Causes of increased numbers of NRBCs:
(1) fetal asphyxia with hypoxia
(1a) uteroplacental insufficiency
(2) intra-uterine fetal blood loss
(2a) fetal to maternal transfusion
(2b) donor in the twin to twin transfusion syndrome
(2c) lacerated blood vessel
(3) maternal diabetes
(4) intra-uterine placental blood loss
(4a) abruptio placenta
(4b) laceration of velamentous blood vessel
(5) intra-uterine hemolytic anemia
(5a) anti-D, anti-C or other anti-Rh antibodies
(6) trisomy 13 or 21
(7) severe hemoglobinopathy
ABO-incompatibility is unlikely to cause a significant increase in NRBCs by itself.
For a transient insult there is a lag time between the onset of the event and the appearance of the NRBCs (as little as 2 hours up to 48 hours). Once the problem is reversed then the NRBCs can disappear within a few hours. With severe intra-uterine asphyxia they can persist for up to a week or more after delivery (suggesting on-going problems with oxygenation).
