Embryo Definition

In IVF and surrogacy, a sperm and egg are combined, and when the fertilized egg divides, it is defined as an embryo.

Once you have viable embryos, they can be frozen until it’s time for them to be transferred to the intended mother’s or surrogate’s uterus, where it may develop into a pregnancy within weeks.

If you need help finding a clinic to create embryos for IVF or surrogacy, you can contact our specialists. If you’re considering surrogacy, we can work with you to complete our surrogacy planning questionnaire so you have a head start on the process when your embryos are ready.

To understand the definition of an embryo and how embryos are involved in surrogacy, below we will share the fertilization process and how embryos are formed in IVF during surrogacy.

Embryos Defined: Gamete vs. Zygote vs. Embryo vs. Fetus

The definition of an embryo is commonly misunderstood. In popular use, “embryo” is used broadly, but the embryo definition biologists and medical professionals use is more precise.

The embryo stage of development happens when a fertilized egg divides, forming a multi-cell clump. Once the embryo has reached the stage of development optimal for implantation, the embryo is transferred to the intended mother’s or surrogate’s uterus. If the embryo implants in the uterine wall a pregnancy is achieved.

Over the next few months, the embryo reaches a stage of development where it is no longer considered an embryo and instead is referred to as a fetus.

Separating these names out is easier when you look at the stages in order:

Gametes (Before Fertilization)

Before an egg and a sperm are combined, they are called gametes. A gamete by itself cannot become a pregnancy—you will need an egg gamete and a sperm gamete.

Zygote (Fertilized Egg)

When the sperm and egg are combined, they form a single cell called a zygote. A zygote contains genetic material that can form an embryo and eventually may become a baby.

Embryo (Fertilized Egg after Cell Division)

As a part of the growth process, a zygote divides into two, then into four cells. When it divides for the first time (usually in the first days of development), it is called an embryo and will continue to be called an embryo for some time.

In IVF, an embryo will be created by injecting an egg with sperm outside of the body (in vitro) and then transferred into the uterus where it will hopefully become a healthy pregnancy.

Blastocyst: After several days of division, the embryo becomes a blastocyst, which is a significant stage in early development. A blastocyst is a more advanced form of the embryo, typically occurring around five to six days after fertilization.

This stage is important because the blastocyst is what implants into the uterine wall. In the context of IVF, reaching the blastocyst stage before transfer can improve the chances of implantation and a healthy pregnancy.

Fetus (Developed Embryo)

The embryo is referred to as a fetus around week 10 of development when the embryo has developed some of the necessary body systems and is around 2 inches in size. This is the term used until your baby is born.

As you can see, to define an embryo, it makes sense to think about embryos as a specific stage of development in the reproductive process.

The Definition of an Embryo in Surrogacy

In surrogacy and IVF, the embryo stage is incredibly important because it is just the beginning of making your dreams of parenthood a reality.

Embryos for surrogacy are created using IVF.  A medical professional known as an embryologist combines your and your partner’s or a donor’s sperm and eggs in a lab environment. Over the next few days, an embryologist observes which sperm and eggs fertilize and develop into embryos.

During this stage, decisions are made about which embryos are most likely to become pregnancies and result in a birth. Not all eggs will fertilize and some embryos may stop developing before they reach the blastocyst stage, meaning that they will not become a pregnancy or result in a birth, and are therefore not viable for transfer.

Embryo Grading and Transfer

Medical professionals do their best to predict which embryos will be most successful, and categorize which embryos are most likely to succeed through embryo grading.

When embryos are graded, embryologists look at development markers like the number of cells, the size and shape of cells, and the development of embryo parts.

The embryos that have the best chances of becoming a successful pregnancy are selected to be transferred to your surrogate’s uterus through a medical procedure and may become a pregnancy within the next couple of weeks.