Decoding IVF: A Day-by-Day Breakdown of Follicle Size and What It Means for Your Fertility Journey

In vitro fertilization (IVF) can be a very efficient and effective treatment option for couples struggling with infertility and can allow for embryo banking as a way to preserve one’s fertility for the future. One of the most critical aspects of the IVF process is the stimulation of growth and development of ovarian follicles, which ultimately determine the quantity of mature eggs retrieved for fertilization. Follicles are fluid filled cysts that each contain a single egg- these bubbles of fluid can be visualized and measured via ultrasound. Each cycle, a different crop – a limited subset of the total number of eggs you have- get recruited to the surface of the ovary and become visible as baseline, or ‘antral’, follicles. As the bubble of fluid expands and the follicle diameter increases, the egg inside matures.

Monitoring follicle size during ovarian stimulation is essential to gauge response to treatment and to optimize timing of the egg retrieval procedure to maximize the number of mature eggs yielded. In this blog post, we will explore the significance of follicle size by day during an IVF cycle and how it impacts your fertility journey.

Understanding Follicle Growth in IVF:

During the IVF process, the ovaries are stimulated using fertility medications to encourage the growth of multiple follicles containing eggs. The aim is to achieve a balance – where the majority of eggs available are pushed to reach the mature stage of development (M2; metaphase II) that is capable of being fertilized by sperm – and they are not retrieved too early (immature eggs) or too late (post-mature, degenerated eggs). We also want to be mindful of minimizing risks associated with stimulating the ovaries too aggressively and for too long, such as ovarian hyperstimulation syndrome (OHSS).

Regular ultrasound examinations are used to monitor follicle size, providing valuable information about egg maturity and the appropriate timing for egg retrieval. By closely tracking follicle development, fertility specialists can fine-tune treatment protocols, increasing the likelihood of a successful IVF cycle.

A Day-by-Day Guide to Follicle Size:

The following table presents a general overview of typical follicle sizes on each day of an 11-day ovarian stimulation period. Keep in mind that these values are approximate and should be used for informational purposes only. Consult with your fertility specialist for personalized advice.

Day of IVF Cycle	Typical Follicle Size (mm)
1-3	< 9
4	7 – 11
5	9 – 13
6	11 – 15
7	13 – 17
8	15 – 19
9	17 – 21
10	19 -23
11	21 – 25

These values represent an approximate range for ideal follicle sizes on each day of the 11-day stimulation period. It is NORMAL to have varying sizes – but the goal is to have the majority of follicles grow. Not all follicles will begin to grow at the same time and not every baseline follicle present is guaranteed to respond to medication. Your fertility specialist will closely monitor your follicle growth and make any necessary adjustments to your treatment protocol to optimize the development of mature eggs and improve your chances of success.

Interpreting Follicle Size Data:

Slow Follicle Growth

If follicles are growing slowly or are smaller than expected, your fertility specialist may adjust the dosage or type of medication to encourage further growth. Beyond a certain dose, the receptors that the medications bind and act upon get saturated – so know that there are dosages beyond which increases in medication are not expected to help or produce a different result.

It’s essential to communicate with your doctor and report any side effects or concerns during the process of ovarian stimulation.

Optimal Follicle Size

Mature follicles typically measure between 18 and 25 millimeters in size. A cohort of mature follicles increases the chances of retrieving multiple mature eggs for fertilization. Once the majority of follicles have reached this size, your fertility specialist will schedule the egg retrieval procedure. It is important to note that even follicles which reach a medium size (14-15mm) by day of trigger have a good chance of yielding mature eggs at retrieval. The practical goal is that there should be at least 2 mature measuring follicles (>18mm) with several others following closely behind in the 14mm-17mm range on the day of the trigger shot. The small to medium sized follicles continue to grow and mature between the day of trigger shot and the retrieval.

Rapid/Robust Follicle Growth

In some cases, follicles may grow too large or too quickly. There is an ideal pace to the stimulation of follicle growth. If there is a dominant or lead ‘breakaway’ follicle that races ahead of the majority of other follicles, we may ignore the lead follicle and make our decisions regarding the dose of medications and timing of retrieval based on how the majority of smaller follicles look. In an ideal world, we would have the majority of follicles growing at a similar pace. In future cycles, we can ‘prime’ with estrogen patch or estrogen containing birth control in an effort to suppress early rise of FSH from the pituitary to reduce the chance of early selection of a dominant, larger follicle and aim for better, more synchronous growth of the follicles. 

If there are many follicles growing and the response to treatment is robust with rapidly rising estrogen levels, this can predispose to a condition called ovarian hyperstimulation syndrome (OHSS). OHSS is caused by factors released by multiple maturing follicles that can lead to blood vessels becoming ‘leaky’ and allowing the fluid to accumulate in the pelvis and abdominal cavity, and even around the lungs. This can cause pain, bloating, and in severe cases, more serious complications such as difficulty breathing. In general OHSS is less common these days to due changes in protocols (with lower doses of HCG trigger, or being able to avoid using HCG in the trigger shot, in favor of Lupron) and the ability to freeze all embryos and wait to do embryo transfer in an unstimulated, more natural hormonal environment. To minimize the risk of OHSS, your doctor may monitor you more frequently than the typical cadence during an IVF cycle, start with lower doses of medication, adjust the medication regimen frequently, and will opt to delay the embryo transfer to a subsequent cycle to allow your ovaries to rest and recover.

Inconsistent Follicle Growth

Occasionally, some follicles may grow faster than others, resulting in a mix of mature and immature eggs. In such cases, your fertility specialist will carefully assess the situation and determine the best course of action, considering factors such as your age, ovarian reserve, and overall treatment goals.

The Importance of Personalized Treatment

Follicle growth and size is a vital metric that fertility specialists use to gauge response to treatment and anticipated yield of mature eggs at retrieval. While the number of eggs at retrieval is an important factor in efficiency and efficacy of an IVF cycle – the quality of the eggs (which cannot be predicted by ovarian reserve or response to treatment) is just as, if not more, important to the overall outcome. The interplay of egg count and quality is such that having a higher number of eggs to start with can help to overcome poor egg quality (the more eggs you start with, the more embryos you may get to test and the better the chance that you will obtain genetically normal, healthy embryos). IVF is truly a numbers game. But conversely, young patients with good egg quality may not need many eggs to get a normal embryo. This is important to keep in mind if you are struggling with low AMH and low response to treatment. It’s important to remember that each patient’s IVF journey is unique, and follicle growth rates may differ significantly between individuals. Factors such as age and ovarian reserve and your genetic makeup can determine your response to treatment. IVF protocols and medication dosing can be tailored according to these individual factors and based on prior response to stimulation in any previous IVF attempts.

You can learn more about IVF protocols and medications in related blogpost IVF Medications and Protocols Explained by a Fertility Doctor.