What Size Follicles are Needed for Trigger in IVF?

If you’re considering in vitro fertilization (IVF), you may have heard that you need a certain number of follicles for the procedure to be successful. But what exactly is a follicle, and how many do you need? Here’s what you need to know.

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In order to ensure that the eggs are mature and ready to be fertilized, the follicles must be a certain size. The most important factor in determining the size of the follicles is the number of days of ovarian stimulation. In general, the larger the follicles, the more days of stimulation required. For example, if your doctor is aiming for follicles that are 18 mm in diameter, you may need anywhere from 6 to 12 days of ovarian stimulation. If your doctor is aiming for follicles that are 23 mm in diameter, you may need 10 to 14 days of ovarian stimulation.

What is the minimum follicle size for trigger?

In order to trigger ovulation, the minimum follicle size is 18mm. However, depending on the retrieval method and reproductive goals, the trigger size may be different. For instance, if you are using IVF with ICSI, a smaller follicle size may be needed in order to prevent ovarian hyperstimulation syndrome (OHSS). If you are trying to conceive naturally or using intrauterine insemination (IUI), a follicle that is at least 20-22mm will increase your chance of success.

Does the size of the follicle matter?

The size of the follicle has been found to be predictive of the number of oocytes (eggs) retrieved in an IVF cycle. A study conducted in 2002 by Tarkkonen and colleagues found that when follicles were > 16mm in size, the number of oocytes retrieved was significantly higher than when follicles were < 16mm.1 In another study from 2006, patients with follicles measuring 18-20mm had the highest pregnancy rates.2 It is unclear if there is a “optimal” follicle size for ovulation induction, but it is generally accepted that larger follicles have a higher chance of producing more eggs.3 Therefore, if the goal of treatment is to increase the chances of pregnancy, it would make sense to trigger ovulation when follicles are larger. One potential downside of allowing follicles to grow too large is an increased risk of ovarian hyperstimulation syndrome (OHSS). OHSS is a condition where the ovaries become enlarged and can leak fluid into the abdomen. This can cause pain, bloating, and in severe cases, hospitalization. OHSS occurs in about 5% of IVF cycles and is more likely to happen when larger numbers of eggs are retrieved.4 It is important to discuss with your doctor what their recommendations are for trigger given your individual situation. In general, you want to balance the risk of OHSS with the potential benefit of increasing your chances for pregnancy by retrievaling more eggs.

How to measure follicle size?

There are two ways to measure follicle size:

1) Transvaginal ultrasound: This is where a small, hand-held ultrasound probe is inserted into the vagina. The probe is then used to measure the size of the follicles.

2) Blood tests: estradiol (E2) and anti-Müllerian hormone (AMH) levels can give your doctor an indication of the number and size of follicles present in your ovaries.

The ideal follicle size for trigger in IVF will depend on the type of medication that you are taking. For example, if you are taking gonadotropin injections, your doctor will likely want to see at least one follicle that is 20 mm or larger before trigger. On the other hand, if you are taking letrozole or clomiphene, your doctor may be happy with follicles that are 18 mm or larger.

How is follicle size related to success rates?

It is unclear how follicle size is related to success rates. Some studies suggest that larger follicles (> 17 mm) are associated with higher pregnancy rates, but other studies have not found this to be true. Follicle sizes vary greatly from patient to patient, and even from cycle to cycle in the same patient. A recent study found that in patients with similar follicle sizes, there was no difference in pregnancy rates.

The decision of when to trigger ovulation is based on many factors, including the number of follicles present, the size of the follicles, the level of estrogen present, and the patient’s specific medical history. In general, it is thought that larger follicles are more likely to result in a successful pregnancy. For this reason, most doctors will trigger ovulation when follicles reach 17-20 mm in size. Some doctors may trigger earlier if the patient has a history of poor egg quality or a bleeding disorder.

What else can affect follicle size?

Many different factors can affect follicle size. These include the age of the woman, the weight of the woman, whether she has had children previously, and many others. In addition, the medications that are taken to stimulate egg production can also have an effect on follicle size. For example, some women who take higher doses of medication may develop larger follicles than women who take lower doses.


Based on the research that has been conducted, it appears that follicles need to be a certain size in order for trigger to be effective in IVF. The size may vary depending on the individual, but follicles need to be at least 17mm in order for trigger to work. In some cases, follicles need to be even larger in order for trigger to be effective.