The Müllerian (paramesonephric) ducts develop at 5 to 6 weeks gestational age in conjunction with the Wolffian (mesonephric) ducts. In the absence of "testis-determining factor" of the Y chromosome, the Wolffian ducts degenerate. At 7 to 9 weeks gestational age, the Müllerian ducts fuse at the midline to form the uterovaginal canal. The proximal segments of the uterovaginal canal are derived from coelomic epithelium; these remain unfused and give rise to the fallopian tubes. At approximately 8 weeks gestational age, the uterovaginal canal extends inferiorly to reach the urogenital sinus (vertical fusion). At the same time, the vaginal plate develops in the distal portion of the uterovaginal canal. The vaginal plate subsequently undergoes canalization. Approximately the upper two-thirds of the vagina derive from the Müllerian ducts and the lower one-third develops from the urogenital sinus. The uterus has assumed its mature configuration by 12 weeks gestational age.
In the neonate, the uterus is relatively large due to the influence of maternal and placental hormones. The normal neonatal uterus is approximately 3.5 cm long and 1.4 cm thick, and the fundus-to-cervix ratio (diameter of the fundus divided by the diameter of the cervix) is 1:2. There is shrinkage to prepubertal morphology over the next few months, assuming a tubular configuration (fundus-to-cervix ratio = 1.0), 2.5 to 4 cm in length and less than 1 cm in thickness (Figures 53-1 and 53-2). A uterine length of greater than 4.5 cm and thickness greater than 1 cm in a prepubertal girl suggest pathological enlargement. At the time of puberty, the uterus enlarges to an adult size and configuration, with a fundus-to-cervix ratio of 2:1 to 3:1. The normal postpubertal uterus is 5 to 8 cm long and 1.6 to 3.0 cm wide.1–5
Normal prepubertal uterus.
This 8-year-old girl was evaluated for clitoromegaly. A longitudinal sonographic image shows an age-appropriate configuration of the uterus. The dimensions are 3.7 × 0.9 cm.
Normal prepubertal uterus.
A. A sagittal T2-weighted image of an 11-month-old girl shows the normal tubular morphology of the uterus (arrows). There is a thin endometrial stripe central to the lower signal myometrium. B. The uterus (arrow) appears as an oval structure between the bladder and rectum on this axial image.
The endometrial stripe is often visible on sonography and MRI in the neonatal uterus, due to maternal stimulation (Figure 53-3). It is otherwise absent on sonographic imaging of the normal prepubertal uterus; a thin stripe sometimes remains visible on MRI of these older children. At puberty, the normal adult zonal pattern of the endometrium becomes evident on sonography and MRI. On T2-weighted MR, the postpubertal uterus has 3 zones: the intermediate-to-high-signal-intensity outer myometrium, the low-signal-intensity ...