Introduction

Investigation of the pediatric female genital tract has been revolutionized since the introduction of ultrasound, which is now the primary imaging modality in use today. Ultrasound is an ideal alternative to examination under anesthesia (EUA) and laparoscopy, with no radiation or anesthetic risk. Magnetic resonance imaging is gaining in importance, particularly for pelvic masses and complex genital abnormalities, but still the initial assessment lies with ultrasound.

Abnormalities of the genital tract are often accompanied by renal (in about 20%) and distal intestinal abnormalities, and these systems should always be examined as part of the ultrasound examination, particularly in neonates.

Successful sonographic examination and evaluation of anatomy and disorders of sexual development in girls, requires an accurate knowledge and assessment of both uterine and ovarian size and morphology as they grow and develop throughout childhood. It is essential that the sonographer is able to assess the normal appearance, because disorders related to premature or late pubertal development form the bulk of the workload related to the female genital tract in childhood. In the young infant with conditions such as ambiguous genitalia or intersex, ultrasound has an important and useful role, so an understanding of what to look for in these disorders is also very important.^1,2

Development of the genital ducts

Both male and female embryos have two pairs of genital ducts. The mesonephric (Wolffian) ducts from the intermediate kidney play an important part in the development of the male reproductive system. The paramesonephric (Müllerian) ducts are those that develop lateral to the gonads and mesonephric duct system and have a leading role in the development of the female.

In male embryos (i.e. those with a 46XY chromosome complement) by 8 weeks, the fetal testes are producing two hormones which have a profound effect on the ultimate sexual and reproductive organs the fetus will have, i.e. its gonadal sex. Testosterone from the fetal Leydig cells stimulates the mesonephric ducts to form male genital ducts which then go on to develop into the epididymis, vas deferens and seminal vesicles. Müllerian inhibiting substance (MIS) also produced by testicular Sertoli cells causes the paramesonephric ducts to disappear, inhibiting the development of the female structures and causing their regression. If MIS is not present these female structures develop passively. So it is the presence of fully functioning testes that determines our reproductive organs or sexual development, in that testes are essential for male development whereas female development does not depend on the presence of ovaries.

In female embryos (i.e. those with a 46XX chromosome complement) the mesonephric ducts regress as there is no testosterone, and the paramesonephric ducts develop as there is no MIS.

The paramesonephric ducts develop into the fallopian tubes, uterus, cervix and upper vagina. The lower vagina develops separately from the urogenital sinus (Fig. 7.1).

Figure 7.1 Normal development of the uterus. (A) Line diagram showing the development of the female genital tract from the paramesonephric duct or Müllerian duct system. The fallopian tubes and uterine body develop from the paired paramesonephric ducts. The vagina develops from the urogenital sinus. (B) Schematic diagram showing the development of a female genital tract and the origin of the fallopian tubes and uterus from the paramesonephric duct system and the vagina from the urogenital sinus.

In a 46XY (i.e. male) fetus with a female phenotype or appearance, there would have been no anti-Müllerian hormone active during intrauterine life, so that the female structures were able to develop passively. If the infant is 46XY and looks like a female but has no uterus (or an abnormal uterus) then there has been an abnormality of the androgen or male testosterone receptors so that the male ducts have not developed. This is otherwise known as testicular feminization.

Development of the ovary

The primary sex organ (gonad) in the female is the ovary. It contains germ cells derived from the yolk sac and somatic cells derived from coelomic epithelium, mesenchyme and mesonephros.

In early fetal development the female germ cells, while migrating to the genital ridge from the yolk sac, undergo a series of mitotic divisions and then differentiate into oogonia. By 12 weeks of development, a proportion of the several million oogonia in the genital ridges enter the first phase of meiosis and then almost immediately become dormant, not completing the first meiosis until the onset of ovulation many years later. Before birth the offspring of the oogonias, with full chromosome complements, enlarge to become primary oocytes. The oocyte with a capsule of epithelium form the primordial follicle. The oocytes which are not incorporated into follicles undergo regression. By 5 months of gestation the number of primordial follicles peaks at about 7 million. Most of these follicles subsequently regress and degenerate before the follicle reaches any appreciable size. By birth only 1 million remain and by menarche less than half that number.

Growth of the ovary

In infancy and childhood follicles are present in the ovary in all stages of development but generally in the earlier stages of development than in later life.

The primordial follicles surrounded by their layer of flat cells start to grow. The flat cells become more spherical and are known as granulosa cells. As the coat becomes multilayered, an antrum appears and the Graafian follicle develops. The granulosa cells differentiate into two layers: the theca interna and theca externa.

The complete unruptured Graafian follicle may regress without expelling the ovum, first by the oocyte dying and then undergoing necrosis with eventual formation of the corpus restiforme; this vanishes without trace. In fetal and postnatal life this generally occurs before the follicle has reached an appreciable size. If the follicle expels the ovum (ovulation) then either a fully developed corpus luteum will result, or the remaining follicle will become luteinized and regress without the development of a corpus luteum. The distinguishing feature of the post-pubertal follicle is the ability of the follicle to liberate its ovum (ovulation) and be converted into a corpus luteum.³

The ovary is poorly vascularized in the infant and young child, but throughout childhood this vascularization slowly increases and by 6 to 8 years of age there is an identifiable medulla and cortex in the ovary. By 11 to 12 years of age primordial follicles are evenly distributed throughout the layers of the cortex. By 12 to 13 years ovulation begins and, from then on, non-primordial ova containing follicles are present, with a gradual reduction of the primordial follicles until none remains in the mature ovary. After 14 years of age the ovary is essentially mature in the majority of females.

Endocrinology

Ovarian activity involves the production of gametes and hormones which determine sexual development and reproductive function. These two functions are closely related and under the control of the hypothalamic–pituitary axis. The hypothalamic–pituitary unit which is responsible for pubertal development is developed and potentially functional by mid-gestation in the fetus. During late gestation the placenta produces large quantities of sex hormones (i.e. estrogens and progesterone) which may inhibit fetal gonadotropin secretion. At birth and with maternal estrogen withdrawal, there is a rise in gonadotropin secretion which occurs at pubertal level for several months.

Gonadotropins fall to low levels in early childhood but start to rise again by 6 years of age. Concurrently, more mature antral follicles develop in the ovaries, producing a subsequent rise in estrogen levels.

In prepuberty, gonadotropin secretion can be measured but occurs in irregular bursts not linked to the night time. At about 7 to 8 years of age, pulsatile gonadotropin secretion becomes steadily linked to the night time and appears to be controlled primarily by the central nervous system. Throughout puberty the amplitude of gonadotropin pulses and gonadal sex steroid secretion gradually increases until late puberty, when adult levels are reached throughout the day. Although nocturnal pulsatility alone may permit menarche, usually anovulatory cycles, the development of diurnal and nocturnal high amplitude pulses is required for ovulatory cycles. Puberty ends with the final stage of ovarian maturation which is the establishment of regular ovulatory cycles. This reflects the maturation of the entire hypothalamic–pituitary–gonadal axis.⁴

Ovarian volume

Studies have shown the ovarian volume to be 0.8 cm³ in the first 3 months of life, increasing in volume from a mean of 1 cm³ at 2 years of age to 2 cm³ at 12 years of age. Normal standards for ovarian volume in childhood and puberty are available.⁶ The volume of the ovary is affected by the presence of large cysts, either primordial or ovulatory follicles, or a corpus luteal cyst, and cannot be reliably calculated when these structures are present.

There appear to be two particular periods of increased growth rate of the ovary. The first occurs at approximately 8 years of age, coinciding with the rise in androgen secretion from the adrenal cortex at adrenarche. The second growth spurt occurs immediately before and during puberty (Fig. 7.2).^6–13

Figure 7.2 Ovarian size. Graph showing the growth of the ovary throughout childhood. There are two periods of acceleration: the first at adrenarche and the second around puberty.⁶

Follicles

In the neonate the size of the ovary and the number of visible follicles is variable although the follicles are often surprisingly numerous. This follicular activity in the infant is related to the maternal hormone influence. Occasionally a follicle may grow large so as to precipitate torsion of the ovary.

Throughout childhood the ovarian follicles increase in size and number. The size of the follicles varies and they may become atretic at any stage of their development. Prepubertally, the ovaries may appear quite active, with follicles of up to 9 mm in size. With the onset of high luteinizing hormone pulses at night, at about 8 to 9 years of age, the ovaries become ‘multicystic’ or multifollicular, defined on ultrasound appearances as more than six follicles of 4 mm diameter. This is a normal phase of development and is the herald of puberty.¹⁴

This coincides with breast development of Tanner stage 2. The follicles continue to increase in size with the progression through puberty. With the constant maturation and regression of the follicles there is a wide range of diameters seen at different pubertal stages. Also, lower levels of gonadotropin in prepubertal girls can result in larger follicles of up to 12 mm. This limits the value of follicular diameter in assessing pubertal status.

Menarche

The principal regulators of ovarian function are luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In response to rising levels of FSH, between 5 and 12 primordial follicles commence to enlarge and are now called primary follicles. The ovarian follicles gradually enlarge until a follicle of at least 16 mm is attained. There is follicular secretion of estrogen, which results in the development of an endometrium. Without ovulating (an anovulatory cycle), the follicle regresses, and the subsequent fall in estrogen results in a withdrawal bleed. Eventually, when a follicle diameter of over 20 mm is achieved, that follicle gains primacy, while the remainder of the other follicles recruited during the cycle degenerate. The remaining follicle is now known as a mature or dominant Graafian follicle. After ovulation the granulosa cells of the ruptured follicle wall begin to proliferate and give rise to the corpus luteum, which is an endocrine structure that secretes steroid hormones that maintain the uterine endometrium in readiness to receive an embryo. If no embryo implants in the uterus, the corpus luteum degenerates after about 14 days (Fig. 7.3).

Figure 7.3 Normal ovarian appearance at different ages. (A) Oblique sonogram of the ovary in a neonate (between calipers). The ovary in a normal neonate can be very follicular and this, as in the uterus, is related to maternal hormone stimulation. (B) Oblique sonogram of the ovary (between calipers) in early pubertal development. The ovary appears very follicular. This multifollicular appearance is a normal stage of development and is the herald of puberty. (C) Oblique sonogram of the right ovary showing a dominant follicle. This is the last follicular change of the ovary as menarche begins. With the development of the dominant follicle, other follicular activity is suppressed.