In vitro fertilization (IVF) is a complex series of procedures used to help people with fertility problems have a baby. During IVF, mature eggs are collected from a woman’s ovaries and fertilized by sperm in a lab. Then the fertilized egg (embryo) or eggs are implanted in the uterus. One key decision in IVF is determining how many embryos to transfer. Transferring more embryos can increase the chance of pregnancy, but also increases the health risks for both mother and baby. Many factors impact whether transferring 2 embryos is a good option for a particular patient.
What is IVF?
IVF was first successfully used in humans in 1978 to help women with damaged fallopian tubes become pregnant. Since then, IVF has evolved into a widely used treatment for many causes of infertility. Here is a basic overview of the IVF process:
Ovarian Stimulation
A woman takes fertility medications for around 10-14 days to stimulate her ovaries to produce multiple mature eggs. During this time, she will have ultrasounds and blood tests to monitor the egg development.
Egg Retrieval
Once the eggs are ready, they are retrieved through a minor surgical procedure. The woman is given sedation while a thin needle is inserted through the vagina to remove the eggs from the ovaries. This takes about 30 minutes.
Fertilization
The retrieved eggs are mixed with sperm in a laboratory dish. This allows fertilization to occur. For some cases of male infertility, a process called ICSI (intracytoplasmic sperm injection) is used to inject a single sperm directly into each mature egg. The fertilized eggs are considered embryos.
Embryo Culture
The embryos are incubated for 3-5 days while scientists monitor their development. The embryos that reach the blastocyst stage, typically by day 5 or 6, have the best chance of implanting successfully.
Embryo Transfer
Typically 1 or more selected embryos are transferred to the woman’s uterus through a thin tube (catheter). This procedure is quicker and simpler than the egg retrieval. The remaining viable embryos may be frozen for potential future use.
Pregnancy Test
Around two weeks after the embryo transfer, a blood test can show if the treatment resulted in a successful pregnancy. If it didn’t, the frozen embryos can be thawed and transferred in another cycle.
How Many Embryos Should Be Transferred?
One of the most important decisions in IVF is determining how many embryos to transfer. Transferring more embryos increases the chance of pregnancy. However, it also increases the health risks from a multiple pregnancy. Twins and triplets from IVF have higher risks of premature birth, low birth weight, disability, and even death. Balancing these risks with the desire for pregnancy is complex. Here are some factors to consider:
Age
A woman’s age is the most significant factor in IVF success rates. Younger women have higher quality eggs and embryos as well as higher implantation and pregnancy rates. Their healthier embryos allow transferring fewer embryos while maintaining good pregnancy rates. Older women often transfer more embryos trying to compensate for lower embryo quality.
Embryo Quality
Good embryo grading parameters like evenly sized cells and a lack of fragmentation are linked to higher potential for implantation and live birth. Transferring fewer quality embryos may be reasonable. Poorer quality embryos often lead to transferring more in hopes that one will implant successfully.
Previous IVF Cycles
Women who have had failed previous IVF cycles often choose to transfer additional embryos in subsequent cycles to try and increase their chances of success.
Desire for Twins
Some families would be very happy with twins and purposefully transfer multiple embryos to increase the odds. However, the medical risks make this choice controversial.
Remaining Frozen Embryos
Having remaining high quality frozen embryos can allow transferring fewer embryos while feeling confident that there are still options if the cycle fails. Those without frozen embryos often transfer more embryos.
Fertility Prognosis
Some diagnoses like blocked fallopian tubes have a very good prognosis with IVF. Other causes like diminished ovarian reserve have much lower IVF success rates, leading some patients to transfer more embryos.
Pros and Cons of Transferring 2 Embryos
Transferring 2 embryos generally offers a higher pregnancy rate than a single embryo transfer, but also increases the risk of twins. Here is an overview of the key pros and cons of transferring 2 embryos:
Pros of 2 Embryo Transfers
- Higher pregnancy rates – Transferring 2 embryos has around a 10-20% higher pregnancy rate than a single embryo transfer.
- Allows freezing extra embryos – Patients must retrieve around 8-15 eggs on average to end up with 2 high quality embryos to transfer. The remaining embryos can be frozen.
- Good option for poor prognosis patients – Those with lower egg quality may transfer 2 embryos to increase their odds of success.
- Increases odds of having at least one baby – If one embryo fails to implant or miscarries, the second may still result in a live birth.
Cons of 2 Embryo Transfers
- Increased risk of twins – Transferring 2 embryos has around a 20-30% chance of twins compared to under 1% with one embryo.
- Higher risk pregnancy – Twin pregnancies have more complications like preeclampsia and preterm labor.
- Higher risk of low birth weight – Average twin birth weights are around 5 pounds compared to 7.5 pounds for singleton IVF babies.
- May result in too many embryos – If the cycle succeeds in twins, extra frozen embryos go unused.
- Not ideal for young, good prognosis patients – Single embryo transfer is recommended for those with a favorable outlook.
Risks of a Twin Pregnancy
While some families may hope to conceive twins intentionally through IVF, most try to avoid it due to the increased medical risks:
| Risk | Singleton Pregnancy Rate | Twin Pregnancy Rate |
|---|---|---|
| Premature Delivery | 6-12% | 50-60% |
| Low Birth Weight | 6-8% | 50-75% |
| Pre-Eclampsia | 3-5% | 8-20% |
| Newborn Death | 0.4-0.6% | 3-5% |
| Cerebral Palsy | 0.1-0.2% | 1% |
| Developmental Delay | 3-5% | 5-8% |
These stark differences between singleton and twin pregnancy risk lead most experts to recommend avoiding embryo transfers likely to result in twins.
Guidelines for Number of Embryos to Transfer
Professional societies have issued guidelines on the recommended number of embryos to transfer based on the patient’s history and prognosis:
SART Recommendations
The Society for Assisted Reproductive Technology (SART) makes these recommendations in women under age 35:
- For the most favorable patients: Single embryo transfer
- For good prognosis patients: 1 or 2 embryos
- For poor prognosis patients: no more than 2-3 embryos
SART recommends no more than 2 embryo transfers for women over age 35. Single embryo transfer may still be considered in exceptional cases with very high quality embryos.
ACOG Recommendations
The American College of Obstetricians and Gynecologists (ACOG) also suggests favorable patients consider elective single embryo transfer. For others, no more than 2 cleavage-stage embryos or 1-2 blastocyst embryos should be transferred.
However, patients who have failed 2 or more IVF cycles with high quality embryos may consider no more than 3-4 embryo transfers after counseling on risks.
Arguments For and Against Transferring 2 Embryos
There are reasonable arguments couples and doctors make both for and against transferring 2 embryos in various IVF cases:
Reasons Some Favor Transferring 2 Embryos
- Most patients desperately want children and prefer maximizing their chances.
- IVF treatment is emotionally, physically, and financially draining, causing patients to take risks.
- Advanced maternal age leads to lower embryo quality and survival rates.
- Previous failed IVF cycles, miscarriages, or genetic issues require multiple embryos.
- Preimplantation genetic testing (PGT) helps select more viable embryos for transfer.
Reasons Some Prefer Single Embryo Transfers
- Most would prefer to safely avoid a high-risk twin pregnancy and birth.
- Excellent frozen embryo survival allows spreading transfers over multiple cycles.
- Single embryo transfer followed by a frozen embryo transfer cycle has similar high cumulative pregnancy rates.
- Patients underestimate the risks of twin pregnancies and births.
- You can’t predict who will get severe complications from twin pregnancy.
There are good arguments on both sides of this complex issue. Patients need unbiased counseling and education on the risks and benefits to make informed choices.
Special Cases to Consider Transferring 2 Embryos
While single embryo transfer is recommended for most good prognosis patients, there are some special cases where transferring 2 embryos can be appropriate. These include:
- Advanced maternal age – Women over 35-40 may consider 2 embryos due to lower conception chances and higher miscarriage rates.
- Previous failed IVF cycles – Multiple failed single embryo transfers may warrant trying 2 embryos.
- Couples using their last embryos – Those with only 2 viable embryos left and no plans for more IVF retrievals.
- Poor embryo quality – When multiple embryos are poor quality, transferring 2 marginal ones may be suggested.
- Genetic issues – Some known problems like translocations are helped by transferring 2 affected embryos.
However, all patients should be fully counseled on the risks prior to proceeding with a 2 embryo transfer.
Examples of Reasonable Cases for Transferring 2 Embryos
Couple with Multiple Failed IVF Cycles
Sarah, age 31, and Mark, age 33, have male factor infertility and have undergone 3 previous IVF cycles. Each cycle had a quality single embryo transfer that resulted in no pregnancy. Given their prior failures, their RE recommends transferring 2 embryos in their next cycle to increase their chances of success.
39 Year Old Patient Using Her Last Embryos
Michelle is 39 and has diminished ovarian reserve. She went through 2 IVF cycles to retrieve 5 eggs, of which 2 fertilized normally. Her doctor recommends transferring these final 2 fair quality embryos together despite the twin risks due to her poor prognosis and limited embryos.
Donor Egg Recipient Couple
Rachel, 44, and Jacob, 47, are using donated eggs due to Rachel going through early menopause. Of the 8 high quality donor eggs fertilized, 3 made it to good quality blastocysts on day 5. Their doctor recommends transferring 2 of these embryos due to Rachel’s uterine age being a factor for implantation.
Patients with Known Genetic Disorder
Karen, 28, and Dave, 31, have undergone PGT testing of their embryos after both being identified as carriers for cystic fibrosis. Of their 9 embryos, only 2 tested normal. Their reproductive endocrinologist recommends transferring both PGT normal embryos together for the highest chance of a successful pregnancy.
Is the Day 3 or Day 5 Transfer Better with 2 Embryos?
Sometimes transferring 2 embryos is decided upon. Another choice is whether to do the transfer on day 3 or day 5. Here is an overview of the differences:
Day 3 Embryo Transfer
- Embryos are 8-cell stage
- Some embryo development issues may still resolve later
- No embryos are lost waiting to day 5
- Used when few embryos fertilize or aren’t growing quickly
- May have lower implantation rate than day 5 transfer
Day 5 Blastocyst Transfer
- Embryos are more developed to 100+ cell blastocyst
- Better match uterine environment and natural timing
- Allows selecting the best quality embryos
- Risk of nothing to transfer if they arrest before day 5
- Considered ideal if enough quality embryos available
Overall, day 5 embryo transfer is generally preferred with 2 embryos if at least 3-4 embryos fertilize and continue to develop well. This allows choosing the best two blastocysts for transfer. However, day 3 transfers can be warranted if growth is poor.
IVF Success Rates with 2 Embryos Transferred
Here is an overview of IVF success rates from the SART national summary based on the number of embryos transferred:
2015 SART IVF Success Rates By Age and Embryos Transferred
| Age Group | 1 Embryo | 2 Embryos | 3 Embryos |
|---|---|---|---|
| <35 | 53.7% | 61.1% | 69.7% |
| 35-37 | 47.7% | 56.1% | 63.4% |
| 38-40 | 41.1% | 48.9% | 61.1% |
| 41-42 | 31.9% | 40.8% | 51.7% |
The data confirms higher success rates with 2 embryo transfers compared to 1 embryo across all age groups. However, the Twin Risks are also substantially increased with 2 embryo transfer. Patients and doctors should review the risks and benefits carefully in each case.
Conclusions
Deciding whether to transfer 1 or 2 embryos in an IVF cycle is complex. Patients need to review IVF success rates and risks with their doctor to make the most informed choice for their individual case. For favorable patients, a single embryo transfer is often recommended to avoid twin risks. However, there are some cases where transferring 2 embryos can be justified after thorough counseling. Those include advanced maternal age, previous failed IVF cycles, or using the last remaining embryos. While 2 embryos does increase live birth rates, all patients must weigh that benefit against the clearly higher risks of twin gestations and births. Whatever the choice, patients need unbiased facts to determine what meets their personal physical, emotional, and ethical needs during assisted reproduction.