10 December 2022

What is a dominant follicle and how to prevent it

dominant follicle ovary
By the Editorial Comitee IVI Blog

Is there a dominant follicle ovary ? In the human reproductive system, the ovaries contain thousands of primordial undeveloped follicles. Each one of them has the potential to develop into an egg which through the ovulation, but very few do. The one, or occasionally two, which make it to maturity each month will have become the dominant follicle through their maturing process. So far, so normal and natural. However in some fertility treatments, particularly In Vitro Fertilisation (IVF), multiple mature eggs are desirable. This tendency for one follicle to dominate can then cause difficulties.

In this article, we look at how the system functions and how to prevent a dominant follicle in IVF. There are also considerations around which ovary has produced the dominant follicle for potential success of IVF. In this context we will examine recent work on what a dominant follicle in the left ovary means. Also, what a dominant follicle in the right ovary means, and the implications for fertility treatment.

What is a dominant follicle?

In order to appreciate the significance of the dominant follicle, we need to look at how the menstrual cycle works. Each complete cycle has two main phases, the follicular phase and the tertiary phase.

The follicular phase starts on day one of your period. This is the time when your body is shedding the build-up of a thick uterus lining that was there in anticipation of the arrival of a fertilised egg. When this doesn’t happen, the lining sheds and by the end of your period the lining is thin. Also during your period your ovaries are starting to prepare for the next ovulation. Your pituitary gland starts to produce Follicle Stimulating Hormone (FSH). This hormone triggers around five or six follicles to begin to mature.

As the follicles grow, they start producing oestrogen and as their size increases, they produce more and more. The presence of oestrogen signals the pituitary gland to slow down the production of FSH. One or occasionally two of the maturing follicles will grow slightly more rapidly than the others. As the supply of FSH decreases, the smaller ones of the group will slow in their development. In the meanwhile, the largest one continues to develop. This becomes the dominant follicle and eventually the others disintegrate and the body itself reabsorbs them.

When the dominant follicle reaches its maximum size, it produces even more oestrogen. This then triggers the endocrine system to release a hormone known as Luteinizing Hormone (LH) which propels the follicle into its final stage of development to become mature. At this stage it bursts open to release an egg, which is the moment of ovulation.

In the second, luteal, phase the follicle that released the egg turns into a corpus luteum which starts to produce both oestrogen and progesterone. The latter is crucial in preparing the uterus lining for the reception of a fertilised egg. During the second phase, progesterone both increases the thickness of the womb lining. It prevents it from breaking down. It also suppresses the production of the hormones that stimulate the ovaries to ovulate. This thereby ensures that if you did conceive, you would not conceive for a second time during your pregnancy.

Why is a dominant follicle a potential problem for IVF?

For people undergoing assisted fertility treatments, this tendency of the ovaries to produce a single dominant follicle can cause headaches. For example, unlike in Artificial Insemination (AI) in which only one or two mature eggs are optimal, the process of IVF requires the production of multiple mature eggs so that these can be fertilised in the laboratory. It’s important to remember that not every mature follicle contains a viable egg. Also, not all embryos resulting from fertilisation in the laboratory are healthy enough to be transferred back to the uterus. Therefore the more eggs, within reason, the better.

How to prevent a dominant follicle in IVF

Experts in IVF have developed techniques to circumvent this potential problem. During the ovulation stimulation phase of the treatment, the administration of hypophysis (that is, the pituitary gland) inhibitors, such as antagonists GnRH and AMP prevents the rise in LH, thus inhibiting the premature development of a dominant follicle, or indeed of any of the follicles undergoing development. This allows all of them to continue in their progress towards maturity.  The maximum possible number of oocytes can then be retrieved at the appropriate time for fertilisation in the laboratory and continuation of the IVF process.

What is the difference between left and right ovaries?

It would be a natural assumption that the ovaries ‘take it in turns’ to be the one that produces a mature egg each month and that eggs from either ovary stand exactly the same chance of being fertilised and leading to a successful pregnancy. However, researchers have found that this is not necessarily the case. A study published by the European Society of Human Reproduction and Embryology showed that ovulation occurs more frequently from the right-side ovary and that a dominant follicle in the right ovary has a significantly higher chance of resulting in pregnancy. The corollary, that a dominant follicle in the left ovary means there is a significantly lower chance of conception, is also true.

This appears to be the case regardless of whether the pregnancy occurred naturally or resulted from assisted fertility treatment. The study found that for women with fertility problems, ovulation from the right ovary occurred between 55% and 60% of the total. They also found that, for women undergoing IVF, the proportion of pregnancies deriving from right-sided ovulation was 64.7%.

As yet, the reasons for this disparity, and the existence of what could be described as a dominant ovary, the right one, are not clear. Obviously this is a fascinating area for further investigation.

Contacting us at IVI

Remember that in the majority of IVF treatments, the production of multiple eggs for fertilisation is achieved through carefully monitored ovarian stimulation. The technique of removing a dominant follicle is a ‘last resort’ intervention only where several cycles have been unsuccessful.

If you’d like to take to us about this or any other fertility concerns, do get in touch using our online contact form.

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