As we show in the example below, during every step of IVF a certain number of eggs or embryos are lost, especially in the middle part of the funnel (growing embryos that are chromosomally normal). As a result, a woman needs to start the process with many eggs.

Ideally, between 15 - 20 mature eggs would be retrieved, though getting a number that high is uncommon. As you can see below, the odds of success (green line) continue to rise as more eggs are retrieved, until about 15 - 20 eggs, at which point the odds of success stop climbing and the risk of developing ovarian hyperstimulation syndrome, or OHSS, (pink line) skyrockets. OHSS can be both painful and dangerous. This comes from a 38,000 patient European registry.

As a result, most fertility doctors shoot to retrieve 15 to 20 mature eggs per cycle. Of course, during a regular cycle most women naturally produce only a single mature egg. To bridge that gap, doctors prescribe drugs that woman take at the start of a cycle to instigate growth of ovarian follicles that contain eggs. This is called “multi-follicular” development and it’s a pivotal step in a successful IVF.

One thing to keep in mind is that every cycle, there is a new cohort of follicles availble to be stimulated, meaning that during different months the same protocol may work more successfully than others simply due to the natural variation in available follicles. Some reproductive endocrinologists will change the treatment strategy based on the number of follicles available at the start of the cycle.

The combination of drugs and their dosing is known as a “protocol” and while it may seem like there are a dizzying number of protocols, the reality is there are only a few core options.

By and large there are two easy ways to think about protocols: how much gonadotropin (the drug that prompts follicles to grow) gets used, and what other drugs get used alongside the gonadotropin which is typically what defines a given protocol strategy. There are 2 - 3 core protocols: the Long Agonist, Antagonist and Flare. Below is an oversimplified way to visualize this.

Let’s start with how much gonadotropin to take. Again, gonadotropin is the injectable hormone that prompts a higher number of follicles, and thus eggs, to grow at any one time.

The dose of gonadotropin is typically measured in “International Units Per Day” and ranges from 0 - 900 with most IVF patients receiving 250 - 450 IUs per day. The dose of gonadotropin matters because, generally speaking, if too little is prescribed, too few eggs are retrieved, and IVF success rates go down. On the other hand, if too much gonadotropin is taken, a woman is at higher risk of hyperstimulation, known as Ovarian Hyperstimulation Syndrome or OHSS.

Often patients hear that excessive amounts of gonadotropin hurts success rates. That data comes from an analysis of over 700,000 IVF cycles run by well-respected investigators at Stanford. The results are below and as you can see, success rates do seem to drop off after 300 IUs per day of gonadotropin.

However, the study has two major shortcomings and for that reason most experts aren’t ready to concede that rates of gonadotropin over 300 IUs per day is harmful.

First, the analysis was retrospective and not prospective. As a result, it’s hard to correct for confounders like the fact that harder cases may (or in our minds, probably) had been given more drug and so the underlying condition, rather than the dose taken, contributed to the lower rates of success.

Second, this study was only done in cycles using a “fresh” transfer. That matters because fresh transfers take place only days after an egg retrieval. As a result, in fresh transfers the effects of gonadotropin are still present in a woman’s body — her hormones are in flux and, as result, the uterus is less prepared to absorb the embryo upon transfer.

Today, most IVF cycles use a “frozen” transfer whereby embryos are frozen and transferred at least a month after the retrieval. In that time a woman’s hormonal balance has been restored and so IVF cycles using a frozen transfer are more likely to work. Thus, the negative impact of taking a lot of gonadotropin may be minimized in a frozen transfer.

Take a look at the data below published on roughly 1,000 fresh transfers and 1,000 frozen transfers. In the case of the fresh transfers, you can clearly see a similar effect to what investigators found above: success rates drop with more drug. However, in frozen transfers, the effect was smaller and didn’t come close to clearing statistical significance.

A fundamental question is whether protocols using a lower dose of gonadotropins do as well as those using a higher dose of gonadotropins. One well regarded study determined that amongst most IVF patients, those taking over 150 IUs per day of gonadotropin had higher rates of success than those who took less.

Another set of investigators looked at a variation of the same question. They studied what happens when you replace gonadotropin with clomid (a cheaper, less potent alternative) for a few days before the retrieval. As you can see below, success rates dropped.

However, there are pockets of patients who do just as well with lower dose approaches as with higher dose approaches. Specifically, “poor responders” (a less than pleasant way of referring to women who produce few eggs per retrieval), do equally well taking 150 IUs of gonadotropin as 450 IUs. Typically, a poor responder is someone who meets two of three criteria: they have collected three or fewer eggs on a previous cycle, is over age 40, or who has a diminished ovarian reserve (antral follicle count below 5 or an AMH below below 0.5).

Some people prefer the term Diminished Ovarian Reserve or Low Egg Reserve for patients who meet this criteria, as the ovarian response to medications for this group is not always necessarily poor, but rather is simply expected to be lower at their given baseline.

Below you can see that when investigators gave poor responders 450 IUs or 150 IUs per day, the groups had nearly identical success rates. Most experts believe these women just don’t seem to respond to increased doses and so above a pretty low threshold of gonadotropin, success rates don’t seem to budge much.

Similarly, when an investigator named Revelli decided to swap out a few days of gonadotropin for clomid in this poor responder population in Italy (and thereafter resumed gonadotropin at low levels), he saw similar rates of success to more conventional levels of gonadotropin use. Today, we’re seeing more well-respected doctors choosing to compliment lower dose of gonadotropin with clomid or letrozole in this group.

Similarly, many doctors believe “low dose” approaches work equally well as high dose approaches on women who are likely to be “hyper responders.” These are women who have a high AMH or had a high number of eggs retrieved in a previous cycle. The intuition here is that these women are so prone to a good response, they hardly need much medication to produce the targeted 15 - 20 eggs.

What’s important to stress here is that just because some “low dose” approaches drive comparable rates of success to conventional approaches, that doesn’t mean all low-or-no dose approaches are effective. As you may recall in the Revelli and Yousef studies, 150 IUs per day of gonadotropin were used, which is well higher than most things marketed as “mini-stimulation” approaches, and any “natural” (no gonadotropin) approach.

While gonadotropin is the critical drug in most every protocol, it’s not the only drug. Often two other types of drugs are needed to accompany gonadotropin: those that block eggs from “maturing” and being ovulated before they can be retrieved, and those that help trigger the eggs to mature so they can be retrieved. As you can see in our summary below, there are multiple drugs that can perform these two functions — the one that’s chosen dictates the protocol strategy. In some cases, a combination of both types of triggers may be used.

Ultimately there are two protocol strategies doctors will use: either the “Long Agonist” or “Antagonist” protocols. This amounts typically to a “Coke vs. Pepsi” kind of decision. Within both, doctors can prescribe as much gonadotropin as they’d like. A third option, the “Flare” protocol is used less often and only in very specific patient types (often poor responders).

The first question is naturally, “which protocol is more likely to deliver a baby,” and when investigators looked at the two most popular strategies, “Long Agonist” and “Antagonist,” it became clear both were equally effective in the general IVF population. Below is data collected on over 3,000 cycles for each protocol approach in the Netherlands.

However, when it comes to specific IVF populations, it’s clear that certain strategies and doses are better than others. Here’s an example from the same study. As you can see below, amongst women with PCOS, the Antagonist protocol drives comparable success rates but with far lower risk of hyperstimulation.

You may wonder how that’s possible. The answer lies in the drug the protocol uses to trigger the eggs to mature so that they can be retrieved. This drug’s known as the “trigger shot”. The Antagonist protocol can use Lupron as its trigger, rather than hCG, and Lupron’s properties dramatically lower the risk a woman will hyperstimulate. On the other hand, the Long Agonist protocol can’t use Lupron as the trigger because it already deploys Lupron elsewhere.

As a result, the Antagonist strategy is generally preferred for women at especially high risk of developing OHSS, namely women with PCOS, younger women, women with high AMH or AFCs, African American women, and those who produced a high number of eggs in a recent cycle.

Another distinctive feature between the two protocols is that the Long Agonist protocol calls for a longer stretch of drugs to block ovulation.

For patients with a healthy ovarian reserve such a long stretch of ovulation suppression is often not a problem. However, for poor responders many doctors worry such a long duration of suppression can hurt outcomes. However, the data doesn’t bare that out. Below is a meta-analysis of 17 (relatively small) studies that, taken together, show the strategies have nearly identical pregnancy rates.

Ultimately, for only a handful of patient types has one protocol shown itself to be superior to the others and we profile those below.

However, given some of the additional features for each protocol (for example, the duration of suppression), some patients might find one preferable to the others. In the next section we’ll walk you through the mechanics of each protocol.