Does follicle size matter? From IVF to IUI, age, twins & more, let’s ask

When it comes to fertility treatment, every detail matters. Having navigated all sorts of treatment experiences on the TRB team, our own search history makes for interesting reading. And follicle size is a significant part of that. 

A fine line, and expert management

Whether you’re considering in-vitro fertilization (IVF) or intrauterine insemination (IUI), understanding how follicles grow and what size they need to reach for optimal conception can empower you, across treatment. Today, we’re diving into the science behind follicle size, addressing key questions like how it impacts IUI, IVF, and even the chances of conceiving twins and multiples.

As an IVF mama who welcomed twins nearly a year ago, our founder Eloise vividly recalls the fine balance of follicle stimulation during her treatment: “I was worried about the smaller ones not having time to catch up to the bigger ones before trigger. And they did! 

We kept up with stims (ovarian stimulation) a little longer – but it was a fine line between stimming and risking the bigger follicles becoming over-mature.” 

Her experience underscores the importance of careful monitoring and expertise in follicle management.

Fresh for 2025 and in partnership with The Prelude Network®, the fastest-growing fertility network in North America, let’s get into today’s actionable insights backed by cutting-edge science and expert care. 

The Prelude Network’s mission is as thoughtful as its name – it’s an introduction, setting the stage for what’s to come. Offering the best in treatment options, science, and patient care, their expert physicians and care teams help anyone aspiring to become a parent navigate their unique path. 

We love unique, and we love specifics. Let’s explore what their experts have to say about follicle size and its role in achieving your family-building goals

What is follicle size, and why does it matter?

Follicles are small fluid-filled sacs in the ovaries that house immature eggs. During a menstrual cycle, these follicles grow in size as they prepare to release a mature egg during ovulation. For those undergoing fertility treatments like IUI or IVF, tracking follicle growth is crucial, as it helps determine the optimal time for intervention (for example, egg retrieval) and increases the chances of conception.

“Follicle size plays a vital role in determining the maturity of eggs,” explains Dr. Sharon Jaffe from the Center for Reproductive Medicine, part of The Prelude Network. “For IVF, we aim for follicles to reach an ideal size of 18-20 millimeters to ensure the eggs are mature and ready for retrieval.”

What is the ideal follicle size for IUI and IVF

Unpacking the specifics, let’s split out IUI and IVF treatment routes, as follicle management may differ between the two. 

IUI (intrauterine insemination)

For IUI cycles, the ideal follicle size ranges between 18 and 22 millimeters. This size indicates that the egg within the follicle has reached maturity, increasing the likelihood of successful fertilization. Dr. Deborah Ikhena-Abel from Aspire Fertility Austin, a member of The Prelude Network, notes, “In an IUI or timed intercourse cycle, aiming for follicles within this range is key. Proper timing of the procedure ensures the highest chances of success.”

IVF (in-vitro fertilization)

In IVF, the process involves stimulating multiple follicles to grow simultaneously. According to Dr. Jaffe, the ideal size for IVF follicles is 18-20 millimeters, though the decision to trigger ovulation depends on factors like estradiol levels, age, and how the patient has responded to previous cycles.

“As patients get older, the ideal follicle size may be slightly smaller. We take a holistic approach to maximize the number of mature eggs while maintaining quality,” she explains.

Can follicle size influence the chances of conceiving twins?

Yes, follicle size and the number of mature follicles directly impact the likelihood of conceiving twins or multiples, particularly in IUI cycles. “The higher the number of mature follicles, the higher the risk of multiple gestation,” says Dr. Jaffe. For IUI, follicles greater than 14 millimeters are considered mature, and careful monitoring is essential to minimize risks associated with multiple pregnancies.

When stimulating follicles for multiple pregnancies, doctors use advanced tools to assess risks based on follicle size, patient age, and other factors. “This allows us to counsel patients effectively about the likelihood of conceiving multiples, and make informed decisions together as a team,” adds Dr. Jaffe.

Recommended read: We found this deep dive study looking into predictors of twin and high-order multiple pregnancy in IUI super handy.

What happens if a follicle is too small or too large?

Follicles that are too small (under 14 millimeters) are unlikely to contain mature eggs capable of fertilization. On the other hand, overly large follicles may indicate cyst formation or other abnormalities. 

Dr. Ikhena-Abel advises, “For both IUI and IVF, we aim for follicles in the range of 18-20 millimeters or more. Follicles outside this range may not yield viable eggs, which could impact the success of the cycle.”

Left or right – does it make a difference?

Patients often wonder if one ovary is more “productive” than the other. 

Dr. Ikhena-Abel explains, “It’s not uncommon for one ovary to be more responsive during a cycle, but this is usually random and not a cause for concern. Unless a patient has had surgery or other issues affecting one ovary, both are equally capable of developing healthy follicles.”

Common abnormalities

Abnormal follicle development can significantly impact fertility. 

Dr. Agard at Pacific Fertility Center says, “Follicle dysfunction in natural conception often involves abnormal growth patterns. Women may experience either unusually rapid follicle growth or slow (or even absent) follicular development. 

In a typical 28-day cycle, ovulation usually occurs around day 14. However, some women ovulate much earlier, even during their period, while others ovulate infrequently, such as once every other month – or less.” 

Conditions commonly associated with these irregularities include: 

• diminished ovarian reserve (DOR), which can cause early ovulation, and 
• polycystic ovary syndrome (PCOS), which is linked to delayed ovulation or the absence of ovulation altogether.”

How can you optimize follicle size for conception?

So, is there anything we can do, to be proactive and make a difference to our follicle state of play? 

Picking up with TRB founder Eloise, three key strategies were front of mind, when discussing follicle growth with her team and prepping for her final round of treatment: 

1. Lifestyle changes: Maintaining a healthy diet, exercising regularly, and managing stress can positively influence hormone levels and follicle development.
2. Medications: Fertility drugs like Clomid or gonadotropins can help to stimulate follicle growth and increase the chances of ovulation.
3. Regular monitoring: Working closely with your fertility specialist ensures that follicle growth is tracked accurately, enabling timely interventions.

Treatment, education & inclusivity

Heading into 2025, The Prelude Network stands for the values we hold close, across our own treatment and family-building pathways. 

Whether you’re single, in a partnership, and/or part of the LGBTQ+ community, they believe everyone deserves the chance to become a parent when the time is right for them. Their inclusive and compassionate approach ensures that all families feel welcomed, supported, and empowered in pursuing their fertility goals.

Touch base with The Prelude Network to learn more about their services. For additional resources and empowering tips direct from the team, check out our collab guide to finding empowerment during IVF or IUI, and across your treatment journey.