Follicle Size

Reviewed by Ashley Wong, MS, Clinical EmbryologistCheckmark
Published: February 21, 2017

What Does Follicle Size Mean?

Follicle size is the diameter of ovarian follicles as measured by transvaginal ultrasound, typically reported in millimeters (mm).

Follicle size is used along with estrogen levels to determine the best timing for ovulation (for intrauterine insemination) or an egg retrieval procedure (for in vitro fertilization).

FertilitySmarts Explains Follicle Size

Follicles are fluid-filled sacs located in the ovaries that contain a maturing egg. The egg is released during the process of ovulation.

With fertility treatments, ovarian stimulation is often used to produce multiple follicles at once. Follicles do not start at the same size or grow at the same speed.

Follicle size typically correlates with follicular maturity. An egg needs to be mature in order to be fertilized. Smaller follicles are less likely to contain mature eggs and those that are mature but come from smaller follicles may have lower implantation potential. It is also thought that follicles larger than 22mm often contain post-mature eggs that are also less likely to result in a pregnancy.

Individual doctors may have different optimal follicle sizes that, depending on the medication used, can typically be 18 to 20 mm. It is possible that follicles ranging between 16mm and 22 mm may have the potential of containing a mature egg.

Along with follicle size, serum estradiol level is taken into consideration. Once target size is achieved, an injection of hCG also known as a trigger shot is used to stimulate the final phase of egg maturity and either ovulation or timing for egg retrieval (36 hours after the injection).

Follicle size and endometrial thickness are related to the success of medicated IUI cycles.

Synonyms

Follicular Size

Folicle size

Follical size

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