What is a left ovary dominant follicle?

What is a Left Ovary Dominant Follicle?

A left ovary dominant follicle is the single largest follicle (typically ≥10 mm) that develops on the left ovary during a menstrual cycle and is destined to ovulate if exposed to the LH surge, while suppressing the growth of other follicles through hormonal mechanisms.

Physiologic Definition and Selection Process

The dominant follicle represents the one follicle that escapes atresia (the fate of all other follicles) and, if exposed to the LH surge, will ovulate and subsequently differentiate into the corpus luteum 1. This selection process occurs around cycle day 6.3 ± 2.3 days, when the dominant follicle reaches a mean size of approximately 9.9 ± 3.0 mm 2.

Key Characteristics of Dominant Follicles

• Size threshold: The dominant follicle is typically defined as ≥10 mm in diameter, distinguishing it from smaller antral follicles 3
• Growth pattern: Once selected, the dominant follicle continues to grow while all other follicles in both ovaries remain <11 mm in diameter 2
• Hormonal dominance: The dominant follicle suppresses growth of nondominant follicles, particularly in the same ovary, through paracrine mechanisms during the late follicular and luteal phases 2

Clinical Significance of Laterality

The location of the dominant follicle (left versus right ovary) has both physiologic and clinical implications:

Ovulation Patterns

• Contralateral ovulation patterns (when the dominant follicle develops on the opposite ovary from the previous cycle) may be associated with improved pregnancy outcomes in IUI and IVF natural cycles 4
• Right-sided dominance appears more common in certain contexts, with evidence suggesting that contralateral right-sided ovulation (following left-sided ovulation in the preceding cycle) shows the best pregnancy outcomes 4

Ultrasound Assessment Criteria

When evaluating a dominant follicle on the left ovary using transvaginal ultrasound, the following parameters should be documented 3:

• Follicle measurement: Total follicle number per ovary measuring 2-9 mm
• Dominant follicle identification: Any follicle ≥10 mm should be reported as a dominant follicle
• Ovarian volume: Three dimensions and volume of the left ovary
• Exclusion criteria: Ensure no corpora lutea or cysts are present that could be confused with a dominant follicle 3

Clinical Applications in Fertility Treatment

IUI Cycle Management

In ovarian stimulation cycles for IUI, treatment should be withheld when more than two dominant follicles >15 mm are present to prevent multiple gestation pregnancies 3. This guideline applies regardless of whether the dominant follicles are on the left, right, or both ovaries.

• Monofollicular growth results in pregnancy rates of 8.4% 3
• Single dominant follicle on the left ovary represents the ideal scenario for natural conception or minimally stimulated IUI cycles 3
• Multiple pregnancy risk increases significantly with 2,3, or 4 dominant follicles (6%, 14%, and 10% respectively) 3

Important Clinical Caveats

• Developmental timeline: A dominant follicle requires at least 3 months from the primordial stage to reach the antral stage, which has implications for fertility preservation planning 5
• Wave dynamics: Multiple waves of antral follicles may develop during a single menstrual cycle, meaning a dominant follicle on the left ovary in one wave may be followed by a dominant follicle on the right ovary in a subsequent wave within the same cycle 6
• Ovarian reserve assessment: The presence of a single dominant follicle does not provide information about overall ovarian reserve, which requires assessment of total antral follicle count (2-9 mm follicles) and potentially AMH levels 3

Common Pitfalls to Avoid

• Do not confuse a corpus luteum or ovarian cyst with a dominant follicle; proper ultrasound technique with transducers including 8MHz frequency bandwidth is essential 3
• Do not assume that the absence of a dominant follicle on the left ovary indicates pathology, as normal ovulation alternates between ovaries in variable patterns 4, 6
• Do not use ultrasound for PCOS diagnosis in those with gynecological age <8 years, as multifollicular ovaries are common in this population 3