What Are Follicles? Number, Growth And Other Characteristics

To maintain a healthy reproductive cycle, it's important that every woman is aware about the processes involved in the biological cycle. To start with, it is crucial to know what exactly follicles are. One of the most common confusions among women is that follicles are eggs. This belief is entirely incorrect. Read on to know what exactly follicles are and what role it plays in ovulation.

What Are Follicles?

A follicle is a fluid-filled sac which contains the oocyte when matured forms the egg. The follicle is also functional in containing other types of cells that produce oestrogen. Oestrogen (or estrogen) is a hormone that is required for normal development of a matured egg.

what is normal follicle size for ovulation
  • The Ovarian Cycle
  • Number And Characteristics
  • Time Of Growth And Size Of Follicle During Ovulation
  • Follicular Monitoring

The Ovarian Cycle

The cycle comprises phases namely follicular phase, ovulation and corpus luteum (luteal phase). The first weeks of a female embryo's life is when the endowment of eggs is determined. From the first menstruation onwards, an egg is selected in each cycle. The first selection is that of higher quality eggs. As time passes, the successive cycles will observe eggs that have deteriorated in quality. This being one of the prime reasons why older women find it difficult to conceive. This also plays a role in affecting the miscarriage rate.

Number And Characteristics

When female fertility is studied, the main concept that forms the basis is understanding the ovarian reserve. Knowledge of ovarian reserve is attained through hormonal analysis for assessment (hormone AMH, LH, FSH, estradoil, etc) and vaginal ultrasound. Ovarian reserve is expressed in terms of the number of follicles per ovary noted in the first days of a menstrual cycle (i.e., 2nd to 5th day). This is done by performing a vaginal ultrasound. These follicles are referred to as 'antral'. Ultrasound helps the observer to note the evolution and growth of the follicles both in a spontaneous cycle as well as in the case of a stimulated cycle when fertility treatments are taken up.

The determination of whether a woman has adequate ovarian reserve is made depending on the number of antral follicles. If the count is 6 to 10 then it is considered that the woman has a normal ovarian reserve. If then count is less than 6, it indicates low ovarian reserve. If the count is greater than 12, it indicates high ovarian reserve. Follicular size during this phase is 2 to 10 mm. Women with low ovarian reserve respond very less to any form of fertility treatment. On the other hand, women with high ovarian reserve respond in an exaggerated manner.

Time Of Growth And Size Of Follicle During Ovulation

Under normal circumstances, when the natural cycle is in progress, a dominant follicle is selected from the antral follicles. The dominant is different from the rest in terms of its size and growth rate. The mature follicle then becomes ready to ovulate. The remaining ones disappear or die eventually. Just before ovulation happens, the average diameter of the dominant follicle would be about 22 to 24 mm. This factor is considered the one and only marker that can predict the occurrence of ovulation with utmost ease.

When hormonal treatments are in progress, it is considered a stimulated cycle. In this process almost all the antral follicles are made to grow. However, the growth rate differs for each of them. When a majority of them have reached a size of about 18 mm, a trigger is administered in the form of hCG hormone, which triggers the ovulation.

The fertility treatment procedure involves scheduling of egg collection about 36 hours after the administration of this hormone. The fertility treatment is done with the aim to collect the most mature eggs that can then be fertilized by sperm. Nevertheless, it is important to know the fact that the growth of a follicle does not necessarily always mean that there exist a mature egg.

Follicular Monitoring

It is a process that involves serial ultrasonic monitoring of the ovarian follicles. This helps in identifying the maturation status of the eggs. This procedure is useful for assessing the size of the follicle, which supports the growing egg. It also helps in determining the thickness of the uterine lining.

Women who seek fertility treatment or are under fertility medication require follicle monitoring so that assessment can be made as to how well she is responding to the treatment/medication. To enable the performing doctor to proceed safely in the fertility treatment, it is important to note the number of eggs that are to ovulate and also the level of estradiol. About 150 to 200 pg/ml of estradiol should be produced by each mature egg. A woman's response to a fertility treatment is better understood by using the available hormone levels along with an ultrasound examination.

The dose of the fertility medication can be adjusted depending on the woman's response to the treatment or in case too many eggs begin to mature. Estradiol levels should not be allowed to reach very high as this can put the woman at a risk of developing ovarian hyperstimulation syndrome.

The prime goal of follicle monitoring is to attain information as to how many mature eggs would ovulate and also to ensure that this is achieved safely. When the eggs are mature, the doctor would advise to take appropriate steps to conceive or to choose to go ahead with intrauterine insemination. Egg collection is also done at this stage if an In-Vitro Fertilization cycle is the chosen medium.

Follicular monitoring might require you to make about 3 to 4 visits to the doctor's clinic. The initial scan would be scheduled on day 2 of your menstrual cycle followed by scans on day 6 and day 9. The later scans are scheduled based on the results of the previous scans.

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