not explain their lower cord volumes. Previous studies have found that mothers with fewer previous births had larger cord blood collection volumes (Ballen et al, 2001) but our study found no difference in the number of previous births among the different ethnic groups, so this would not explain cord volume differences. One study (Jones et al, 2003) found that induction of labour was associated with larger cord volumes, but our study found no difference in the frequency of induced labours among the different ethnic groups, so would not contribute to cord volume differences. The same study found that emergency caesarean section was associated with larger Correspondence