The optimal follicular size for fertilization can vary depending on the specific treatment and individual patient factors. However, several studies have provided some guidance on the ideal follicular size for ovulation and fertilization:

• Intrauterine insemination (IUI) cycles with clomiphene citrate (CC) or letrozole: A retrospective study of 1,075 women undergoing IUI cycles found that the optimal size of the leading follicle before human chorionic gonadotropin (hCG) administration was in the 23 to 28 mm range for both CC and letrozole. The study also found that each additional millimeter of endometrial thickness increased the optimal follicular size by 0.5 mm.

• Oocyte retrieval: Studies have shown that follicles with a diameter of 16-24.5mm are the most likely to yield mature oocytes, which are capable of fertilization and best suited for development into high-quality embryos30198-0/fulltext). However, some studies have found that follicles of 12-19mm on the morning of trigger administration were most likely to yield a mature oocyte following hCG, GnRHa, or kisspeptin.

• Natural cycle: In a natural cycle, the dominant follicle that is selected for ovulation differs from the others by its size and rapid growth. Before ovulation occurs, the average diameter of the dominant follicle is 22 to 24 mm (range 18-36 mm) .

It is important to note that follicular size is just one factor in determining the optimal time for ovulation and fertilization. Other factors, such as endometrial thickness and hormone levels, also play a role. Additionally, individual patient factors, such as age and ovarian reserve, can impact the ideal follicular size for fertilization. Therefore, it is important to consult with a healthcare provider to determine the best course of treatment for each individual patient.