Medical diagnostics company LipoScience’s (NASDAQ:LPDX) stock has fallen since its January initial public offering and the company has an idea why.

Shareholders who are not subject to the post-IPO lockup – the period after an IPO during which certain inside shareholders are prohibited from selling their shares – have been selling shares. An executive for Raleigh-based LipoScience communicated that belief with three securities analysts after the close of trading Friday, the company said in a securities filing.

In the filing, LipoScience said that the company believes that some of the stockholders who held preferred stock had begun to sell their shares. LipoScience’s in-house attorneys had issued 12 opinion letters to the company’s transfer agents that authorized removing restrictions on the shares in order to release them for potential resale. An estimated  80,000 to 100,000 shares so far have been removed from these restrictions. Another 60,000 shares will be removed from these trading restrictions this week.

“The Company understands that the individual stockholder holding these shares has pledged these shares to an institution that is likely to sell them quickly,” LipoScience said in the filing. “ The Company believes that these sales by long-time stockholders who were not subject to lockup restrictions have put pressure on the price of its stock, particularly given its low trading volume.”

LipoScience has developed and commercialized a diagnostic test and device that assesses a person’s cardiovascular disease risks. The company’s technology was originally developed at North Carolina State University.

LipoScience went public in January at $9 a share. It’s stock traded as high as $11.33 on Feb. 11 but the stock price has fallen since. LipoScience closed at $9.60 on March 15.