In “Maggie’s Plan,” writer-director Rebecca Miller’s screwball comedy out Friday, a young woman named Maggie (Greta Gerwig) faces an unusual rom-com hurdle: her desire to start a family has worked out a little too well. After she recruits an acquaintance (Travis Fimmel) to knock her up, destiny, ever ironic, takes over: A college professor (Ethan Hawke) whose marriage is on the rocks falls in love with her—or rather, her enthusiasm for his unwritten novel. He leaves his kids and his wife (Julianne Moore), an icy intellectual whose academic reputation overshadows his own, to be with Maggie and her child. It’s all more than she bargained for, so Maggie sets about trying to put the home she wrecked back together. The film is a skewering portrait of New York intellectuals, but also, in its warmth and humor, a reminder that sometimes the best-laid plans go awry, wonderfully.