"According to the court chronicles, Jean d'Ailleboust began his labors as King Henri's physician in 1593, when he was already quite an old man; it is unlikely that he was born later than 1518.
In 1594, he was ordered to examine Gabrielle d'Estrées, the king's mistress, who was feeling unwell. When the king inquired what ailed her, Jean d'Ailleboust bluntly said that she was pregnant. The king was furious, to say nothing of Mlle d'Estrées, but the elderly physician did not budge; he even had the effrontery to predict, much to the king's displeasure, the exact day the royal bastard would be born.
Very near the day he had predicted, on June 7, 1594, Gabrielle d'Estrées delivered a healthy boy, the future César de Vendôme. Jean d'Ailleboust did not have long to enjoy the victorious outcome of his dispute with his royal master; he himself died under highly mysterious circumstances on July 24 of the same year. According to the chronicles of Sully and d'Estoile, he was poisoned by the spiteful Gabrielle d'Estrées. The king grieved the death of his honest old physician and regretted that he had spoken harshly to him before."
[The Two-Headed Boy, Jan Bondeson, 2000]