Yahoo CEO Marissa Mayer just announced she is pregnant, due to deliver identical twin girls in December.

The 40-year-old CEO has spent the past few years trying to turn Yahoo's fortunes around, making several changes and running the company with a firm hand. Now, Mayer's pregnancy announcement comes as Yahoo is gearing up to spin off its 15 percent stake in Chinese e-commerce titan Alibaba, which went public last year. The spinoff is expected to complete during the fourth quarter of 2015.

Mayer gave birth to her first child, a boy, back in 2012, and resumed her position at the company just two weeks after that. At the time, the CEO sparked plenty of interest for setting unrealistic expectations for new mothers, but Mayer calmed the waters soon after that, announcing a more attractive package of family leave benefits. More specifically, Mayer offered both mothers and fathers paid leave of up to eight weeks, as well as eight weeks of paid leave for mothers after pregnancy.

Now at her second pregnancy, Mayer doesn't seem eager to take advantage of those generous packages. The CEO still aims to take limited time away from work throughout and after the pregnancy, just as she did when she had her son, Macallister Bogue.

Mayer further says that both she and her husband, investor Zachary Bogue, were surprised at the news of having twins, as she had no history of twins in her family, nor other "predisposing factors." Nevertheless, the couple has "embraced the surprise" and is excited about the pregnancy and receiving great encouragement and support from Yahoo.

In a new post on her Tumblr, Mayer says she has already shared the news of her pregnancy with the Board of Directors and the executive team and all have been "incredibly supportive."

"Moving forward, there will be a lot to do for both my family and for Yahoo; both will require hard work and thoughtful prioritization," Mayer noted.