Embryo Transfers

What is an embryo transfer?

An embryo transfer is just that: the transfer of an embryo into the uterus. Embryos can either be transferred as fresh (5 days after your egg retrieval while they are still in culture) or frozen (thawed for transfer during a later cycle). A gestational carrier can carry a pregnancy to term when a woman cannot do so.

What happens before an embryo transfer?

On the days leading up to your embryo transfer, you may take medications that help your uterine lining thicken. These medications consist of estradiol (Estrace) and progesterone (PIO). Some transfers are unmedicated, or natural, and do not require medications. Your doctor can help determine which type of transfer is best for you.

Your uterine lining must be thick with blood and nutrients in order for the embryo to implant into it. Your uterine lining thickness is measured using an ultrasound and will be monitored prior to your scheduled transfer. If your uterine lining does not thicken prior to your embryo transfer, the transfer may need to be canceled and attempted again during a later menstrual cycle. You may also require some blood work prior to your transfer to ensure that your hormone levels are within normal ranges for your embryo transfer.

What happens on the day of an embryo transfer?

On the day of your transfer, an embryologist will either observe your embryo (if it is a fresh embryo transfer) or thaw your embryo (if it is a frozen embryo transfer).

If you are having a fresh transfer and no embryos are adequate for transfer (they are not at the right stage of development and/or are poor quality), your transfer will likely be canceled and your embryos will be cultured in an incubator overnight to (hopefully) continue growing. If any embryos grow overnight, they can be frozen and stored for a later frozen embryo transfer. If your embryo is thawed for a frozen embryo transfer and is of poor quality or degenerate, your doctor may ask if you would like to thaw a different embryo. This is not common and occurs in <5% of frozen embryo transfers.

You should arrive at Pelex 30 minutes prior to your scheduled transfer time and try to have a full bladder when it is time for your transfer. A full bladder provides a better view of your uterus on the ultrasound monitor and a straighter path for the catheter to enter the uterus through.

The transfer procedure is typically not performed under anesthesia, though some women do elect to take a Valium (if prescribed) prior to the transfer to calm their minds and bodies. Most women do not report feeling pain during their transfers, though many do report feeling pressure from the speculum, cramping when the catheter enters the uterus, and discomfort from their full bladders. Fortunately, the embryo transfer process only takes a few minutes to complete.

During the procedure, you will lay on an exam table with your feet in stirrups. The doctor will insert a speculum in order to see the cervix and then will clean the cervix (it can have mucus on it from the medications). Once the cervix is clean, the physician will locate the cervical os (a small hole in the cervix that leads to the uterus). Using an abdominal ultrasound, the physician will locate the cervix, uterus, and uterine lining on an ultrasound monitor. The physician will then insert a trial catheter (which does not contain the embryo) through the cervical os and into the uterus. The trial catheter consists of a plastic sheath (sleeve) with a flexible catheter inside of it. A trial catheter is necessary because it creates a clear path for the embryo into the uterus. If the trial catheter cannot enter the uterus, the embryo will also not be able to enter the uterus.

Once the trial catheter is properly placed, the physician will remove the flexible catheter but will leave the plastic sheath (sleeve) inside of your uterus. This creates a tunnel for the catheter that will contain the embryo.

The embryologist will then load your embryo into a new catheter (called a replacement catheter) using a syringe. This catheter consists of the same flexible material that the trial catheter consists of. Once the embryo is loaded into the replacement catheter, the catheter is inserted through the "tunnel" created by the plastic sheath of the trial catheter and enters the uterus.

Using the ultrasound as a guide, the physician will advance the catheter (with the embryo inside) into the uterus. Once the catheter is in the proper position, the physician will inject the embryo (and some fluid) into the uterus. Many patients will see a brief flash of light when the fluid is injected.

Afterward, the physician will remove the catheter that contained the embryo, and the embryologist will take it back to the lab. The embryologist will flush out the catheter with media to ensure that the embryo was successfully transferred from the catheter. If the embryo is retained inside the catheter, it is reloaded into the catheter and the transfer process repeats itself.

After the transfer, your physician will review your rules and restrictions for the next few days. You will have a scheduled blood test ~10 days after the embryo transfer. This blood test detects the level of hCG in the blood. hCG is a hormone that is secreted by an embryo after it implants into the uterine lining. If there is no hCG in the blood, it means that the embryo did not implant. If there is hCG in the blood, it means that the embryo did implant.

Day 3 versus day 5 embryo transfers

Years ago, IVF clinics performed most embryo transfers 3 days after an egg retrieval. At the time, IVF culture media (the fluid that embryos grow in inside of an incubator) was not as advanced as it is today. Therefore, many embryos did not survive in culture media until day 5.

However, there have been many advancements in culture media, and embryos are now able to survive up to (and sometimes more than) 6 days inside an incubator. With this advancement, many clinics began transferring embryos 5 days after egg retrievals. At the same time, many clinics began freezing embryos on days 5 or 6 rather than day 3.

Statistically, day 5 transfers have higher implantation rates compared to day 3 embryo transfers. This is likely because embryos may not grow past day 3 (even when they are of good quality on day 3) regardless of whether they are in an incubator or a uterus. For these reasons, we at Pelex do not perform routine day 3 embryo transfers.


Fresh versus frozen embryo transfers

For many years, IVF clinics performed fresh day 5 embryo transfers on the majority of their IVF patients. Fresh transfers do allow for faster pregnancies and are often more affordable than frozen embryo transfers. However, many studies have shown that frozen embryo transfers have higher implantation rates than fresh transfers. There are a few possible reasons for this:

1. Embryos that undergo PGT must be frozen, and only normal (and some mosaic) embryos are transferred. These embryos have higher implantation rates and lower miscarriage rates. Fresh embryos that are transferred cannot be tested, so there is a higher chance of transferring an abnormal embryo (this increases with age). Since PGT has become very common in IVF clinics, this can account for the higher implantation rate for frozen embryos.

2. Giving the body a month or so to "reset" has shown to improve implantation rates. This is especially true for women that have had a high number of eggs retrieved (and are at risk for OHSS), women with PCOS, and women with DOR.

For these reasons, we at Pelex strive to perform frozen embryo transfers, though fresh transfers are performed in certain circumstances.

What are my chances of success during an embryo transfer?

Unfortunately, this isn't an easy question to answer. There are so many variables to consider when it comes to success rates. For example, your diagnosis, age, medical history, and embryo quality. Further, embryos that undergo PGT have higher success rates than those that do not, especially for women over 35.

Why do embryos fail to implant?

The primary reason that embryos do not implant into the uterine lining once they are transferred is because they are genetically abnormal (they do not have the correct number of chromosomes). However, there are a few other factors that can hinder embryo implantation:

  1. The uterine environment: the presence of large fibroids, polyps, or scarring can impact embryo implantation. Additionally, a thin uterine lining can prevent embryo implantation.
  2. An infection: primarily endometritis, which is an infection in the uterine lining. This can often be treated with antibiotics and/or steroids but must be diagnosed by your doctor.
  3. An immunological, clotting, or genetic factor: you can discuss these factors with your doctor if you have multiple failed embryo transfers. These factors are uncommon, but they can impact embryo implantation.


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