The episode opens with Julianne Moore's character giving birth. Moore was actually 62 when she filmed the scene.
Mary Villiers is actually quite a minor character in Benjamin Woolley's book, "The King's Assassin", upon which the series is based.
Although King James I was married and his wife bore him seven live children (as well as several stillbirths), he was generally renowned for preferring the company of male courtiers. There is some speculation as to whether these relationships were sexual - sodomy was illegal in England at the time. George Villiers was generally thought to be his favorite.