From the Guidelines
A mature egg is typically found in a follicle that measures between 18-25 millimeters in diameter, with the optimal size being around 18mm when the dominant follicle(s) reaches a mean diameter, as indicated by the use of HCG injections to trigger ovulation in IUI-OS cycles 1. When a follicle reaches this size during the ovulation process, it usually contains an egg that has completed its maturation and is ready for ovulation and potential fertilization.
Key Points to Consider
- During a normal menstrual cycle, several follicles begin to develop, but typically only one becomes dominant and grows to this mature size before releasing its egg.
- In fertility treatments, doctors monitor follicle growth through ultrasound to determine the optimal timing for procedures like egg retrieval in IVF.
- The size is important because follicles that are too small (under 18mm) often contain immature eggs with lower fertilization potential, while those that grow too large (over 25mm) may contain post-mature eggs with decreased quality.
- The growth of follicles to this mature size is driven by follicle-stimulating hormone (FSH) and other hormones that regulate the complex process of egg development and ovulation.
Clinical Implications
- The optimal timing of IUI is crucial, and the method of triggering in IUI stimulated with gonadotrophins can be determined by the provider, with single IUI performed any time between 24 and 40 hours after HCG injection without compromising pregnancy rates 1.
- In IUI in natural cycles, IUI should be performed 1 day after LH rise, highlighting the importance of precise timing in achieving successful outcomes.
From the Research
Follicle Size and Mature Egg
- The size of the follicle is an important factor in determining the maturity of the egg, with follicles between 12-19 mm in diameter being most likely to yield a mature oocyte 2.
- A study found that follicles in the size range of 19-24.5 mm in diameter were associated with significantly greater than average yields of good-quality blastocysts 3.
- Another study found that the incidence of MII oocytes was significantly associated with larger follicle size, and that the development of quality blastocysts was also associated with larger follicles 4.
- However, the same study found that the incidence of euploidy among biopsied blastocysts was not associated with follicle size 4.
Optimal Follicle Size
- The optimal follicle size for yielding a mature oocyte is between 12-19 mm in diameter, with follicles in this size range contributing the most to the number of oocytes and mature oocytes retrieved 2.
- Follicles larger than 19 mm in diameter may also yield mature oocytes, but the yield of good-quality blastocysts may decrease slightly for follicles larger than 24.5 mm in diameter 3.
- It is worth noting that the relationship between follicle size and oocyte maturity is not entirely dependent on the size of the follicle, and other factors such as the quality of the oocyte and the fertilization process may also play a role 4.
Follicle Size and Embryo Quality
- The size of the follicle is associated with the quality of the embryo, with follicles in the size range of 19-24.5 mm in diameter being associated with significantly greater than average yields of good-quality blastocysts 3.
- However, the incidence of euploidy among biopsied blastocysts is not associated with follicle size, suggesting that other factors such as the quality of the oocyte and the fertilization process may be more important in determining embryo quality 4.