The first treatise on gynecology written by a Portuguese author, the work was written in two parts: Part one, about theory, was titled De natura mulierum (On female nature) and was divided into four books: (1) Anatomy of the uterus and the breasts; (2) Semen and menstruation; (3) Intercourse, conception, and pregnancy; (4) Childbirth and breastfeeding. Part two, titled De morbis mulierum (On female diseases) was more practical in nature, but was also divided into four books: (1) Diseases common to all women; (2) Diseases of widows and virgins; (3) Diseases related to generation and pregnancy; (4) Puerperal and wetnurses’ diseases.

In exploring issues in physiology and anatomy, embryology, conception, sex, pregnancy, abortion, infertility, childbirth, monstrous beings, etc de Castro evaluated classical and Arabic traditional thinking on these subjects in the writings of Hippocrates, Aristotle, Pliny, Soranus, Galen, Averroes, Avicenna, etc., He also established a multivocal dialogue between traditional ideas and new ideas, engaging with the work of more contemporary authors such as Du Laurens, Amato Lusitano, Mercado, Akakia, Paré, Rousset, Mercuriale and others. 

Digital facsimile of the 1604 edition from Google Books at this link.