But leave it to Ellen DeGeneres to get the details.
"I don't know, apparently protection was just thrown out the window!" Simpson said in an interview airing tomorrow on Ellen, when asked just how her surprise pregnancy came about.
"We were definitely extremely shocked," the Fashion Star judge added.
Simpson said that she and Johnson are also quite happy, although they certainly already had plenty to do with daughter Maxwell at home.
"Maxwell was our focus, and then I was puking and lying on the couch," Simpson said, noting that her second pregnant has been the "complete opposite" of her first.
"Like with Maxwell, I felt amazing," the pregnant star said. "Like, I could do everything, eat everything. Do whatever I wanted. I had a lot of energy. This time around...I'm like, exhausted, eating Tums. That's my snack of choice!"
Speaking of snacks, Simpson reiterated her intention not to gain too much weight this time around.
"This time around I think because I was doing Weight Watchers and I felt super healthy, I'm not really craving as much and I know how hard it is [to take the weight off] so I'm trying to make healthier, wiser decisions," she told DeGeneres.
But, Simpson added happily, Johnson obviously had no issue with her appearance at any stage of the game.
"Apparently Eric still wanted me so...I can gain as much weight and I will have a husband-to-be that loves me!"
Cheers to that!