She also explained that she and her husband like two baby names but they haven't chosen one yet.
In response to another question she said her sons are "mega excited," and "it's so lovely that they understand".
Another follower asked whether she was suffering from morning sickness and Pippa responded: "Not in the mornings but every evening from maybe 4/5pm I felt like I'd been hit by a bus. It just stopped at 12 weeks.”
Asked whether this pregnancy was very different from her other two, she said: "I nearly forget how the others felt. I was 28 having Ollie and feel like I breezed through it without even thinking much.
"I had gestational diabetes on that pregnancy at the very end but I managed it with diet.
"On Louis, I had a giant cyst that needed to be removed. I had keyhole surgery at 11 weeks. That was scary but I recovered okay after it.”