Vasa previa placentas are rare 1:2500 pregnant women have them therefore they do not check for them as standard however detected before labour mum and baby have 99% chance of survival. Undetected if mum goes into labour baby's chance of NOT surviving is between 55-95%.
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This is when the vessels that feed the baby oxygen are on the outside of the placenta so when mum dilates and baby's head moves down the vessels burst starving baby of oxygen (with 2mins to get baby out) and causing mum to haemorrhage. Detected, mum will be hospitalised from 29 weeks and have a c section between 35-36 weeks. Undetected results can be devastating. A few extra minutes on a scan could save mum and baby's life.