I wonder how the production might handle the situation if, mind you "IF", Whittaker were to become pregnant during her run? With Lucille Ball, they simply wrote it into the show. Doctor Who, being science fantasy, could get a bit more creative. I mean, if Chibnal and staff wanted to get strange, they could write it as the Doctor getting "infected" with a parasitical lifeform, one slowly growing over the course of the series. On the other hand, a real life pregnancy could be "ignored" with careful editing, cinematography, wardrobe and in some case, a bit of digital "jiggery pokery". Ironically, the arguably most daring choice would be to treat it as the condition it is, a "conventional" pregnancy (whatever might be considered "conventional" for Gallifreyans).