The "30 Rock" star first revealed that she and her husband, Jeffrey Richmond, 50, were expecting during an appearance on Oprah Winfrey's show in April.
"My husband and I really decided that we felt rather than risk having '30 Rock' end in several years and feeling like part of our family is missing that we were going to prioritize our family," Fey explained.
In her new memoir, "Bossypants," Fey described feeling guilty and anxious about "the second baby issue," and worried that her 5-year-old daughter, Alice, needed a sibling.
"I get up to go to the bathroom and study myself in the mirror,” she writes in the best-selling book. “Do I look like someone who should be pregnant? I look good for 40, but I have the quaggy jawline and hollow cheeks of a mom, not a pregnant lady. It's now or never. This decision can't be delayed."
Concerned that, "science shows that fertility and movie offers drop off steeply for women after 40," she writes that family is still more important than a career at the end of the day.