What is the significance of a simple dominant follicle on the ovary observed during a transvaginal ultrasound (TVUS)?

From the Guidelines

A simple dominant follicle on transvaginal ultrasound is a normal finding that represents a developing egg within the ovary during the follicular phase of the menstrual cycle, and no specific treatment is needed as this is a physiological finding. The dominant follicle typically grows to about 18-25mm before ovulation occurs, releasing the egg for potential fertilization. This structure can be identified on ultrasound as a round, fluid-filled (anechoic) structure within the ovary. During a normal menstrual cycle, several follicles begin developing, but usually only one becomes dominant and proceeds to ovulation, while the others undergo atresia (degeneration). The presence of a dominant follicle indicates normal ovarian function and suggests that ovulation is likely to occur or has recently occurred.

Key Points to Consider

• The diagnosis of polycystic ovarian morphology (PCOM) is based on the presence of an excessive number of follicles (≥20) per ovary, as defined by the Rotterdam classification, and can be assessed using transvaginal ultrasound with a transducer frequency >8 MHz 1.
• The identification of a dominant follicle on transvaginal ultrasound can help predict ovulation, which typically happens about 24-36 hours after the follicle reaches its maximum size 1.
• Transvaginal ultrasound (TVUS) is the preferred imaging modality for evaluating ovarian morphology and follicular development, as it provides high-resolution images of the ovaries and can accurately measure follicle size and number 1.

Clinical Implications

• The presence of a simple dominant follicle on transvaginal ultrasound does not necessarily indicate polycystic ovary syndrome (PCOS) or any other pathological condition, and should be interpreted in the context of the patient's clinical presentation and medical history.
• For women undergoing fertility treatments, monitoring follicular development using TVUS can help determine the optimal timing for procedures or medication administration 1.
• The use of anti-Müllerian hormone (AMH) as a surrogate marker for elevated follicle number per ovary (FNPO) can be useful in defining PCOM, especially in cases where FNPO is not available 1.

From the Research

Ovarian Simple Dominant Follicle on Transvaginal Ultrasound

• The appearance of a simple dominant follicle on transvaginal ultrasound can be an indicator of ovulation induction in women with polycystic ovarian syndrome (PCOS) 2, 3.
• Studies have shown that letrozole is a more effective ovulation induction agent than clomiphene citrate, with higher ovulation rates and pregnancy rates 2, 3.
• The size of the dominant follicle can also be an indicator of ovulation, with larger follicles (≥22 mm) associated with higher incidences of luteinized unruptured follicle syndrome (LUF) 4.
• Transvaginal ultrasound can be used to monitor follicular development and ovulation in women with PCOS, with the presence of multiple small follicles and increased ovarian volume being characteristic of the condition 5.
• However, the absence of these characteristic ultrasound features does not necessarily rule out a diagnosis of PCOS, as the condition can present with varying degrees of follicularity and ovarian volume 5.

Factors Affecting Ovarian Follicle Development

• The use of letrozole and clomiphene citrate can affect ovarian follicle development, with letrozole resulting in higher monofollicular development and lower rates of multiple pregnancy 2, 3.
• The size of the follicle can also affect the outcome of ovulation induction, with larger follicles associated with higher rates of LUF 4.
• Hormonal profiles, such as LH and FSH levels, can also impact ovarian follicle development and ovulation 6, 5.

Monitoring Ovarian Follicle Development

• Transvaginal ultrasound is a useful tool for monitoring ovarian follicle development and ovulation in women with PCOS 2, 3, 5.
• Regular ultrasound examinations can help identify the presence of a dominant follicle and monitor its growth and development 4.
• The use of ultrasound can also help identify potential complications, such as LUF, and guide treatment decisions 4.