Sometimes the mothers I think of are fictional, like the one in “The Thornbirds.” She continually gets pregnant for the whole duration of this seven million page book, and each time, her eldest, an (Irish? Australian?) teenager, howls stuff at the husband like, “Ah! Can’t you keep your goat hooves off of her, you devil?” But even she works: she’s constantly beating dirty underthings with a rock or rolling out biscuit dough for two hundred around her swollen belly, in the sweltering Irish-Australian heat.