3249734 PNMR perinatal mortality rate 45 3249739 pNMR pulsed nuclear magnetic resonance 15 3249735 PNMR post-neonatal mortality rate 13 3249733 pNMR paramagnetic NMR 10 3249736 PNMR postnatal myelomeningocele repair 1 3249737 PNMR primary non-refluxing megaureter 1 3249738 PNMR pulse NMR 1