Embryo

Embryo development is a delicate, multi-stage process. From the moment of fertilization, growth follows a pattern that can be influenced by many factors. With in vitro fertilization (IVF), that pattern closely mimics what happens in the human body during a natural pregnancy, when an embryo grows from a single cell to the blastocyst stage. However, rather than occurring in a fallopian tube and the uterus, the initial phases take place in a petri dish in a lab while being monitored by trained embryologists.

How an embryo develops in the first few days — specifically, its rate of growth and changes in structure — directly impacts whether the embryo will result in a healthy live birth. Learning how embryos grow during the IVF process can help patients understand what a desirable outcome looks like and whether their embryos have achieved certain milestones.

Preimplantation genetic testing (PGT) is the general term for genetic testing performed on human embryos used for in vitro fertilization (IVF) and before transfer to the uterus. PGT aims to improve the chances of a successful ongoing pregnancy after embryo transfer.

While genetic testing does not impact or change the genetics of an embryo, it gives doctors and prospective parents more information and may help them select the most viable embryo(s) for transfer. It is important to understand that PGT will not improve the cumulative live birth rate of a given IVF cycle and that there are both advantages and disadvantages to PGT.

Preimplantation genetic testing (PGT) is the general term for genetic testing that is performed on embryos produced by in vitro fertilization (IVF) before the embryos are transferred to the uterus. The purpose of PGT is to improve the chances of having a successful embryo transfer. Genetic testing does not impact or change the genetics of an embryo, but it does give doctors and prospective parents more information and may help them select the most viable embryos for transfer.

There are three types of PGT: PGT for monogenic disorders (PGT-M), PGT for structural chromosomal rearrangements (PGT-SR), and PGT for aneuploidy (PGT-A). PGT-A is the most routine type performed. As such, it is an important area of education for prospective parents.

To increase the chances of a successful pregnancy, embryo grading is completed by an embryologist during an in vitro fertilization (IVF) cycle. Assessing embryos for quality at specific stages of embryo development can be a valuable tool in reproductive medicine to help the doctor decide which embryo to transfer first (when there is more than one embryo available for transfer).

After an egg retrieval for a fresh cycle, or when contemplating a frozen embryo transfer, patients often wonder how many embryos should be placed into the uterus. Usually, single embryo transfer (SET) is promoted, especially if an embryo is genetically tested, meaning it undergoes preimplantation genetic testing (PGT). However, there are cases where a patient may want to transfer two or more embryos. Examining the pros and cons of multiple embryo transfers can help patients make an informed decision.

An embryo transfer is usually the final step in the process of in vitro fertilization (IVF), in which the embryo is transferred to the patient’s uterus. The embryos used in this procedure may either be fresh or thawed (after being frozen).

During IVF, mature eggs are fertilized, either by conventional IVF (insemination) or intracytoplasmic sperm injection (ICSI). Next, the resulting embryos are grown in an incubator in the embryology laboratory. Finally, an embryo transfer is the procedure whereby one or more of the embryos that have been grown in the lab are then transferred into the patient’s uterus in the hopes of initiating a pregnancy.

Fresh embryo transfers involve transfer to the uterus several days after egg retrieval within the same IVF cycle. The embryo is never frozen.ⁱ Alternatively, embryos can be frozen and preserved (known as cryopreservation), then stored to be transferred in a subsequent cycle. This process is known as a frozen embryo transfer (FET) and requires the cryopreserved embryo to be thawed before transfer into the uterus.ⁱⁱ

Donor eggs (oocytes) are derived from an individual other than the intended parent(s). This involves in vitro fertilization (IVF), where the recipient uses healthy and fertile donor eggs to conceive. Recipients of donor eggs are often uncertain as to whether they should choose to use fresh or frozen donor eggs, and the differences between the options.  This article explores the pros and cons of using fresh vs. frozen donor eggs.

Exploring epigenetics, the study of how environmental factors affect gene expression, reveals complex dynamics in the context of donor eggs used for fertility treatments. For mothers seeking a genetic connection to their donor egg–conceived child, epigenetics can provide some insight.

Epigenetic changes are reversible alterations to deoxyribonucleic acid (DNA, the genetic code) that do not alter the code itself but can influence how an embryo develops.ⁱ The dynamic interaction between inherited genetic information and environmental influences shapes the overall epigenetic profile of gametes (oocytes and sperm), the developing embryo, and fetus.ⁱⁱ Epigenetic changes are especially important during critical periods like prenatal development, where the epigenetic landscape is particularly sensitive to external cues.ⁱⁱⁱ

Conceiving through in vitro fertilization (IVF), or other assisted reproductive technology, is a multifaceted process that involves several steps. Once eggs have been retrieved, successfully fertilized, and the embryos have developed, the embryo transfer procedure is planned by the reproductive endocrinologist (REI, also called RE).

IVF embryo transfer processes vary depending on multiple factors, including whether fresh or frozen embryos are used and how many embryos are planned for transfer. Successful embryo implantation into the uterine cavity requires careful planning before the transfer procedure itself. It may also involve lifestyle adjustments for the female during the weeks immediately following the procedure.

Embryo freezing, also known as cryopreservation, is an assisted reproductive technology (ART) that involves storing preimplantation-stage embryos that were created through in vitro fertilization (IVF). The fast-freeze process, known as vitrification, is done at an extremely low temperature (-196°C or -321°F) to halt embryo development and preserve cellular vitality. Following vitrification, these embryos can be safely cryopreserved for extended periods until they are thawed for transfer back to the uterus (called a frozen embryo transfer).

Cryopreservation is necessary when preimplantation genetic testing (PGT) is chosen. Typically, fresh embryos undergo a biopsy before freezing to allow time for the genetic tests to be conducted and results to come back. However, there are cases where patients decide that they want to perform genetic testing of previously frozen embryos. In these circumstances, frozen embryos must be thawed for biopsy, then refrozen until a future date when they might be used. That means the embryos will undergo an additional freeze-thaw round compared to the typical protocol.

Chromosomes carry DNA, which forms genes that determine multiple aspects of a human’s makeup. A chromosome analysis is often performed to give a healthcare provider more insight into a range of issues, from potential genetic conditions to factors that may be causing recurrent miscarriage.

Embryonic and fetal genetic testing may be recommended for a variety of reasons. Sometimes, individuals or couples have genetic conditions that run in their families. In that case, in vitro fertilization (IVF) with preimplantation genetic testing (PGT) can help select embryos that do not carry the condition. In other cases, patients who have had multiple pregnancy losses may pursue genetic testing of an embryo to help discover a cause. Genetic testing can also give an early glimpse into the sex of the fetus as well as whether the fetus has extra/missing chromosomes that could cause conditions such as Down syndrome or Turner syndrome.

A dilation and curettage (D&C) is a surgical procedure that removes tissue from the uterus. While it is used for a variety of reasons including diagnostic purposes, it is typically performed following a miscarriage, or for elective termination of a pregnancy. An understanding of what to expect before, during, and after a D&C is beneficial for navigating this procedure, as well as to learn about potential risks and alternative treatments.

Following an IVF cycle, any resulting embryos that are not transferred to the uterus (fresh transfer) are frozen, or cryopreserved. These frozen embryo(s) will remain frozen until the patient is ready to transfer them (called frozen embryo transfer). However, sometimes there are surplus embryos after the patient has completed their family, or plans change, and embryos remain unused for many reasons. Hundreds of thousands of surplus frozen embryos are currently stored at fertility clinics worldwide – that is, embryos created for fertility treatment and then are not needed by the person(s) that created them.

When a person has frozen embryos in storage that they do not plan to use, it can be difficult to decide what to do with those embryos. When people begin their IVF journey, they may not consider that this is something they may need to think about in the future. Alternatively, if they have considered this, their thoughts on what they plan to do with any unused embryos may have changed. This article outlines the logistical and emotional considerations for the four main options for additional or unused embryos: donation (to medical research, or to another family), destruction, compassionate transfer, and perpetual/indefinite storage.^i,ii,iii

This article will look at broad outlines, practices, and legislation, which do vary depending on where someone lives. Some countries have comprehensive regulations surrounding the handling of surplus embryos, including guidelines on informed consent and storage limits, while others have fewer rules in place, which leaves the decision largely down to clinics and individuals.^iv,v,vi

A Reproductive Endocrinology and Infertility (REI) specialist or fertility clinic can provide more information on local regulations surrounding the options for unused embryos.