Antral Follicle

Definition - What does Antral Follicle mean?

An antral follicle is a fluid-filled sac containing an immature egg that is developing within a woman's ovaries. It is called an antral follicle because at this stage the follicle contains a fluid-filled cavity called an antrum. Antral follicles are larger than follicles at earlier stages of development, so they can be visualized on an ultrasound. Although not all antral follicles will continue to mature and be released during ovulation, the number of antral follicles a woman has in her ovaries is a good indication of how many eggs she has left. A woman with a low antral follicle count may struggle with infertility.

Antral follicles are also called resting follicles.

FertilitySmarts explains Antral Follicle

A girl is born with all the eggs that will ever go on to mature and be released through ovulation in her lifetime. Over time, most of the eggs die while a small number of others will mature and be ovulated. This process continues until menopause when there are no eggs remaining in the ovaries.

The eggs within a woman or girl's ovaries are contained in fluid-filled sacs, called follicles. During each menstrual cycle, some of the follicles will develop and one will mature to the point of being ovulated. At the beginning of their development, the follicles are called primordial follicles. Primordial follicles are too small to be seen by the naked eye, so they are not counted in fertility testing. If the follicles survive and continue to mature, they will grow larger in size. Some of the follicles will reach the antral stage. At this stage, they are between 2 and 10 mm in size, which is large enough to be seen on an ultrasound.

The number of antral follicles within a woman's ovaries relates to the number of eggs remaining. For this reason, fertility testing often includes an antral follicle count (AFC). This test involves counting the number of antral follicles seen on an ultrasound and is a good indicator of a woman's ovarian reserve.

Antral follicle counts will normally lower as a woman ages. However, if a woman has a very low antral follicle count before the age of 40 she may be diagnosed with premature ovarian failure (POF). An abnormally high antral follicle count may be a symptom of polycysitc ovary syndrome (PCOS).

In vitro fertilization (IVF) involves stimulating a woman's ovaries to mature more follicles so that a number of eggs can be retrieved from them. A woman with a high antral follicle count usually responds well to the ovarian stimulating medications and produces many mature follicles. This allows more eggs to be retrieved and increases the chances of a successful pregnancy. A woman with a low antral follicle count may not respond as well to the drugs and fewer mature follicles will be produced. If not enough eggs can be retrieved in this process, the IVF cycle may be canceled, as there is a low chance of conception with the woman's eggs. In such cases, the use of a donor egg may be offered as an option.

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