By 5 weeks' gestational age, the wolffian (ie, mesonephric) and the müllerian (ie, paramesonephric) ducts have formed from intermediate mesoderm. In the absence of testosterone and müllerian inhibitory substance, the mesonephric ducts regress and the paramesonephric ducts continue to form the female reproductive structures with fusion of the distal portions of the paramesonephric ducts to give rise to the uterine fundus, the cervix, and the upper vagina. These developmental changes are genetically controlled in large part by a series of complex transcriptional signaling pathways including Wnt signaling, Hox genes, and many others. In a female fetus, the wolffian duct disappears except for nonfunctional vestiges. The müllerian duct is lined by a columnar epithelium. This includes the entire cervix and upper vagina to the vaginal plate (ie, sinovaginal bulb). Through a process of squamous metaplasia, the vagina and a variable portion of the ectocervix become covered with squamous epithelium. This process is complete by the fifth month of pregnancy.