Here are a list of precedents for the case.
Los Angeles Department of Water & Power v. Manhart (1978): The incremental cost of employing members of one sex cannot justify a discriminatory refusal to hire members of that gender.
Phillips v. Martin Marietta Corp. (1971): The Court found sex discrimination (at a company) because the policy established one hiring policy for women and another for men - each having pre-school-age children.
Newport News Shipbuilding & Dry Dock Co. v. EEOC (1983): The Pregnancy Discrimination Act has now made clear that, for all Title VII purposes, discrimination based on a woman's pregnancy is, on its face, discrimination because of her sex
Dothard v. Rawlinson (1977): The Court indicated that danger to a woman herself does not justify discrimination.