Emily Oster opens by examining how many standard pregnancy guidelines and restrictions, when carefully investigated, lack the rigorous scientific evidence backing many expectant parents assume they have.

Why this evidentiary gap matters:

• Many commonly given pregnancy restrictions are based on weak, outdated, or overly cautious interpretations of limited evidence, rather than strong, well-established research
• Oster's economics training in careful data analysis provides a distinctive lens for examining this evidence more rigorously than typical popular pregnancy guidance
• This opening establishes the book's central methodological project: applying careful, rigorous data analysis specifically to pregnancy-related guidelines and decisions

This opening establishes Oster's distinctive analytical approach, differentiating the book from more conventional pregnancy guidance that often doesn't examine underlying evidence quality as rigorously.