What is an embryo transfer? 

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Embryo transfer is the process that occurs in the last step of IVF, by which an embryo is transferred into the uterus to achieve a pregnancy.ⁱ  

A fresh embryo transfer refers to a protocol whereby the embryo is transferred from the culture dish into the uterus after an egg retrieval, in the same cycle. Alternatively, a frozen embryo transfer (FET) involves transferring embryos that were frozen (cryopreserved embryos), in a previous egg retrieval cycle. Frozen embryos are stored for use later; thus, FET requires thawing of the cryopreserved embryo prior to transfer to the uterus.ⁱⁱ

Fresh embryo transfer is usually done on either day 3 or day 5 following egg retrieval and fertilization. On day 3, an embryo is referred to as being in the cleavage stage or as a cleavage-stage embryo and is approximately eight cells in total. On day 5, the embryo should be at the blastocyst stage and is more developed.ⁱⁱⁱ

Importantly, embryo transfer is not the same as implantation. While embryo transfer describes the process of transferring the embryo into the uterus, embryo implantation occurs when the developing embryo attaches to the uterine lining, which is what establishes a pregnancy. ^iv

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What is the embryo transfer process?

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The process for fresh embryo transfers differs from an FET when it comes to the monitoring and drugs that may be used to support the body throughout the process.  

Fresh embryo transfer procedure

Following ovarian stimulation, eggs are retrieved and fertilized in vitro to create embryos. The embryos will be cultured, usually for 3-5 days, and then one or more embryos will be transferred into the uterus. Any remaining embryos can be vitrified (frozen) for a future FET.  

The patient typically will be instructed to take estrogen (vaginally, orally, or transdermally) and progesterone (vaginally or via injection) to support the lining of the uterus, also called the endometrium.^v Once the endometrium is ready, an embryo that is developing in the lab is transferred directly into the uterus on day 3 or day 5 of embryo growth. The patient will typically continue progesterone and estrogen at a minimum until the day of a pregnancy test, or for 8-12.^vi,vii

Frozen embryo transfer (FET) cycle

Natural and modified natural cycle for FET:

This protocol works best in patients with a regular menstrual cycle, since the embryo transfer day is based on the timing of their natural cycle. Because fewer or no medications are used, it is often considered more patient-friendly and may have lower costs. The protocol involves regular transvaginal ultrasounds to check the thickness of the endometrium, and regular monitoring of hormones in the blood, namely estrogen, progesterone, and luteinizing hormone (LH), to track ovulation.  

Tracking the day of ovulation is important because embryo transfer occurs a specific number of days after ovulation (typically 5-6 days after blood samples show LH surge for a blastocyst).^viii  

In modified natural cycles, ovulation will be triggered with injection of human chorionic gonadotropin (hCG) when a dominant ovarian follicle reaches approximately 16-20 mm in diameter.^ix This trigger helps narrow down the timing of the transfer since the transfer will usually occur 6-7 days after hCG trigger injection.^x  

Even though in a natural or modified natural cycle the ovaries and corpus luteum (follicle cells that are modified after ovulation) produce adequate progesterone to support implantation, typically luteal phase support (e.g., progesterone medication) will still be prescribed.  

Medicated Cycle for FET

Medicated frozen embryo transfers are also known as hormone replacement, artificial, or programmed FET cycles. Medicated FET cycles are more common than natural FETs. They involve taking hormone medications such as estrogen and progesterone, which control the hormone levels in a cycle therefore allowing for predictability.

Medicated FETs begin with daily estrogen supplementation starting on cycle days 1-3. The estrogen supplements will suppress natural follicle development and ovulation. Progesterone is started prior to embryo transfer.^xi Typically, progesterone is taken for three days prior to a day 3 embryo transfer and five days prior to a blastocyst transfer. It can be taken as vaginal suppositories or progesterone in oil (PIO) injections or both.^xii

Although less common, some patients may take mild ovarian stimulation medication, similar to stim meds taken to help grow multiple follicles for IUI or egg retrieval (e.g., Letrozole^®).^xiii This is believed to potentially improve endometrial lining thickness and/or receptivity. However, due to overall paucity of clinical evidence, it remains unclear as to whether this improves live birth rates in those with regular menstrual cycles.

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What tests are done before the transfer protocol?

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To heighten the probability of embryo implantation after transfer, several tests will be done before and during the transfer protocol. These tests assess everything from the hormone levels in the blood to the size of follicles and thickness of the uterine lining.  

Blood testing is completed prior to embryo transfer, as ensuring optimal hormonal range is important to successful implantation (described below). An ultrasound will also be completed prior to transfer, to assess the endometrial thickness and endometrial pattern. This helps predict if the endometrium will be receptive for implantation.^xiv Sometimes an extra ultrasound will be performed the day before embryo transfer to ensure that compaction of the uterine lining has occurred.^x  

Blood monitoring  

The level of progesterone in the blood (P4) is measured prior to embryo transfer.

For fresh embryo transfer, progesterone levels may be measured even before egg retrieval, typically the day of hCG trigger. At this point, it is desirable to have very low P4.  In cases where progesterone levels are prematurely elevated (i.e., above 1.5ng/mL) on the day of trigger, a decision to freeze the embryo(s) instead of transferring fresh is often made.^{xvi,xvii,xviii}

By the time of fresh or frozen transfer, progesterone levels will be increased. Some studies suggest that blood progesterone level should be above 10 ng/ml directly before the transfer in order to support implantation.^xix,xx Physicians will often use a combination of published data, internal clinic data and individualized recommendations to determine optimal progesterone ranges at the time of transfer.    

It should also be noted that significantly elevated progesterone at transfer could potentially have a negative impact too. One study by Kofinas et al (2015) observed an ongoing pregnancy rate of 70 percent for P4 at 10-15ng/mL vs 33 percent for P4 >40ng/mL on the day of transfer.^xxi However, not all studies have observed this same trend for elevated P4 levels.^xxii Note that these data refer to progesterone levels at the time of transfer, which will be lower than P4 bloodwork at the time of beta-hCG.

Blood estrogen level is also measured prior to embryo transfer. A study by Diluigi et al (2005)^xxiii found that a higher estrogen level is associated with better odds of implantation and pregnancy. They found that the odds of pregnancy increased as levels of serum estrogen increased. A 2020 study showed that measuring estrogen and progesterone the day prior to embryo transfer improved live birth rates, since cycles with imbalanced hormones prior to FET were re-scheduled.^xxiv

Thyroid hormone levels are often monitored during IVF, as thyroid stimulating hormone (TSH) may increase during controlled ovarian stimulation (COS).^xxv During IVF and ovarian stimulation, the levels of serum estrogen are much higher than normal which puts stress on the thyroid gland and can cause TSH levels to rise. Because normal thyroid function is important for the health of a pregnancy, TSH levels are often monitored to ensure that they are within normal range.^xxvi

Lining measurement  

Prior to embryo transfer, endometrial thickness will be measured through an ultrasound to ensure that the endometrium is adequately prepared for the embryo to implant. A meta-analysis that included 88 056 IVF cycles found that a thinner endometrium is associated with lower pregnancy rates, decreased implantation rates, and lower live birth rates.^xxvii  

Aside from endometrial thickness, the endometrial pattern may also be an important consideration in embryo transfer. The endometrial pattern describes the anatomic appearance on ultrasound of the endometrium, which changes throughout the menstrual cycle in response to estrogen and progesterone. Specifically, a pattern of three lines, known as a trilaminar pattern, is favorable just prior to starting progesterone supplementation.^xxviii

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What is the success rate of an embryo transfer? 

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The success rates of embryo transfer vary based on factors that include age, whether pre-implantation genetic testing is completed, the patient’s medical conditions, the indication for IVF, and the transfer protocol used. The success rates relative to these various factors are discussed below.^{xxix,xxx,xxxi,xxxii}

Age  

A study by Yan et al (2012) evaluated the success of embryo transfer of non-PGT tested embryos in 11 830 transfer cycles in different age categories. They found that the pregnancy rate was highest in the 21-30 age category (60 percent), and lowest in the 41+ age category (27 percent). The pregnancy rate was 57 percent in patients aged 31-35 and 47 percent in patients aged 36-40. They also found that the rate of miscarriage was lowest in the 21-30 age category (9 percent), and highest in the 41+ age category (36 percent).^xxxiiiBased on a study by Cheung et al (2021), the usage of donor eggs may mitigate this decrease in success rate of embryo transfer, since the average age of egg donors is younger. This study of 1 080 cycles found no significant difference in the rate of pregnancy and live birth rates in four groups of patients categorized by age (<40, 40-44, 45-49, and ≥ 50 years old). They did, however, note a higher rate of first trimester pregnancy loss in patients ≥ 50 years old (20 percent).^xxxiv

Genetically tested embryos  

Pre-implantation genetic testing (PGT) is a technique used to genetically screen embryos in IVF in order to select which embryo(s) to transfer.^xxxv Research has shown that PGT-A, a form of PGT that screens for an abnormal number of chromosomes (aneuploidy), may help reduce the total number of embryo transfers, promote single embryo transfers, and reduce the rate of miscarriage during IVF.^xxxvi,xxxvii

Some studies have suggested that PGT-A may improve outcomes in IVF patients.^xxxviii,^xxxix One recent study by Sato et al (2019) found that the use of PGT-A improved the live birth rate per transfer in patients with recurrent implantation failure to 63 percent compared to 32 percent.^xl  

However, it is important to note whether studies are looking at improved success rates “per transfer” versus “cumulative live birth rates”. A recent large prospective randomized controlled trial showed that cumulative live birth rates were slightly lower in the PGT-A group (77%) than in the untested group (82%).^xli Other studies have similarly noted that PGT-A does not always provide a benefit,^xlii,xliii particularly for those producing fewer than four embryos per cycle.^xliv  

Fresh vs frozen transfers  

Some studies show that there are no differences in success rates for fresh versus frozen transfer. A meta-analysis of several studies by Wong et al (2017) found that there was no significant difference in the cumulative live birth rate between women who underwent fresh embryo transfer compared to frozen embryo transfer. They found that the live birth rate for fresh embryo transfer was 58 percent, compared to a live birth rate of 56-65 percent for frozen embryo transfer.^xlv

A study by Boynukalin et al (2020)^xlvi analyzed the live birth rate in 2 523-day 5 embryo transfers; they found that success rates comparing fresh vs frozen transfers depended on the number of eggs retrieved. They found that in patients with 11-25 eggs retrieved, the live birth rate was significantly higher in frozen embryo transfers compared to fresh embryo transfers. There was no significant difference in fresh versus frozen embryo transfers in patients with 1-10 eggs retrieved.^xlvii

Day 3 (D3) vs day 5 (D5) embryos  

In general, transferring day 5 blastocyst embryos is believed to have a better chance of success than day 3 cleavage stage embryos. This is because they have already been through a selection process due to natural attrition rates between day 3 and 5. In other words, some embryos will arrest between day 3 to day 5, and by waiting until day 5, the embryologist will know which ones did not make it, thereby avoiding the transfer of those embryos. On day 3, it is difficult to predict which embryos will make it to the blastocyst stage.  

A Cochrane review of 4 031 couples showed that pregnancy and live birth rates were higher in day 5 embryo transfers compared to day 3 when for fresh embryo transfers. Their evidence suggests that if 29 percent of patients had a live birth following a day 3 transfer, 32-42 percent would have a live birth following a day 5 transfer.^xlviii In contrast, a smaller study of 190 women who underwent intracytoplasmic sperm injection (ICSI) during IVF found that the success rate of fresh embryo transfers was similar between day 3 and day 5 embryos. The implantation rate was 47 percent for day 3 embryos, and 45 percent for day 5 embryos. The live birth rate for day 3 transfers was 37 percent, compared to 35 percent for day 5 transfers.^xlix

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What drugs are used for an embryo transfer?

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There are multiple medications that are routinely used in embryo transfers, and it depends on the protocol used. Each drug given is meant to support some biological function that would support the successful implantation of the embryos.  

Preparing the endometrium

Estradiol (Estrace®) is the active form of estrogen. It usually starts on day 1-3 of the natural period in a FET and after retrieval in a fresh transfer. Estradiol is continued until the day of the pregnancy test following embryo transfer or until 10-12 weeks' gestation if pregnant. Estradiol works to prime the endometrium and suppresses the natural growth of the ovarian follicles. It also suppresses ovulation in FETs.^l

Progesterone usually starts the day after egg retrieval in a fresh cycle. It is an important hormone that facilitates implantation and helps maintain a pregnancy once it is established. It is commonly continued to 10-12 weeks' gestation.^li In an FET, progesterone is usually started 3-5 days before transfer, once the uterine lining has reached an appropriate thickness and patterning.  

Trigger shots  

In modified natural FET cycles, ovulation will be triggered with hCG when a dominant ovarian follicle reaches approximately 16-20 mm in diameter. The trigger helps narrow down the timing of transfer. HCG (e.g., Ovidrel®, Novarel®, or Pregnyl®.) functions in a manner similar to luteinizing hormone (LH) and can trigger ovulation.  A gonadotropin releasing hormone (GnRH) agonist can also be used to trigger ovulation in antagonist cycles, particularly when there is a risk of ovarian hyperstimulation syndrome (OHSS).^lii

Fresh transfer protocols  

Fresh transfer protocols are used in order to both retrieve eggs and attempt pregnancy within the same cycle. A fresh embryo is selected and transferred into the uterus at either day 3 or day 5 post-retrieval. Any remaining embryos are cryopreserved for upcoming FET cycles.^liii

One concern of fresh embryo transfer is an increased risk of OHSS. This is the leading cause of morbidity in IVF treatment, and in rare cases can even lead to patient death (an estimated 3 deaths per 100,000 cycles). The use of hCG to trigger ovulation in COS can result in increased permeability (leakiness) of the blood vessels. This causes the ovaries to swell and leak fluid into the body, causing pain, shortness of breath, swelling, bloating, increased risk of blood clots, and nausea among other symptoms. Frozen embryo transfer protocols prevent OHSS by eliminating ovarian hyper-stimulation.^liv

Frozen transfer protocols  

In a patient undergoing IVF transfer with frozen embryos, hormone therapy is often required to prepare the lining of the uterus to receive the embryo for implantation. This starts with taking an estrogen such as Estrace® on day 1-3 of the cycle.^lv Progesterone is then administered to make the uterus receptive to implantation of the embryo. It is commonly administered vaginally, using suppositories such as Endometrin® or Prometrium® but it may also be injected into the muscle.^lvi  

A large study by Ernstad et al (2019) identified possible risks associated with frozen embryo transfer. They found that frozen versus fresh embryo transfer is associated with higher risks of postpartum hemorrhage, delivery after full term (post-term birth), macrosomia (larger baby), and hypertensive disorders of pregnancy.^lvii There is currently a multi-center randomized clinical trial to investigate the impact of medications used for FET on risk of preeclampsia during pregnancy.^lviii

Other drugs  

Other drugs may also be used for embryo transfer to support a specific aspect of the process. Some of the most common drugs used during this point in the IVF process are below.  

Prednisone: Prednisone is a steroid medication that is involved in modulating the activity of the immune system. Multiple studies have shown that prednisone can suppress the activity of certain immune cells within the uterus and promote embryo implantation.^lix However, the use of prednisone in embryo transfer is still being debated and should be done on a case-by-case basis. A study by Robertson et al (2016) argues that it should only be used if identified immune dysfunction is present.^lx  

Intralipid: Intralipid infusions are an emulsion of fats given intravenously. They have been hypothesized to suppress natural killer immune cell activity for patients with RIF in IVF. The effectiveness of intralipids is still controversial. A 2020 review study concluded that intralipids may improve outcomes in women with previous implantation failures, but the results did not show any significant differences in live birth rates with or without intralipids.^lxi  

Metformin: Metformin (Glumetza^®, Fortamet^®, Glucophage^®,) is an oral medication that is often used in patients with Polycystic Ovary Syndrome (PCOS) to improve the body’s sensitivity to insulin and decrease excess androgen production (e.g., testosterone).   One study observed that metformin increased pregnancy rates for PCOS patients with a BMI of ≥26.^lxii However, a review by Tso et al (2020) involving a total of 1 132 women with PCOS found no conclusive evidence to support the use of metformin to improve live birth rates.^lxiii  

Heparin and derivatives: Enoxaparin (Lovenox ^®, Clexane ^®) is a type of blood thinner medication that acts as an anticoagulant, meaning that it prevents blood clots from forming. Heparins are sometimes used in patients with RIF or those with known thrombophilia (increased chance of blood clots), with the intent of improving implantation rate.^lxiv The utility of heparin in embryo transfer remains controversial. A small 2018 study of 83 women with known thrombophilia found that live birth rates were increased with enoxaparin 40mg/day treatment (24 percent) compared to controls (no enoxaparin, 3 percent).^lxv

However, when analyzing women with no known clotting issues, a 2018 meta-analysis found no difference in outcomes when heparins were used.^lxvi,lxvii

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What happens on embryo transfer day? 

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Patients are usually instructed to arrive approximately 45 minutes before the embryo transfer. During the procedure, a speculum is inserted into the vagina in order to visualize the cervix. The cervix is then cleaned, and a catheter is inserted through the opening of the cervix into the uterus so that the embryo can be loaded and transferred through the catheter into the uterus. The process is typically not painful, but some experience mild discomfort and cramping. The entire process is typically completed without sedation.^lxviii  

The embryo transfer is most often completed using ultrasound for direct visualization during the transfer process.^lxix Patients are instructed to drink water prior to the appointment in order to fill their bladder, which improves the ultrasound image. The ultrasound probe is usually placed over the abdomen and allows the physician to see the placement of the catheter in the uterus. This allows the doctor to directly observe the location of the transferred embryo(s) and confirm that it is successfully in the uterus, to maximize chances of implantation and pregnancy.^lxx At many clinics, air bubbles are inserted into the catheter before and after the embryo. This is used to help visualize entry of the embryo into the uterus on the ultrasound.^lxxi,lxxii

Once the ultrasound is complete, the embryologist will check to ensure that the embryo did in fact get transferred to the uterus and did not get “stuck” in the catheter. If one or more embryos were retained in the catheter, the transfer process is repeated immediately. Studies have shown that there is no impact on success rates when this occurs in cleavage stage embryo or blastocyst transfers.^lxxiii The insertion of the catheter is typically done very slowly and gently in order to prevent uterine cramping, which can potentially lead to expulsion of the embryo from the uterus.^lxxiv  

Fertility clinics generally advise that the time required for the actual transfer procedure is 5-15 minutes.   Many clinics will allow patients to watch the actual embryo process on a large monitor. They may also provide photos of the embryos to be transferred, and/or allow individuals to take pictures of the monitor.

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What should you do after embryo transfer during the two-week wait (TWW)?

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Medical providers will give direct instructions on what to do in the days following the protocol to support successful embryo transfer and implantation.

Some older research had previously suggested that bed rest was necessary following embryo transfer, but more recent evidence does not support this. A study by Frankel et al (2016) found no significant difference in implantation rates between the bed rest group (65 percent) versus the group that moved around immediately after transfer (66 percent).^lxxv  

Some clinics recommend abstaining from intercourse after embryo transfer due to the potential for infection and the possibility of an additional conception. However, there is also evidence suggesting that intercourse around the time of transfer, specifically exposure to semen, may improve clinical pregnancy rates. An argument for abstaining is that sexual intercourse during the TWW poses the risk of an additional pregnancy. This could be a heterotopic pregnancy^lxxvi,lxxvii which occurs when there is one pregnancy in the uterus and one outside of the uterus (e.g., fallopian tube), which poses serious complications. In addition, a 2014 study observed that patients who had sexual intercourse in the days following embryo transfer had higher odds of miscarrying.^lxxviii

Conversely, some studies indicate that seminal plasma interacts with the endometrium to promote immune regulation and tolerance, therefore facilitating implantation.^lxxix,lxxx  Data from various studies suggests that exposure to seminal fluid around the time of embryo transfer can improve clinical pregnancy rates compared to women not exposed to seminal fluid.^{lxxxi,lxxxii,lxxxiii} Overall, there is a lack of consensus as to whether sex, and thus exposure to semen, may increase or decrease chances of IVF success. There is currently a randomized controlled trial recruiting patients in an attempt to answer this question.^lxxxiv  

Most clinics will also have suggestions on avoiding things such as alcohol, certain undercooked foods, heavy lifting, and some types of exercise.

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What happens on beta test day? 

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HCG is the hormone that is tested in a home urinary pregnancy test and is one of the earliest markers of embryonic implantation.  It may be present in a woman’s urine and/or blood by 6-8 days after implantation occurs,^lxxxv but blood levels of beta-hCG are typically tested 9-14 days after embryo transfer. In fresh embryo transfer, data from several studies suggested that a beta-hCG level of 111-213 IU/L at 10-12 days after fresh blastocyst transfer was a good indicator to predict an ongoing pregnancy. In frozen embryo transfer, the beta-hCG threshold was 137-399 IU/L in predicting successful pregnancy at 10-12 days.^lxxxvi

Progesterone is often tested on the day the beta test is completed, as it is important for maintaining a pregnancy once it occurs. Progesterone does so by modulating a woman’s immune system, preventing the uterus from contracting, and allowing for good circulation between the uterus and the placenta. A study by Kim et al (2020) suggests that a progesterone level of at least 25.2 ng/mL 14 days after embryo transfer is associated with a higher rate of ongoing pregnancy.^lxxxvii Progesterone levels on the day of beta-hCG are higher than on transfer day (discussed above).

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Pregnant and next steps

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After embryos are transferred, care providers will continue to monitor hormone levels and wait for the fetal heartbeat to be detected to confirm pregnancy. In a normally progressing pregnancy, the beta-hCG is expected to approximately double every 48 hours.^lxxxviii Biochemical pregnancies and ectopic pregnancies are also possible in the weeks following implantation, punctuating the importance of continuing monitoring. A chemical pregnancy is a pregnancy loss that happens after heightened hCG levels have been detected, but before an embryo can be detected by ultrasound.^lxxxix

The fetal heart rate is usually detectable by approximately six and a half weeks gestational age on ultrasound.^xc In most cases, discharge from the fertility clinic to an obstetrician usually occurs at about 10 weeks' gestation.^xci

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Not pregnant and next steps 

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After a negative beta-hCG test, the physician will likely suggest that the patient stops transfer medications, though all should be continued until directed by a physician. The patient will usually begin menstrual bleeding within a few days of stopping medication, although the exact number of days is variable.

If a woman has frozen embryos, her next steps after a negative beta-hCG test will likely be to try another frozen embryo transfer (FET) with or without additional testing. Some women do this in the next cycle, while others wait. If there are no frozen embryos after an unsuccessful cycle of IVF, another IVF cycle can be tried. Although the process may be discouraging, a study by Smith et al (2015) of 156 947 women showed that approximately 63 percent of women under 40yr achieved a live birth by the 3^rd IVF cycle.^xcii An IVF cycle is defined as one egg retrieval and transfer of all embryos made from the eggs retrieved.  

A woman can try a second egg retrieval cycle directly following a failed cycle, although some evidence indicates it may be better to wait. One study by Reichman et al (2012) compared 192 women who started a second cycle fewer than 55 days from their previous egg retrieval to 557 women who started a second cycle between 56-140 days following their prior egg retrieval. They found that the rate of implantation was slightly lower in the first group (9 percent) compared to the latter (13 percent).^xciii

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Conclusion  

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While every IVF cycle may not result in pregnancy, the procedure continues to grow more advanced and the rates of successful pregnancies with IVF have grown substantially. Becoming familiar with what to expect helps ensure the embryo transfer process has the best chance of a positive outcome. While the process can be physically and mentally challenging, advance preparation, education, and understanding will help.