An employment policy interfering with employee pregnancy, in particular, is illegal (but only because it was explicitly made so in 1978 - https://www.eeoc.gov/statutes/pregnancy-discrimination-act-1...).

In general, when an employment discrimination is not specifically forbidden via the Civil Rights Act (or interpreted as equivalent to something explicitly forbidden by the Court), it is legal to discriminate based on it in the United States. Beyond that, the Court and the legislature are (from a political philosophy standpoint) generally willing to let individual employees and employers (or employers and unions) hash out the messy details.