Under the title "happy personal news", Mayer said she was expecting twin girls in December 2015.
She wrote: "With great happiness, Zack and I have some exciting news to share – I’m pregnant! In fact, I’m expecting identical twin girls, likely arriving in December.
"The twins part was quite a surprise, because I have no family history of twins or any other predisposing factors. However, as I’ve now learned, identical twins occur by random chance in roughly 1 out of approximately every 300 pregnancies. Zack and I have embraced the surprise and are very excited about these new additions to our family."
However Mayer - who controversially scrapped working from home when she started as boss of Yahoo - won't be taking much time off.
"Since my pregnancy has been healthy and uncomplicated and since this is a unique time in Yahoo’s transformation, I plan to approach the pregnancy and delivery as I did with my son three years ago, taking limited time away and working throughout.
"I’ve shared the news and my plans with Yahoo’s Board of Directors and my executive team, and they are incredibly supportive and happy for me. I want to thank them for all of their encouragement as well as their offers of help and continued support."
Mayer already has a son Macallister, nearly three.
The blog finishes with a comment from Yahoo staff, which reads: "We are extremely happy for her and supportive of her plans and approach."
Mayer's first pregnancy was announced on her first day at Yahoo in July 2012. What are your thoughts around the time she's taking off around her pregnancy? Any advice from mums with twins?