Relative contraindications include:

Lethal chromosomal disorder/anomaly (trisomy 13, 18)
Severe brain damage
Severe irreversible neurological injury or high-grade intracranial hemorrhage
Prematurity or low birthweight (newborns < 34/40 gestational weeks should be considered on an individual basis; <32/40 is contraindicated)
Severe congenital anomalies or conditions such as malignancy with poor survival prospects
Contraindications to anticoagulation or severe or uncontrolled systemic bleeding
Irreversible organ damage (unless considered for organ transplant)
< 2 kg (vessel size does not always correlate with body weight)
< 34 weeks post-menstrual age
(increased risk of intracranial hemorrhage)