One law can prevent thousands of babies dying per day.

I disagree - this is not known. Particularly the magnitude of the expected effect is hard to predict and a cost-benefit analysis requires a prediction.

• The law would affect conception rates as well as birthrates, and we don't know how ahead of time.
• It would also affect rates of unwanted babies born and given up for adoption or raised in unloving or too-poor or single-mother homes. These factors affect life expectancy (also through hightened poverty and crime), which have moral weight by the "biological human life" criterion.
• Some women would still do illegal or at-home or out-of-state abortions. Some of the women would also die or be injured thereby.
• Enforcement of the law costs money and resources and also depends on cultural support for the law in each community.