Although more eggs retrieved can help increase the chances of one or two normal embryos implanting and leading to pregnancy, implantation rates have more to do with egg quality than quantity. The fresh, day five (blastocyst) stage of an embryo has a better chance of being genetically normal and resulting in a pregnancy than the day three embryo.

How many eggs are retrieved?

When a woman experiences ovulation during her menstrual cycle, several eggs develop within the ovaries. During ovulation, one of these will become dominant and is released into the uterus. The other eggs are reabsorbed back into the ovary. If she has enough eggs, her doctor can collect them in a lab and mix them with her partner’s sperm. Typically, this will yield a few top-quality embryos. Not all of these embryos will implant (or lead to a successful pregnancy). This is due to genetic abnormalities, implantation failure, and several other factors. Eggs also deteriorate over time. On average, about 80 percent of the eggs retrieved will be fertilized. Most doctors will advocate for transferring only one embryo, known as single-embryo transfer or SET, to increase the odds of a singleton pregnancy. This is because multiple pregnancies carry a higher risk of obstetric complications. It’s important for patients to understand that attrition is a normal part of the IVF process.

How many embryos are produced?

The process of IVF involves taking fertility drugs to increase the number of eggs your body produces. Once enough eggs develop, your doctor will retrieve them with a procedure called follicular aspiration. This requires the use of anesthesia. A needle is inserted into each egg-containing follicle to suction out the eggs and fluid. This procedure can cause bleeding, swelling, and discomfort. The embryos are then screened for PGT-A, a genetic test to ensure they have the full set of chromosomes which is a requirement to achieve a healthy pregnancy. Then, only normal embryos are transferred. For women under 35, 12 mature eggs should be enough to produce 3-4 normal embryos that have about a 65 percent chance of leading to a successful pregnancy. However, this is not a guarantee and will vary from person to person.

How many embryos are transferred?

The number of embryos that are transferred depends on your doctor’s plan. Most doctors don’t transfer more than two embryos because that increases the chance of multiple births, which can cause health complications. After you have an ultrasound to check your ovaries and a blood test to measure your hormone levels, you’ll take fertility drugs to increase the number of eggs that your body produces each cycle. Then, you’ll undergo a procedure called egg retrieval. During this procedure, your doctor uses an ultrasound wand to guide a needle through the vagina into an egg-containing follicle. During IVF, your doctor can produce several top-quality embryos in one cycle. This is because fertility drugs can improve your chances of getting more than one normal embryo if you are young and have good egg quality. However, many patients don’t have enough top-quality embryos to achieve a pregnancy using just one IVF cycle. That’s why many IVF programs offer to freeze additional embryos for future cycles.

How many embryos are implanted?

If you’ve had a normal number of eggs retrieved, the chances of having one or more healthy embryos to transfer are very good. However, it’s important to remember that not all cycles of IVF are the same. Each ‘batch’ of eggs recruited will have a mix of normal and abnormal ones, affecting pregnancy rates. After the eggs are retrieved, they are mixed with sperm from your partner or a donor or the sperm can be injected directly into the eggs in a process called intracytoplasmic sperm injection (ICSI). The fertilized eggs become embryos and are monitored by laboratory staff. Guidelines suggest that physicians transfer only a single embryo, and some practitioners do so. However, many do not, and research is needed to examine the factors that influence these choices. This research should include a thorough exploration of the factors, uncertainties, and challenges involved in this area. It also should examine why, and to what extent, providers deviate from the current guidelines on transferring multiple embryos.