Fetal hemoglobin level at postmenstrual age is unaffected by gestational age at birth.

Background Our aim was to determine whether the postnatal age or postmenstrual age is a more appropriate criterion for evaluating foetal haemoglobin concentrations. Methods Blood samples (n=1095) were obtained from 394 infants and were divided into two groups based on gestational age at birth: <37 weeks (n=491) and 37 weeks (n=604). (1) Foetal haemoglobin concentrations divided by one month at age after birth were compared between the groups. (2) Foetal haemoglobin concentrations divided into 9 months from last menstruation and one month thereafter were compared between the groups. Results In samples from infants 37 weeks' gestational age at birth, the median foetal haemoglobin concentrations were 69.5%, 21.4% and 3.6% at 0-1 month, 2-3 months and 5 months after birth, respectively. The median foetal haemoglobin concentrations in infants <37 weeks' gestational age at birth were 75.5%, 62.7% and 5.1% at 0-1 month, 2-3 months and 5 months after birth, respectively. The median foetal haemoglobin concentrations in infants <37 weeks' gestational age at birth were significantly higher than that in infants 37 weeks' gestational age at birth at all postnatal age points. (2) There was no significant difference between the groups at all age points after nine months of postmenstrual age: 72.5 and 75.3% at 9-10 months, 25.1 and 26.6% at 11-12 months and 5.5 and 4.6% at >13 months after last menstruation in infants 37 and <37 weeks' gestational age at birth, respectively. Conclusions Evaluation of foetal haemoglobin concentrations at postmenstrual age is unaffected by gestational age at birth.