Women who do this are selfish, evil bitches. And that's putting it mildly and politely.
It happens more than you think.
I wanted to add a few more thoughts.
It may be considered 'old-fashioned' but to my way of thinking the ONLY way to approach having a child is when it's a thought-out and agreed upon step by two mature adults in a loving, committed relationship.
Otherwise, don't do it. Because when all is said and done, it is the child that will have to suffer for the actions of the selfish parent...in this case, the mother.