Try to ask yourself why you can't keep the baby. If it is due to financial, you can adjust current family living styles e.g. less eat out, less traveling and etc. There will be a solution definitely. If it is due to no one to take care of the baby, you can find alternative arragements e.g. nanny, IFC.
Each life is precious. Just because the baby is in no.3 sibling sequence doen't mean he or she don't have the right to stay in this world. I have a friend who aborted her 3rd baby and regretted until now and the relationship with husband ends up a lot of blaming and regrets. Whenever she sees other people's announced no.3 pregnancy, she can't help to feel sad and think about "what if" she keep the baby. But it's too late already. Do think twice.
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