IVF Funnel: Why Not All Eggs Become Embryos

To understand the funnel better, let’s do a brief recap of the five main stages of IVF:

• Ovarian stimulation. A woman takes injectable hormones that prompt her body to produce multiple eggs.
• Egg retrieval: A fertility doctor removes the eggs from the ovaries during an egg collection procedure.
• Fertilization: The eggs are fertilized in a fertility lab with sperm provided by the partner or donor sperm.
• Embryo growth: The fertility lab monitors the growing embryos for a few days, ideally until they reach the blastocyst stage.
• Embryo transfer: The embryos are then transferred into the woman’s uterus. Approximately two weeks later the woman takes a pregnancy test to check if implantation was successful.

Read more: IVF Process: Your Step-by-Step Guide from Start to Finish Now that you understand the basics, let’s take a look at how the IVF funnel works at each stage.

Ovarian stimulation & egg retrieval

As mentioned, hormones are given during ovarian stimulation. This coaxes the ovaries to grow as many follicles as possible. Each follicle is a fluid-filled sac that houses an egg. However, not all follicles develop into mature eggs. Roughly 30% of all eggs retrieved are immature. Immature eggs are more likely to have implantation issues or lead to miscarriage. So they’re not ideal for fertilization. So say a woman goes through ovarian stimulation and ends up with 20 follicles. Out of those 20, about 14 may develop into mature eggs. Why some eggs mature and others don’t isn’t clear. However, age may play a role. Research shows the number of eggs retrieved declines with age, while the number of immature eggs increases. In general, the more eggs retrieved, the higher your chances of an IVF cycle resulting in a life birth. So what’s the magic number? One study compared the pregnancy rates for 1,615 IVF cycles. They found that pregnancy rates were best when at least 10 eggs were retrieved.

Number of eggs retrieved	Pregnancy rate (in %)
1-4 eggs	30.8%
5-9 eggs	36.2%
10 or more eggs	44% or higher

In addition, they found that the ideal number of eggs retrieved varies by age. For women under 35, pregnancy rates were best when at least 10 eggs were retrieved. For women between the ages of 35 and 39, retrieving 5-9 eggs yielded the best results. And for women over 40, the more eggs retrieved, the higher the pregnancy rate. That said, no two IVF cycles look the same. Some women may have an unsuccessful IVF cycle with 20 eggs retrieved. Others may have a successful pregnancy with only 3 mature eggs. It’s all relative. Read more: Mini IVF: Success Rates, Side Effects, and More

Fertilization

After the eggs are retrieved, they’re fertilized in a fertility lab. There are two routes here. The first is conventional fertilization. In this case, an egg and sperm are mixed together in a petri dish. The second option is intracytoplasmic sperm injection – or ICSI for short. ICSI involves injecting sperm directly into an egg. No matter which option you choose, typically 70-80% of eggs get fertilized. So if 14 eggs are retrieved, roughly 11 may get fertilized.

Embryo development

The fertility lab observes the fertilized eggs over the next few days as they develop into embryos. Embryos typically grow until day 3 or day 5. Day 3 embryos have roughly 8-10 cells, while day 5 embryos (blastocysts) have between 80 to 100 cells. Transferring embryos at the blastocyst stage tends to be more successful than at earlier stages. Even so, not all embryos will grow enough–or be of high enough quality–to be suitable for embryo transfer. Sadly, less than 50% of embryos make it to the blastocyst stage. Many factors play a role here, including laboratory conditions, sperm quality, and egg quality. Meaning, that out of 11 fertilized eggs, only about 6 embryos will survive. Read more: Day 3 vs. Day 5 Embryo Transfer: Which is Better?

Genetic testing results

For the embryos that do reach the blastocyst stage, you have the option to do genetic testing. This is called preimplantation genetic testing, or PGT testing. And it tells you whether the embryos are genetically normal. To do PGT testing, you need to freeze the embryos. This adds to the IVF timeline, but since it can offer peace of mind, many find it’s worth the wait. Studies show about 35% of day 3 embryos are genetically normal (euploid), while around 55% of day 5 embryos are genetically normal. This means if 6 embryos reach the blastocyst stage, only about 3 of them may be chromosomally normal. These rates stay the same between ages 24 to 35. However, after age 35, the number of genetically normal embryos drastically decreases. At age 35 roughly 60% of embryos are genetically normal. But by age 40, that number drops to 35%. And by age 45, only about 15% of embryos are euploid. This is where PGT testing can be useful. That way, you know which embryos are the healthiest and have the best chance of implantation. If egg quality is an issue, remember donor eggs are always an option.

Embryo implantation

Following embryo transfer comes what many consider the hardest part of IVF: “the two-week wait.” This is the 11-14 days after embryo transfer when you’re waiting to find out if an embryo got attached to your uterine wall. Unfortunately, not all embryos successfully implant. This can make the two-week wait a difficult and nerve-wracking time. Read more: Your Complete Guide to Surviving the Two Week Wait One study found implantation rates per embryo transferred range between 10 to 40%. According to this study, the average implantation rate is about 26%. However, other studies are more encouraging. More on this later. Many factors play a role in whether or not an embryo implants. These include endometrial receptivity, the techniques used during embryo transfer, and embryo quality. Embryos are graded and given a blastocyst morphology score based on the shape and appearance of their cells. Typically, the higher the score, the higher the implantation and live birth rates. To illustrate this, one study looked at embryo quality from 1,766 IVF cycles. Here’s what they found:

Embryo score	Pregnancy rate (implantation rate)	Live birth rate
“Good” embryos	55.6%	46.8%
“Fair” embryos	47.2%	39%
“Poor” embryos	43.6%	34.1%

With these stats in mind, if 3 embryos were transferred, it’s likely only one or two of them may implant. Funnel example for a woman starting with 20 mature follicles To help you see the funnel in action, here’s a recap of the example we shared:

IVF stage	Number of eggs/embryos	Attrition rate	Actual material useful for the next stage
Ovarian stimulation	20	30%	14
Egg retrieval	14	20-30%	11
Fertilization	11	50%	6
Embryo growth	6	50%	3
Embryo transfer	3	60-90%	1

Keep in mind, this is only an example and each situation is unique. Your attrition stats will likely look different than the ones above. The number of eggs and embryos varies among women and from cycle to cycle. But hopefully, this will help you set expectations and understand the process better if you’re considering IVF.