Your guide to embryo transfer

  • Guides
21 Jan 2025

The final step in your fertility journey

Embryo transfer is an important step in your IVF journey, bringing you closer to your goal of having a baby! This guide will explain what embryo transfer involves, the different types of embryo transfer available, and what to expect during the process.

What is embryo transfer?

Embryo transfer is a procedure in IVF where a fertilised embryo is placed into the uterus to help achieve pregnancy. It is the final step of the IVF process, following fertilisation in the lab, and aims to establish a successful pregnancy.

In this guide, we’ll explain the different types of embryo transfer and what you can expect.

Before your embryo transfer

Before getting to the stage of embryo transfer, you’ll need to undergo a few steps to create the embryo. This is what the process looks like:

Egg retrieval

    • During the IVF process, fertility drugs stimulate your ovaries to produce multiple eggs.

    • When the follicles (egg-containing sacs) reach the right size, your doctor schedules the egg retrieval.

    • During the procedure, the doctor extracts eggs from the ovaries using a needle guided by ultrasound.

Fertilisation in the lab

    • The retrieved eggs are combined with sperm (from a partner or donor) in the lab.

    • Fertilised eggs are monitored in an Embryoscope, a specialised incubator with continuous video surveillance and artificial intelligence to track embryo development.

    • Embryos are typically cultured for 5 to 6 days until they reach the blastocyst stage, an advanced stage of development with higher implantation potential.

Below is a visual to show you the steps you’ll go through during your IVF journey, depending on your treatment.

An image showing the different stages of IVF treatment from test and ultrasounds, to ovarian stimulation, sperm retrieval, fertilisation, embryo culture, PGTA testing and embryo transfer.

What happens during embryo transfer?

The embryo transfer is a straightforward but delicate procedure that places one or more embryos into the uterus to help achieve pregnancy. Here’s what happens:

Selecting the embryos

    • Your doctor carefully chooses embryos based on their quality, your age, health, and IVF history.

    • The aim is to maximise the chance of pregnancy while reducing the risks of multiples like twins or triplets.

The procedure

    • A thin catheter is loaded with the selected embryos.

    • The doctor gently guides the catheter through your cervix and into your uterus.

    • The embryos are then released into the uterine lining with care.

What happens after embryo transfer

    • You will rest briefly after the procedure before going home.

    • Your doctor may prescribe medications to support the uterine lining and improve implantation chances.

    • Around two weeks after the transfer, you can take a pregnancy test to find out if it was successful.

How do embryos develop after fertilisation?

When undergoing IVF, it’s helpful to understand the different stages of embryo development. Here’s a brief overview:

Fertilisation (24 hours):

    • What happens: A sperm cell joins with an egg cell to form a zygote (a single cell).

Zygote (Day 1)

    • What happens: The single-cell zygote starts dividing into more cells.

Cleavage Stage (Days 1-3)

    • What happens: The zygote divides into 2, 4, then 8 cells, continuing to multiply.

    • Key feature: The embryo looks like a small ball of cells.

Morula Stage (Day 3-4)

    • What happens: The embryo, now called a morula, consists of about 16 cells.

    • Key feature: Cells continue to divide and stick together tightly.

Blastocyst Stage (Days 5-6)

    • What happens: The morula becomes a blastocyst, with three parts:
        • A fluid-filled cavity.

        • An inner cell mass (which will become the baby).

        • An outer layer (which will become the placenta).

    • Key feature: This stage is now ready for implantation in the uterus.

Implantation (Days 6-10)

    • What happens: The blastocyst attaches to the uterine lining and begins to implant.

    • Key feature: Successful implantation marks the start of pregnancy.

Here’s a visual representation of how the embryo develops after fertilisation.

An image showing how embryos develop from fertilisation. The image shows how the embryo develops from day 1 to day 5 or 6.

Types of embryo transfers: fresh and frozen

There are two main types of embryo transfers, depending on when the embryos are transferred after fertilisation: fresh embryo transfers and frozen embryo transfers (FET).

Fresh embryo transfer

In a fresh embryo transfer, the embryo is placed into the uterus within days of fertilisation. The embryo has never been frozen; instead, it is transferred once it reaches day 3 or day 5 or 6 of development, depending on your individual case. According to Dr. Vladimiro Silva, Scientific Director of Ferticentro, at Ferticentro, 99% of fresh embryo transfers are done on day 5 or day 6.

Advantages of fresh embryo transfers:

    • Immediate timeline: Fresh transfers happen in the same cycle as egg retrieval and fertilisation, avoiding delays and helping reduce anxiety by progressing quickly to the next step.

    • Simpler process: There is no need to freeze or thaw embryos, which eliminates additional steps and potential costs. While modern freezing techniques are highly effective, fresh transfers remove even the minimal risks associated with the freezing and thawing process.

Disadvantages of fresh embryo transfers:

    • Hormonal effects: The high hormone levels from ovarian stimulation can make the uterine environment less ideal for implantation. This may lower the chances of success for some patients, as the body might need more time to recover.

    • Limited genetic testing: Fresh embryo transfers don’t allow time for preimplantation genetic testing (PGT-A). This testing, used to check embryos for genetic conditions, requires freezing the embryos while the analysis is completed.

Frozen embryo transfer (FET)

A frozen embryo transfer (FET) involves freezing embryos after fertilisation and storing them for later use. The transfer can take place in a future cycle, allowing time for the body to recover from ovarian stimulation or to align with personal and medical needs.

Advantages of frozen embryo transfers:

    • Flexible timing: Frozen embryo transfers can be scheduled when the uterine lining is in the best possible condition for implantation. This flexibility makes it easier to plan around your natural menstrual cycle, a controlled hormonal cycle, or personal commitments.

    • Recovery time: Frozen embryo transfers allow your body to recover from ovarian stimulation, creating a healthier uterine environment for implantation. It also reduces the risk of complications like ovarian hyperstimulation syndrome (OHSS).

    • Genetic testing: Frozen embryo transfers allow you to take advantage of preimplantation genetic testing (PGT), which checks embryos for genetic abnormalities. This testing improves the chances of a healthy pregnancy and lowers the risk of miscarriage, making it especially valuable for couples with known genetic conditions or recurrent pregnancy loss.

Disadvantages of frozen transfers:

    • Additional steps: Frozen embryo transfers involve more steps compared to fresh transfers. After the embryos are created, they need to be frozen using a technique called vitrification and stored until it is time for the transfer. This process requires extra handling and careful laboratory work, which adds complexity to the IVF cycle. Every step is carefully managed to ensure the highest success rates, relying on skilled professionals and precise protocols.

    • Potential for loss: While freezing and thawing techniques are very advanced and effective, there is a small chance that some embryos may not survive the process. Occasionally, embryos can be damaged during freezing or thawing, which may result in the loss of viable embryos. However, the risk is very low—around 2%—thanks to modern cryopreservation methods. It is something to keep in mind when planning your IVF journey.

Types of frozen embryo transfers

Frozen embryo transfers can be done using different protocols: natural cycle FET, medicated FET, or modified natural FET. Let’s learn more about them.

Natural cycle frozen embryo transfer

A natural cycle frozen embryo transfer (FET) aligns the timing of the embryo transfer with your natural menstrual cycle. Unlike medicated cycles, it does not use additional hormones. Instead, it relies on your body’s natural production of oestrogen and progesterone to prepare the uterus for implantation.

How it works

    1. Monitoring ovulation
      The process starts with tracking your menstrual cycle to pinpoint ovulation. This is done using blood tests to check hormone levels and ultrasounds to monitor the growth of ovarian follicles.

    1. Timing the transfer
      After ovulation is detected, the embryo transfer is scheduled to match the natural rise in progesterone. This ensures your uterine lining is ready to support implantation.

    1. Embryo transfer
      A few days after ovulation, the embryo is thawed and transferred into the uterus, following your body’s natural rhythm.

Pros of Natural Cycle FET

Less medication: A natural cycle FET uses your body’s natural hormonal changes, so you need fewer medications. This means fewer injections and less stress, making the process simpler and more comfortable. With fewer drugs, there’s also a lower risk of side effects from hormonal treatments.

Natural environment: Using your natural cycle may create a better environment for implantation. Studies suggest that natural hormones can improve the chances of a successful pregnancy. When the body prepares the uterine lining naturally, it often responds better than with artificial hormone treatments.

Corpus luteum support: After ovulation, the corpus luteum in your ovary produces natural progesterone. This helps keep the uterine lining healthy, giving more support for implantation and early pregnancy.

Cons of Natural Cycle FET

Requires regular cycles: A natural cycle FET works best if you have regular menstrual cycles. Irregular cycles can make it harder to predict ovulation, which is essential for timing the embryo transfer.

More monitoring: You’ll need regular monitoring to pinpoint ovulation, including blood tests and ultrasounds. This might mean more visits to the clinic, which can feel inconvenient for some.

Less control: The timing of a natural cycle transfer is less predictable than a medicated cycle. This can make scheduling trickier for both you and the clinic.

Medicated (Artificial) frozen embryo transfer

A medicated FET uses hormones to prepare the uterus for embryo transfer, without relying on your natural menstrual cycle. Medications create the ideal conditions for implantation, ensuring the uterine lining is ready for the embryo.

How it works

Oestrogen supplements
You’ll start with oestrogen, taken as pills, patches, or injections. Oestrogen helps build a thick uterine lining (endometrium) to support implantation.

Progesterone supplements
Once the lining is ready, progesterone is added to stabilise it and make it receptive to the embryo. Progesterone can be taken as injections, vaginal suppositories, or oral medications.

Timing and monitoring
The timing of oestrogen and progesterone is carefully controlled and monitored. When your lining is ready, the embryo transfer is scheduled to give the best chance for success.

Pros of artificial FET

Greater control: One of the main advantages of a medicated FET is the ability to precisely control the timing of the embryo transfer. By using supplemental hormones, doctors can schedule the transfer at the most convenient time for the patient, ensuring the uterine lining is perfectly prepared for implantation. This control can lead to higher chances of success.

Flexibility: Medicated FET is suitable for women with irregular menstrual cycles or those who have had issues with the thickness or quality of their uterine lining in the past. Since the process does not rely on natural ovulation, it can accommodate a wider range of patients, including those who do not ovulate regularly or at all.

Consistency: Using medications ensures a consistent and reliable preparation of the endometrium, which can lead to better outcomes, especially for women who have had difficulties with natural cycle FETs.

Cons of artificial FET

More medications: Medicated FET requires taking several medications, which can have side effects such as bloating, mood swings, and fatigue. The increased use of drugs also makes the process more complex and may be daunting for some patients.

Increased monitoring: The need for regular monitoring through blood tests and ultrasounds to check hormone levels and the condition of the uterine lining can be time-consuming and may require frequent clinic visits.

Cost: The use of additional medications and increased monitoring can raise the overall cost of the treatment, which may be a consideration for some patients.

Modified natural frozen embryo transfer

A modified natural frozen embryo transfer (FET) combines aspects of natural and medicated cycles. It may include medication to trigger ovulation while relying on your body’s natural hormones to prepare the uterine lining. This approach aims to balance minimal medication with the precise timing of a medicated cycle.

How it works

    1. Ovulation induction
      Medication like letrozole or clomiphene is used to stimulate ovulation, ensuring it happens at a predictable time. This is key for scheduling the embryo transfer.

    1. Monitoring
      After ovulation is induced, your natural hormone levels (oestrogen and progesterone) are monitored with blood tests and ultrasounds. This helps determine the best time for the transfer.

    1. Timing the transfer
      When ovulation is confirmed and your uterine lining is ready, the embryo transfer is scheduled to align with your body’s natural cycle. This ensures the uterus is prepared for implantation with minimal reliance on additional medication.

Pros of Modified Natural FET

Balanced approach
A modified natural FET provides more control over the timing of the embryo transfer than a fully natural cycle while using less medication than a fully medicated cycle. It balances natural hormonal processes with the precision of medical intervention.

Flexibility and control
This method is ideal for women who ovulate naturally but need some assistance to optimise the timing of the transfer. It can be especially helpful for women with slightly irregular cycles or those needing support with ovulation.

Cons of Modified Natural FET

Moderate medication use
Although a modified natural FET uses less medication than a fully medicated cycle, it involves more than a purely natural cycle. This can cause some side effects, though they are usually mild compared to fully medicated cycles.

Increased monitoring
Close monitoring of hormone levels means more visits for blood tests and ultrasounds. This can be time-consuming and inconvenient for some people.

Understanding day 3 and day 5/6 frozen embryo transfers

One important decision is the timing of the embryo transfer. Embryos are usually transferred on day 3 or day 5/6 after fertilisation. Dr. Vladimiro Silva says:

“Day 3 embryo transfers have been progressively abandoned (at our clinic they correspond to less than 1% of all embryo transfers), but can be justified in some situations.”

3-Day embryo transfer

What is it? A 3-day embryo transfer involves transferring embryos that have been allowed to grow in the lab for three days after fertilisation. At this stage, the embryo is typically at the cleavage stage, consisting of around 6-8 cells.

Why is it done?

    1. Faster transfer: Some clinics prefer a quicker transfer to place the embryos back into the natural environment of the uterus sooner. This can be important in some specific periods of the year (holidays, for example), or because of individual constraints from the patients (work, personal life, etc.).

    1. Limited resources: In situations where the laboratory environment might not support extended embryo culture, a 3-day transfer is preferable.

    1. Absence of high-quality embryos: sometimes, when patients just have one embryo in development, or when they want to give their lower quality embryo a chance for implantation and we see that it is unlikely to form a blastocyst, we end up transferring a day 3 embryo. This is never a first option, but can happen in very specific circumstances.

Day 5/6 embryo transfer

What is it? A 5/6-day embryo transfer involves transferring embryos that have been cultured in the lab for five or six days after fertilisation. By this time, the embryo has usually reached the blastocyst stage, characterised by a higher cell count and a more complex structure, including an inner cell mass and an outer layer of cells.

Why is it done?

    1. Better selection
      Allowing embryos to grow to the blastocyst stage helps embryologists select the most viable ones. Embryos less likely to result in pregnancy often do not develop to day 5/6.

    1. Higher implantation rates
      Blastocysts have higher implantation rates as they are more developed and closer to the stage they would naturally be in the uterus. Only the best embryos typically reach this stage.

    1. Natural uterine environment
      By day 5/6, the uterine environment aligns with the natural timing of embryo arrival, which can improve implantation chances.

    1. Reduced risk of multiples
      With higher success rates, fewer embryos usually need to be transferred, lowering the risk of multiple pregnancies.

Differences between 3-day and 5/6-day transfers

    1. Development stage: 3-day embryos are at the cleavage stage with fewer cells, whereas 5/6-day embryos are at the blastocyst stage with more complex development.

    1. Selection process: 5/6-day transfers allow for better selection of the healthiest embryos, which can increase the chances of successful implantation and pregnancy.

    1. Timing and environment: 5/6-day transfers align more closely with the natural timing of embryo arrival in the uterus, potentially creating a more favourable environment for implantation.

Which one to choose?

The choice between a 3-day and a 5/6-day embryo transfer depends on several factors, including:

    • Embryo quality: The number and quality of embryos available.

    • Laboratory conditions: The lab’s ability to support extended embryo culture.

    • The need to do PGT: Preimplantation genetic testing should always be done on blastocysts.

    • Clinician’s recommendation: Based on experience and success rates.

Your fertility specialist will help determine the best timing for your embryo transfer based on your unique circumstances and the specifics of your IVF cycle. Nowadays, more than 99% of our embryo transfers are done with 5 or 6 day blastocysts.

Frequently asked questions

How long after a frozen embryo transfer can I take a pregnancy test?

You should wait 10 to 14 days after a frozen embryo transfer (FET) before taking a pregnancy test. This waiting period gives your body enough time for the embryo to implant in the uterine lining and for hCG (human chorionic gonadotropin), the pregnancy hormone, to reach detectable levels.

Here’s a quick overview of what happens during this waiting period:

    1. Days 1-5: Implantation
      The embryo implants into the uterine lining within the first five days after the transfer.

    1. Days 6-10: hCG production
      Trophoblast cells around the embryo start producing hCG after implantation. The hormone levels rise as the pregnancy progresses.

    1. Days 10-14: hCG detection
      By day 10 to 14, hCG levels in your blood or urine are usually high enough for a pregnancy test to detect.

Types of Pregnancy Tests:

    • Home pregnancy test (HPT): These tests can be used around 10 to 14 days after the FET. They detect hCG in the urine and are convenient for at-home use. For more accurate results, it’s best to follow the instructions carefully and take the test first thing in the morning when hCG levels are most concentrated.

    • Blood test: Your clinic may schedule a blood test around 10 to 14 days after the FET. Blood tests measure the exact amount of hCG in your bloodstream and are more accurate than home pregnancy tests.

Why Wait 10-14 Days?

    • Accuracy: Testing too early can result in a false negative because hCG levels might not yet be high enough to detect.

    • Reducing stress: Waiting the full recommended period can help reduce the emotional stress and uncertainty that comes with testing too soon.

It’s always best to follow the specific guidance provided by your fertility clinic regarding the timing of your pregnancy test after an FET. They may schedule a blood test to confirm pregnancy, which provides the most reliable results.

If I get a positive result, how many weeks pregnant would I be?

If you get a positive pregnancy test after a frozen embryo transfer (FET), figuring out how many weeks pregnant you are can be tricky because it depends on how the weeks are counted. In IVF, pregnancy weeks are calculated from two weeks before the egg retrieval or ovulation, similar to how natural pregnancies are dated from the last menstrual period (LMP).

The dating of the pregnancy also depends on whether you had a day 3 or day 4 embryo transfer:

    • 3-day transfer: the embryo is 3 days old at transfer.

    • 5-day transfer: the embryo is 5 days old at transfer.

Example calculations:

5-day embryo transfer

    • Positive test at 14 days post-transfer = 5 days (embryo age) + 14 days (post-transfer) + 14 days (pre-retrieval) = 33 days (4 weeks, 5 days pregnant).

3-day embryo transfer

    • Positive test at 14 days post-transfer = 3 days (embryo age) + 14 days (post-transfer) + 14 days (pre-retrieval) = 31 days (4 weeks, 3 days pregnant).

Simplified rule of thumb:

    • 5-day transfer: Add 2 weeks to the number of days post-transfer.

    • 3-day transfer: Add 2 weeks plus the embryo’s age at transfer.

Confirm with your clinic:

Your fertility clinic can provide the most accurate dating based on your specific treatment and confirm your pregnancy stage with a follow-up ultrasound.

Deciding what’s best

Embryo transfers are a key step in IVF, offering options like natural, medicated, or modified natural FET. Understanding these choices helps you have informed discussions with your fertility specialist, who will guide you to the best approach based on your needs. At Ferticentro, we combine advanced technology with personalised care, supporting you every step of the way as you confidently navigate your path to parenthood.

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