Well, either as you said, this is a fresh idea from Fujino, or there will be no pregnancy itself. There will be a ritual and then Yui will just appear "out of thin air". Asuna won't walk around with a belly and so on. In fact, that would be stupid. A ritual can simply create a child with the given parameters.
"Attention! Now, because you are pregnant, you cannot participate in battles, have sex, or perform active missions for a month."
Some people here assumed that Asuna would not be able to have sex in the game due to pregnancy. The idea is interesting, but still stupid. If Fujino makes a realistic pregnancy in the game, it will look like it's not a game at all. Or maybe it's not a game.
I remember reading a Chinese novel where people thought they were playing a virtual game, but in fact the virtual reality helmet moved their minds to another planet in an organic clone created specifically for the player. And all NPCs and enemies real and alive.