In addition to HIV, other sexually transmitted diseases should be screened. While doctors are testing pregnant women, they can do genetic testing on mother and child as well. Gene testing is becoming less and less expensive, so why not have some additional tests done? The eradication of diseases is a priority for the medical community and a pregnancy is the perfect way to test for stuff in both mother and child.
Forcing testing for HIV for every pregnant women would be both intrusive and ridiculous. What would this solve? To what end would such a practice take us? To do this would assume we're all living in such an infected and diseased population that mandatory testing for someone in pregnancy wouldn't achieve anything anyway.