Source? Given that for generations unwanted pregnancies were carried to term and produced productive law abiding citizens the burden of proving such an assertion is on you.
That was before the cradle-to-grave entitlement welfare system came into being. Feel free to pay a visit to an NYC public school in a predominantly welfare-dependent neighborhood. You can decide for yourself how many of these barely literate, appallingly behaved kids are going to grow up to be productive and law-abiding citizens. And decide if our society would really be better off with twice as many of them.