MetLife shares were at their lowest since December on Wednesday as the company faces a possible S&P downgrade over the acquisition of a unit of American International Group Inc.

According to Reuters, MetLife shares dropped more than 4 percent, a day after the company confirmed it was discussing the possibility of acquiring AIG's American Life Insurance Company (Alico).

Shares were down $1.50 at $34.87 on the New York Stock Exchange.

MetLife's Chief Executive Robert Henrikson told Reuters in a conference call there is "no certainty" MetLife will reach a deal for Alico.

"Management added on the call the company would not sell any current businesses to finance a possible deal, nor would it use capital in an off-shore reinsurance unit toward a deal," according to Reuters.

"We placed our ratings on MetLife on CreditWatch negative because of the sheer size of ALICO and to reflect our view that the potential acquisition could have a material adverse impact on MetLife's financial metrics, such as capitalization and fixed-charge coverage, as well integration risks," said Shellie Stoddard, an S&P credit analyst, in a statement.

S&P said MetLife's capitalization and financial flexibility are below expectations for its rating, according to Dow Jones Newswires. S&P currently has the counterparty credit rating at A-.

The MetLife/AIG deal could value at around $15 billion, according to reports.