Late Sunday, the House approved a $940 billion health care reform package that will, among other things, require all Americans to have insurance and prevent insurers from denying coverage based on gender or pre-existing conditions. President Obama will sign the measure, which was passed in December by the Senate, into law.

"The health care legislation is a big anchor today," said Derek Hoffman, chief executive and founder of Wall St. Cheat Sheet. "Investors are in the process of determining how the reforms will unfold and impact Wall Street."

Shares of health insurers Cigna (CI, Fortune 500), Aetna (AET, Fortune 500), MetLife (MET, Fortune 500), Humana (HUM, Fortune 500), UnitedHealth (UNH, Fortune 500), and WellPoint (WLP, Fortune 500) were all slightly lower in pre-market trading.

But other factors could weigh on stocks besides health care, including the ongoing concern about Greek debt, following reports that Germany may not support European Union efforts to help resolve the crisis.

Stocks snapped an eight-session winning streak Friday as a strong dollar dragged on commodity prices, giving investors a reason to step back from the recent run up. The Dow, S&P 500 and Nasdaq composite had soared to 18-month highs in the previous session.