The goal of any assisted reproductive technology is to safely maximize each patient’s chances of having a healthy baby. For a long time, doctors and researchers believed that to achieve this goal, they should aim to retrieve 6-15 eggs during the ovarian stimulation phase of in vitro fertilization (IVF). However, new research is showing that, in fact, safely retrieving as many eggs as possible may result in a higher chance of having a baby. Let’s break down why that is.
During IVF, clinicians use hormone medications to carefully stimulate a patient’s ovaries to grow multiple eggs to maturity (rather than one) in a process called ovarian stimulation. The number of eggs a person can safely be stimulated to produce is highly dependent on the person and their ovarian reserve, which is the number of eggs remaining in their ovaries. Their goal is to stimulate as many follicles as possible without risking ovarian hyperstimulation syndrome (OHSS), a potentially dangerous condition in which the ovaries swell and leak fluid into the abdomen.
A patient’s success rate with IVF is defined as their chance of having a baby following the transfer(s) of all viable embryos to the uterus from one egg retrieval cycle. Clinicians commonly transfer only one embryo per transfer attempt. If the IVF cycle produces more than one viable embryo, the cycle can include multiple attempted transfers, which can take some time to complete.
Aside from disorders that may affect their fertility, a patient’s chances of having a baby with IVF depends on their ovarian reserve, their age (which affects the quality of their eggs), their hormones, the age and quality of sperm being used, and the number of IVF cycles they undergo.
Until recently, a held belief was that the maximum number of eggs a person could produce without a decline in overall birth rate was around 15 eggs (1). A major reason for this was that, in the past, clinicians primarily transferred fresh embryos, which meant that after ovarian stimulation and egg retrieval, patients would immediately prepare their bodies for embryo transfer.
During ovarian stimulation, patients are stimulated with hormone medications. It was believed that the raised hormone levels required to stimulate a patient to retrieve more than 15 eggs would negatively affect the patient’s endometrial receptivity (a.k.a. the endometrium’s ability to attach to the embryo and nourish it soon after egg retrieval (2).) This concern about the endometrium may be why the data that researchers were using in 2011 showed a plateau or decrease in success rates with egg retrievals higher than 15 eggs.
Today, while there is still a plateau in success rates for fresh transfers when many eggs are retrieved, embryo culture and cryopreservation techniques have improved so much over the past decade that patients experience greater success with frozen transfers than they could before. With a frozen transfer, patients’ hormone levels have time to return back to normal before preparing their body for embryo transfer. Because of this, clinicians may be able to stimulate a patient to produce the optimal number of eggs for the patient without worrying about a 15 egg limit.
In fact, Alife’s recent research shows that a patient’s average rate of success will continue to increase rapidly up to about 16-20 eggs retrieved, after which it continues to increase, just at a slower rate. In other words, each additional egg retrieved improves the average success rate, or CLBR. The reason is simple: retrieving more eggs gives a patient more opportunities to transfer a high quality embryo. This results in a higher cumulative chance of eventual pregnancy and live birth.
A patient’s chance of success with IVF still depends in part on whether they decide to do a fresh or frozen embryo transfer. For fresh embryo transfer, the ideal range of eggs retrieved was between 11-30 eggs. However, many patients today choose to culture their embryos in a lab and freeze all of the resulting embryos, in part because it allows them to do pre-implantation genetic testing (PGT) on embryos to ensure that only chromosomally normal embryos are transferred. Researchers found that freeze-all IVF cycles that use PGT have the highest success rates during IVF.
With this knowledge, your clinician can focus on safely optimizing your ovarian stimulation protocol to maximize the eggs you can develop while minimizing your chance of developing OHSS. Knowing that freeze-all cycles have high success rates may also affect your decision on how you want to complete your IVF cycle.
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