Bamber was born to a vicar's daughter who had had an affair with a married army sergeant, a comptroller at Buckingham Palace. She gave the baby up for adoption in 1961, the year of his birth, through the Church of England Children's Society. It was only after Bamber's conviction, when his adoption records were published, that his biological parents were told by reporters that Bamber was their son. They were by then married to each other, and both were working at Buckingham Palace.
That was from
http://en.wikipedia.org/wiki/Jeremy_BamberI think there's probably a lot of reasons why a mother would give up their child, especially up to probably recently. Things have changed and there are probably better 'services' and link ups running between services to keep families together.
Way back it was a stigma to have a child out of wedlock to start with and there may be other circumstances like the parent being ill, have an addiction problem, the victim of rape or have a child with a disability that the parent cannot cope with due to circumstaces (having other children or having their own disability etc).
I wouldn't slate anyone for making this decision because it's so difficult. There was a programme on recently,'Protecting our children', that was an eye opener.
http://www.youtube.com/watch?v=7A-2xD3ILrs (there were 3 episodes in total, it looks like they're further broken down to 4 parts per episode).