In women the lower urinary and genital tracts develop in close proximity. The gut is formed by an invagination of the yolk sac and the most caudal part (hindgut) develops a diverticulum, the allantois (Fig. 49.1a). That part of the hindgut connected to the allantois is the cloaca. At about 28 days after fertilization a mesenchymal wedge of tissue, the urorectal septum, starts to migrate caudally and divides the cloaca into a ventral part, the urogenital sinus and a dorsal part, which will become the anorectal canal (Fig. 49.1b). The two are eventually separated from one another when the septum fuses with the cloacal membrane some 10 days later.
At the same time the pronephros develops within the mesoderm but this undergoes early degeneration. The mesonephros initially forms a primitive kidney draining into the mesonephric duct on each side. The tubules undergo degeneration but the ducts remain and grow cau-dally to enter the anterior part of the cloaca on each side. This divides the urogenital sinus into two parts: the area lying between the mesonephric ducts and allantois is the vesicourethral canal and the area below the mesonephric ducts is the urogenital sinus (Fig. 49.1c). The ureteric bud develops as an outgrowth from the mesonephric duct by proliferation of cells. It grows towards the caudal end of the nephrogenic ridge and initiates the development of the metanephros (later to become the kidney) between 30 and 37 days after fertilization.
Dilatation of the cranial portion of the vesicourethral canal leads to the development of the bladder. The area of the bladder bounded by the ureteric orifices cranially and the termination of the mesonephric ducts caudally gives rise to the trigone. The caudal part of the vesicourethral canal narrows to form the upper urethra. The urogenital sinus gives rise to the distal part of the urethra and part of the vagina. These developments occur by 42 days after fertilization (Fig. 49.1d).
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.