What Are My Chances of IVF Twins with Single Frozen Embryo Transfer?

Ever wondered if transferring just one frozen embryo could lead to the joy—and chaos—of twins? While single embryo transfers are designed to minimize the risk of multiple pregnancies, IVF patients still have a 1-3% chance of twins. Here's everything you need to know about this fascinating phenomenon.

In this article:

• Does IVF increase my chances of twins?
• How common are identical twins with IVF?
• ​Why is there a higher chance of twins with IVF?
• Possible Reasons for IVF Identical Twins
• Types of Twins
• Family History of Twins & IVF
• What is eSET?
• The Risks of Multiple Pregnancy
• Increased Risks with Identical Twin Pregnancies
• How to Choose Which Embryo(s) to Transfer
• PGT and Multiple Pregnancy
• How many embryos should I transfer?
• Transferring Multiple Low-Quality Embryos
• Guidelines for Multiple Embryo Transfer
• What is selective reduction?
• Can a single embryo transfer result in fraternal twins?
• Making the Best Choice for Your Family

Does IVF increase my chances of twins?

The risk of having a twin pregnancy following in vitro fertilization (IVF) treatment has been steadily declining over the past 10 years, but is still an important consideration for fertility patients and their doctors. 

Currently, most IVF cycles involve the transfer of just a single embryo, and the risk of having fraternal twins is extremely low. Here at Illume Fertility, a single embryo is transferred in over 90% of IVF cycles.

The vast majority of twins that result from fertility treatments are fraternal twins, associated with treatments that stimulate more eggs to grow each month, as well as with IVF cycles during which two or more embryos are transferred.

However, there is still an increased risk of having identical twins with IVF treatment. Let's explore how even a single embryo transfer could lead to a twin pregnancy.

How common are identical twins with IVF?

The risk is low—a recent analysis demonstrated that identical twins occur in 1-3% of pregnancies (1-3 per 100) following IVF treatment. This is known as monozygotic (identical) twinning, where a single embryo splits into two early in development.

In the general population, identical twins occur in only 0.4% (4 per 1000) of pregnancies. This means there is a significantly greater risk of identical twinning with IVF pregnancies.

While it is encouraging that identical twins are now seen less frequently with IVF, there always remains a small chance of this phenomenon occurring. Despite trying to reduce twin risks by transferring only a single embryo into the uterus, parents-to-be can still be surprised by identical twins when the first pregnancy ultrasound is performed.

​Why is there a higher chance of identical twins with IVF?

While researchers still aren't certain why there is an increased risk of identical twins with IVF, research has shown that identical twins occur more commonly with extended culture in the laboratory, as almost all clinics will allow embryos to develop for up to 5-7 days prior to transferring or freezing them.

Pregnancy rates are higher when we can select the better quality embryos after extended culture, but research suggests there may be effects on the outer shell of the embryo (impacting how the embryonic cells function) after growing in the laboratory for several days.

Can I choose to have twins with IVF?

There is no guaranteed way to have twins through IVF, even with multiple embryo transfers. Each embryo has a chance of implanting, but it's not a certainty. Additionally, the decision to pursue twins must be made carefully due to the increased risks involved.

How long does IVF take?

Learn everything you need to know about in vitro fertilization (IVF) from start to finish with our free Step-by-Step Guide to IVF Treatment.

Possible Reasons for IVF Identical Twins

While there isn't one specific reason we can pinpoint, research suggests that the following factors may play a role in increasing the risk of identical twins following IVF:

Laboratory Practices

• Assisted hatching: A technique that allows embryologists to help an embryo "hatch" from its shell by creating a small crack in the zona pellucida (ZP).
• Intracytoplasmic sperm injection (ICSI): A technique used in cases of severe male factor infertility which may involve zona pellucida manipulation methods.
• Embryo biopsy for preimplantation genetic testing (PGT).

Other Factors

• Conditions in which the embryo and blastocyst are cultured
• Transferring a blastocyst (embryo) on Day 5–6 of development
• Maternal age (the age of the mother) at embryo transfer
• Body mass index (BMI) of the mother (source)

What is eSET?

eSET stands for elective single embryo transfer, where only one embryo is transferred during an IVF cycle. This practice began gaining traction in the early 2000s and has now become the standard of care for most fertility clinics.

The main reason behind promoting eSET is to reduce the risk of multiple pregnancies (twins, triplets, or higher order multiples).

Types of Twins

There are several variations of twin types, which are primarily categorized based on how they are conceived and how they develop during pregnancy. Let's break it down:

1. Based on Zygosity (Fertilization Process)

• Monozygotic (identical) twins: Develop from a single fertilized egg that splits into two embryos (or, in the case of IVF, when a single embryo splits in two after embryo transfer). Identical twins share the same DNA and are usually of the same sex. 
• Dizygotic (fraternal) twins: Develop from two different eggs fertilized by two different sperm cells. They have different genetic makeups, just like regular siblings, and can be of the same or different sexes.

2. Based on Chorionicity & Amnionicity (Placental Development)

• Monochorionic twins: Share the same placenta. This is common in monozygotic twins and can lead to complications like twin-to-twin transfusion syndrome (TTTS).
• Dichorionic twins: Have separate placentas. Most dizygotic twins fall into this category, but monozygotic twins can also be dichorionic if the split happens early.
• Monoamniotic twins: Share the same amniotic sac, a rare and high-risk situation mostly seen in monozygotic twins.
• Diamniotic twins: Each twin has its own amniotic sac. Most twins, whether monozygotic or dizygotic, are diamniotic.

How can a single embryo transfer result in twins?

A single embryo can result in monozygotic (identical) twins during IVF if the embryo splits into two separate embryos after being transferred to the uterus. Here's how it happens:

• A single fertilized egg (zygote) divides into two separate cell masses at an early stage of development.
• This results in two embryos that share the same genetic material, leading to identical twins.

The timing of the split determines whether the twins will share a placenta or amniotic sac. There are three potential options:

• Early split: Each twin has its own placenta and amniotic sac (dichorionic-diamniotic twins).
• Later split: Twins may share a placenta but have separate amniotic sacs (monochorionic-diamniotic twins).
• Very late split: Results in monochorionic monoamniotic (mono/mono) twins, sharing both a placenta and amniotic sac. These types of twins share a chorion, placenta, and an amniotic sac. This is the rarest type of twinning and carries the highest risk of complications.

A High-Risk Twin Pregnancy

After struggling through years of infertility, parents Christa and Aland were shocked to learn their single embryo transfer resulted in identical twins.

The Risks of Multiple Pregnancy

While desired by some, multiple pregnancies also carry significantly higher health risks for both the mother and babies, including:

• Premature birth: Multiple pregnancies are far more likely to result in babies born prematurely. Over 60 percent of twins and nearly all higher-order multiples are premature (born before 37 weeks).
• Low birth weight: Babies in multiple pregnancies often have lower birth weights, which can lead to health problems.
• Pregnancy complications: The risk of gestational diabetes, preeclampsia, and other complications increases with multiples.
• Delivery complications: Multiple pregnancies are more likely to require C-sections, which carries its own set of risks.

By transferring only one embryo during an IVF cycle, the chances of a healthy singleton pregnancy are maximized, and maternal risks are greatly reduced. As we often say here at Illume, our goal is to help families have one healthy baby at a time!

Increased Risks with Identical Twin Pregnancies

​Identical twins are associated with greater risks (to both mother and babies) when compared to fraternal twins. Here's why:

• Fraternal twins develop in completely separate embryos, usually with two separate placentas and pregnancy sacs, making it easier for each embryo to receive the nutrients it needs to develop properly.
• Identical twins often share a placenta and may be developing within the same pregnancy sac. There are blood flow abnormalities that can occur when twins share a placenta, creating a higher risk for poor fetal growth and loss of the pregnancy.

Additionally, since both embryos share a common placenta, when one embryo fails to grow, the other embryo/fetus will also be affected. The risk of pre-term labor with fetal loss is also elevated, similar to the risk associated with fraternal twins.

How to Choose Which Embryo(s) to Transfer

A highly-skilled embryologist at your fertility clinic will provide recommendations on the best embryo(s) to transfer based on a variety of factors, which will typically include the following:

PGT and Multiple Pregnancy

Preimplantation genetic testing (PGT) can identify whether an embryo has the correct number of chromosomes and is free of heritable conditions. This can lead to higher success rates with eSET, further reducing the need for multiple embryo transfers.

Note: PGT itself does not directly increase or decrease the chances of twins.

How many embryos should I transfer?

The decision of how many embryos to transfer during an IVF cycle is made on a case-by-case basis, considering factors like embryo quality, the patient's age, medical history, and previous IVF outcomes. The ultimate goal is always to balance the chances of a successful pregnancy with the risks associated with multiples.

At Illume Fertility, the decision to transfer more than one embryo would involve a comprehensive counseling process. The factors we consider include:

• Embryo quality: The quality and developmental stage of the embryos play a significant role.
• Patient age: Younger patients may be more likely to have eSET recommended.
• Medical history: Any medical conditions or history of complications in previous pregnancies would be taken into account.

Transferring Multiple Low-Quality Embryos

While it might seem like a good way to increase your odds of pregnancy, transferring multiple low-quality embryos can also increase the risk of multiple pregnancies and their associated complications. Always discuss the best course of action with your doctor.

Some studies have shown that in women 42 years of age or older, transferring a single euploid blastocyst resulted in pregnancy rates similar to those achieved by transferring two untested blastocysts - while also dramatically reducing the risk of twins.

A euploid blastocyst is an embryo that has the correct number of chromosomes (46) and is considered "normal." Preimplantation genetic testing for aneuploidy (PGT-A) must be performed in order to assess the chromosomal makeup of each embryo.

Guidelines for Multiple Embryo Transfer

While there is a strong recommendation by the American Society of Reproductive Medicine (ASRM) for single embryo transfer in most patients, not all fertility clinics follow these guidelines. In addition, not all patients will choose to proceed with single embryo transfer, even after professional counseling. 

Elective single embryo transfer (eSET) is an increasingly common practice, but some fertility clinics may still offer multiple embryo transfers (MET), especially in cases where a patient's embryos have a lower prognosis, or after repeated failed eSET cycles.

This decision is typically made after careful consultation with the patient and a thorough review of their medical history. It is also important to consider the following factors:

Patient Autonomy

The transfer of more than one embryo requires a comprehensive discussion of all ethical and medical considerations to ensure that the patient is making a fully informed decision. However, respect for each patient's autonomy remains a critical aspect of the conversation.

Embryo Quality

Preimplantation genetic testing for aneuploidy (PGT-A) can be a helpful tool for reducing the rate of multiple pregnancy, especially in women 37 years of age or younger. By gaining additional information about the genetic makeup of each embryo through PGT-A, your Care Team can offer the best guidance on which embryo to transfer first. 

Financial Restrictions

Many patients still lack insurance coverage for fertility treatment, and even if you have a  policy that covers IVF, it will typically only allow for a limited amount of cycles. Given these facts, it is understandable why financial considerations can be the tipping point in favor of elective multiple embryo transfer.

Studies have shown that insurance coverage for IVF is associated with the transfer of fewer embryos and leads to significantly lower rates of high-order multiple births.

Two sets of IVF twins:

An uninsured couple shares how the financial burden of infertility led to decisions that resulted in the birth of two sets of fraternal twins in three years.

FAQs About Twins

Want to dive a little deeper? Here are some of the most frequently asked questions we hear from our patients about multiple pregnancy:

What is selective reduction?

Selective reduction (also known as multifetal pregnancy reduction, or MFPR) is the practice of reducing the number of fetuses in a multiple pregnancy, such as quadruplets, to a twin or singleton pregnancy. It is typically considered in cases where there are serious health risks for the mother and/or babies. 

Benefits of Selective Reduction

• Increases the chances of survival and decreasing the risk of disabilities for the remaining fetus(es).
• Reduces the risk of maternal complications, such as gestational diabetes, preeclampsia, and anemia.
  

Personal & Psychological Considerations

Making the decision to proceed with selective reduction is emotionally complex for most patients. For those who have undergone fertility treatment to achieve pregnancy, having to consider making this choice can be exceptionally difficult. 

It can also raise deep moral, ethical, or spiritual concerns for some.

Erin Bulcao, an IVF mom and vocal advocate in the fertility community, described her experience with selective reduction in this story shared with Cozy Warrior:

"We went into the clinic and had our first ultrasound. With my legs wide open and my heart pounding fast, worried we had miscarried, the nurse looked up and said 'Congratulations! You are having TRIPLETS.' We were in shock.

Immediately following the ultrasound, we were called into the doctor’s office to discuss. He told us that I would be at high risk, meaning my own life would be at risk along with the babies’ lives if we decided to carry all three. He said we should consider a reduction and that’s what he would recommend.

Our hearts sunk. How on earth could we make this decision and yet how would I survive mentally worrying that our lives were at risk if we moved forward with three?"

Though uncomfortable to consider, it's important to have open discussions with your doctor so you can make the most informed choice possible. It can also be helpful to have the support of a mental health professional as you navigate the decision-making process.

Can you choose the gender with IVF?

It is possible to determine the gender (or rather, biological sex) of an embryo through a process called preimplantation genetic testing (PGT) after in vitro fertilization has been performed.

A small sample of cells is carefully biopsied from each embryo and sent off to an outside laboratory for extensive genetic testing, which can include identifying its sex.

Note: Gender selection for non-medical reasons is legal in the United States, although some fertility clinics have their own policies about whether or not they will perform these procedures. This practice is not legal in all countries.

Can a single embryo transfer result in fraternal twins?

Fraternal (dizygotic) twins could theoretically result from a single embryo transfer, though it is exceptionally unusual. Here are the ways this might occur:

1. Embryo fission and oocyte fusion describe unusual processes affecting embryonic development, typically associated with IVF and cellular anomalies.
2. Heterogeneous twinning and superfetation involve separate ovulations or fertilizations and can result from natural conception, IVF, or both. In these cases, the twins would be fraternal (dizygotic) but may have distinct genetic origins—one from the IVF process and the other from natural conception.

How likely are these scenarios?

These situations are extremely rare because IVF cycles usually include hormonal treatments to prevent ovulation during embryo transfer and the timing required for both events to coincide is very specific.

In rare cases, ovulation may still occur even during an IVF cycle, particularly if the woman hasn’t undergone ovulation suppression as part of the treatment protocol. If ovulation occurs and unprotected sex happens, a natural pregnancy can occur (superfetation).

Making the Best Choice for Your Family

There's no doubt that twins and triplets are an awe-inspiring occurrence (and a very cute one at that). However, it is essential to understand the potentially serious risks of multiple embryo transfers and carefully consider all of your options before making a final decision.

As research on assisted reproductive technologies (ART) continues to unfold, it is important to work with a reproductive endocrinologist who is not only board certified, but also dedicated to pursuing ongoing education and following current best practices. Doing this will give you the best possible chances of success.

Don't be afraid to ask questions and discuss any concerns openly with your doctor and Care Team. Transparency and communication will both be crucial components of making informed choices as you walk this path to parenthood.