Dear Lou, whilst your sons bilirubin level was elevated, it was by no means an uncommon level, so try not to worry yourself unduly.
Basically, as a rough guide, a level over around 250 is usually in the 'phototherapy' range, although this does depend on the age of the baby etc. For example, a level of 347 in a baby less than 24 hours old would be classed as very high, and possible further investigations as to cause would need to be carried out. In a baby 3 days old or older, it is not so worrying so no further investigations would be necessary at this time, but phototherapy would be recommended.
Briefly this is because raised bilirubin levels (which result in that jaubdiced look), is the normal physiological process - jaudice usually becomes apparent around day 2-3, peaking around day 4-5, reducing by day 7. Therefore, if jaundice occurs outside of these normal parameters, it should be further investigated (ie before day 1, or if it persisting beyond about 2 wks of age).
PHYSIOLOGY: Whilst in utero, the baby has a very high iron level (hb), in order that he/she may attract enough oxygen across the placenta. Once born however, the baby no longer need this high level of iron, so these excess red blood cells are broken down naturally by the body, and a by-product of this process is bilirubin. Because the baby's liver is still immature, it cannot necessarily deal with these sudden levels of bilirubin, and consequently cannot get rid of it quickly enough. Excess bilirubin in the system therefore leads to this yellow tinge - normal physiological jaundice. This type of jaundice, which is by far the most common, can vary in degrees - some babies seem unnafected, others I fondly term as 'little corn fed chickens!'. Bear in mind, the worst the jaundice, the longer it will take to fully reced, so a very jaundiced baby requiring phototherapy will not usually be totally jaundice free by day 14.
Other types of jaundice are less common - they could be caused by infection, incompatability with baby blood group and your own, infection, prematurity or liver problems - all of which I must stress are rare. Similarly, those who remain jaundiced beyond 14 days old, and who have 'prolonged jaundice screening' are more often than not diagnosed with a form of 'breast milk jaundice' - a funny term, and noone really understands the whys or wherefores here - suffice to say, something in your breastmilk results in this jaundice, which is perfectly safe, and usually resolves within a few months.
Phew, hope this isn't too long winded!
Back to the original question. The worry with untreated high levels of jaundice, is that it could possibly cause brain damage - when levels of unconjugated bilirubin (ie the bilirubin not broken down by the liver) are so high they escape into the basal ganglia of the brain. This is called 'kernicterus', which would result in signs of cerebral palsy.
A level of 347 would by no means be very unusual Lou33, just treated with a stronger form of phototherapy I suspect, until it receded.
Just to say as well Lou, midwives and paediatricians all refer to a chart when deciding if a bilirubin level is high enough to treat/ observe etc, and these figures are in reality, very outdated. Newer research has suggested that we treat jaundice far too readily, and figures which we act upon are incorrect - in fact the research suggested that only when levels exceed 500 should we become concerned about side effects of jaundice.
Hope this has been of some help Lou33. Difficult to say what could have prompted the cerebral palsy, but I feel it is unlikely to have been the jaundice. All the best.