Chrissy was interacting with her fans on Twitter Friday when she explained before she had the surgery, they administered a pregnancy test ... which came back negative.

Here's what she said after the surgery ... "A few weeks after surgery, I took a test. And for many years now, I've taken pregnancy tests almost every month, praying to see a positive one day. Just wishful thinking. I never had a positive before."

She says the test she took on the day John Legend's album came out was when she got a positive test result, adding, "I was scared s**tless. Was pretty positive you shouldn't get your boobs out while pregnant? Pretty sure."

She explains how scary it was ... "So we prayed to the boob surgery gods that everything would be okay. Went to every appointment terrified. Even without the surgery, I didn't think I could get pregnant naturally anyway. So the odds just felt ... bad."

Chrissy broke the news she was pregnant with their third child in John's music video, "Wild."

They have 2 kids ... 4-year-old Luna and 2-year-old Miles.