Differences between human ‘zygote’, ’embryo’ and ‘fetus’

Within the world of Reproductive Biology, we can find many terms and concepts that people without specific health knowledge may confuse. This is the case of the words zygote, embryo and fetus, which definition and main differences will be explained hereunder.

It should be noted that these terms are used during the early stages of human development, known as Carnegie stages, which are based on the external and internal features of the embryo. The human embryonic period comprises 23 Carnegie stages that cover the first 8 weeks after fertilization.

The various sections of this article are assembled in the following table of contents.

Medical definition of ‘zygote’

By zygote or zygocyte we refer to the very first stage of life, which starts after the union of egg and sperm (male and female gametes). When gametes fuse together, a new cell with a core and 46 chromosomes is formed: 23 of these chromosomes are of paternal origin and 23 of maternal.

About 24 hours after the fusion, the first division of the new cell happens, and it is no longer called zygote. The embryonic period begins.

At what stage is a zygote called ‘morula’?

At the beginning, the embryo has two cells that keep on dividing themselves. When so many cells that cannot be distinguished exist, the embryo is called morula, because it acquires the appearance of a mulberry.

It is different from a blastocyst in the fact that a the term morula is used when the embryo is 3 to 4 days post fertilization, while it can be called blastocyst from day 4-5 onwards. Also, a morula is a 16-cell mass with a spherical shape.

It turns into a blastocyst through a process knows as compaction, which involves the following steps:

  • Desmosomes and gap junctions begin to be formed within a few days after fertilization. It means that the cells on the outer part of the morula become bound tightly together.
  • After that, a cavity forms inside the morula, which results in a “ball” of cells (inner cell mass or ICM). This group of cells and the outer part of the morula lead to the blastocyst, a stage which will be explained below.

Human embryo: definition & stages

From the moment two cells exist up until the 8th week of gestation, the development of the embryo takes place. At this stage, intense cellular changes occur.

Once the zygote is formed, the first cleavage of the human embryo occurs. Two cells are obtained from this process. Then, successive cleavage divisions will occur. The amount of cells and how rapid these divisions take place is known as morphokinetics.

Also, while there are certain embryonic cleavage patterns, each embryo is unique and can divide in a more or less quick way depending on its quality. Thus, it will be qualified as a high-quality embryo provided that it follows the common patterns of embryonic cleavage.

A day-2 embryo comprises usually 4 cells, which should be of similar size. Likewise, a day-3 embryo of optimal quality is usually composed of around 7-9 cells.

Blastocyst stage

When the embryo is on day 5, it is called blastocyst. It is from this moment on when it acquires a particular shape with different cell types.

A cavity forms inside the embryo and a series of cells called trophoblast will be responsible for placental development. The inner cell mass, from which the baby’s body will form, is located within a section of this cavity. When it is at blastocyst stage, the embryo breaks down the membrane that is protecting it to enter the endometrial lining.

Embryo implantation occurs between day 7 and 8. This is a crucial moment, since some embryos won’t be able to implant. Implantation stands for the fixation of the embryo—at blastocyst stage—in the woman’s uterus, in particular, in an inner layer called endometrium.

Implantation will finish (more or less) on day 14 after fertilization. On this stage, we can say the endometrium is invaded by the embryo. If this process goes as expected, one can confirm that the woman is indeed pregnant. From this moment on, production of the beta-hCG hormone begins.

From this point on, the embryo grows at a rapid pace and goes from having a rounded and unrecognizable form to acquiring a slightly elongated shape, similar to the silhouette of a baby but without any recognizable details.

What is a human ‘fetus’?

From week eight of your pregnancy onwards, the embryonic period ends and makes space for the so-called fetal period of pregnancy.

Throughout the first month, overall body proportion is unequal and head volume is bigger than other parts of the body. As the months go by, the rest of the body will gradually acquire a larger proportion until the silhouette of the fetus is fully formed.

The fetus has very specific levels of cell specialization. This gives rise to tissue formation, from which the body’s major organs, such as the brain, the liver, and kidneys develop.

This is the longest stage of pregnancy. The baby will be called fetus until childbirth, that is, between weeks 38 and 40 of pregnancy. After childbirth, the fetus is not called like that anymore and is renamed baby.

Comparison with other animals

Vertebrates, that is, animals with backbones share a common ancestor, and the genetic information that guides their development is almost the same. However, every living organism has a molecule known as deoxyribonucleic acid (DNA), which is the set of codes that act as the “blueprint” of each being.

That is why, when they begin to grow, all vertebrates look so much alike: fish, chickens, dogs, lizards… If we see a picture of any of them at the same embryo stage, we might find almost no difference between their external characteristics:

At the early stages of development, a human fetus may look like a fish or a chicken embryo, but the different parts of the DNA molecule of the human will play their role and build the particular molecules that a human growing body needs.

Although it may seem that the folds of a fish embryo resemble those of a human embryo to a large extent, this resemblance stops as they grow. These are some of the most common resemblances between human embryos and other vertebrates:

  • Gills: Humans are not related to fish. What seems to be the “gills” of a fish end up developing into the glands and structures of the neck and ears, not into the lungs, mouth and trachea.
  • Tail: When the coccyx is developing, it resembles the tail of a reptile or other mammal. That is the reason why it is also known as tailbone, although it is not literally a tail. In fact, it is a unique part of the human body: the final segment of the vertebral column.
  • Yolk sac: What seems to be the source of nourishment as in a bird egg is actually the source of the first blood cells of the human embryo.

Even though all human and animal embryos go through similar stages of development, the main difference is how long it takes for each specie to reach the same stages.

FAQs from users

What is the pronunciation of ‘zygote’?

The right pronunciation of this medical term is /ˈzaɪgəʊt/ or /zīˈgōtˌ/. The term comes from the root Greek word zygōtos, which means to yoke or to join. Some synonyms are zygospore or zygocyte.

Zygote or embryo, which comes first?

As explained above, the zygote is the first stage of human development right after fertilization occurs. It is the result of the fusion between gametes: the haploid ovum cell from the female and the haploid sperm cell from the male. The resulting diploid cell is what we know as zygote.

What does a human fetus look like?

The development of a human fetus changes by weeks or even by days. It is not possible to provide a detailed description of the appearance of a human fetus, because it changes as the different parts of the organism develop.

At 4 weeks, it begins to develop the structures that will form its face and neck, but it does not look like a baby yet. At 8 weeks, it is more or less half an inch in size and, by week 12, it measures about 2 inches and the sex organs start to become clear.

For more details, see also: The stages of pregnancy.

What is the difference between ‘zygote’ and ‘gamete’?

Gametes are reproductive cells that unite at fertilization and create a new being: the zygote. The female gamete is what we know as egg cell, ovum or oocyte; the male gamete is the sperm or spermatozoon.

Gametes are haploid cells, while the zygote is a diploid cell: while the former has just one complete set of chromosomes, the latter contains two complete sets. Another difference is that diploid cells reproduce by mitosis, while haploids do so by meiosis.

Do all human fetuses begin as female?

No, that is not true. The sex of humans is determined from fertilization, that is, as soon as it becomes a zygote: if it is XX, it will be a female, while, if it is XY, a male. However, at the beginning, human embryos have the precursors of both female and male gonads.

Taking this into account, if there is no Y chromosome, the embryo will proceed with the development of Müllerian ducts (which give rise to the ovaries). Also, when there is no Y chromosome, the embryo lacks the SRY region: the sex determining region.

To sum up: it is true that one could say that, in mammals, the initial plan for all embryos is female, which is only altered if the SRY region is present.

Suggested for you

All these changes and developments take place during the beginning of pregnancy. Would you like to know what are the most common symptoms and changes a woman experiences throughout this period? Do not miss this post: First moth of pregnancy.

Also, once the fetus is renamed baby, the moment of childbirth is closer, and new phases of your journey to motherhood begin. If you would like to find out what to expect during the next months, see also: Pregnancy month by month.

If you are about to undergo in vitro fertilization (IVF), you might be interested in getting further information about embryos. We suggest you read the following articles:

On the other hand, a zygote is the first stage of human development right after fertilization, but how does implantation occur? This sought-after step toward pregnancy is an amazing process that we explain in detail here: How does embryo implantation occur?

16 comments

  1. usuario
    AmAnDa

    Hello, I’m terrified, since my doctor told me that on week 12 we would be able to see a kind of structure, and once it is seen, we would be able to rule out chances of our baby having Down Syndrome… But we’ve done the ultrasound scan and this “structure” is so small that we are unable to see anything!! I’m terrified, and the back of the neck is kind of thick, do you think this could be due to the Down Syndrome or any other abnormality? I’m suffering panic attacks and stress, help me please.

    • usuario
      Pro Life 4ever*

      My dear, why are you panicking so early? It just does not mean anything yet, maybe your fetus is developing slower, it could be as simple as that, nothing else. You are not a doctor, therefore you should wait to see your doctor and get his confirmation. There’s no reason to be so stressed! And what if it were the Down’s Syndrome? Would you get an abortion for that reason? Please, come on! The world is still plenty of immature people 🙁

      • usuario
        Provoice

        And if she decided she indeed wanted to terminate, that would be ok as well. It is already a stressful enough and stigmatized decision to have people/strangers that impose their opinions and judgements under the disguise of helpful advice.

      • usuario
        I

        Nobody said anything about an abortion. She is just concerned about whether or not her baby has Down Syndrome. Any mother would be reasonably concerned in this situation because raising a child with an intellectual disability presents unique challenges. When someone needs support, simply give them support and don’t bring up your ethical views to make it worse.

  2. usuario
    lamiah

    I wanted to know the difference between foetus and embryo.

    • avatar
      Sandra F.Fertility counselor

      Hello Aurora,

      that information is thoroughly explained on this article. Anyhow, keep in mind that embryo is until week 8, and fetus is from week 8 onwards.

  3. usuario
    Gail M. Travis

    Actually, it’s more than just 24-hours that a zygote is a zygote; it’s more around 4-days. Zygotes don’t have brains, nor bones, nor skin, nor functioning internal organs; they are, more or less, blood-clots with potential. Embryos don’t have all of their parts, yet either. As for fetuses – there has never been one, survive even with assistance, prior to 3 months, outside of the uterus. And, there has never been a fetus that has survived outside of the uterus, unassisted, prior to 6 months.

    • usuario
      Beverly Fields

      I beg to differ. The youngest is 21 weeks and 6 days.

      • usuario
        Geez

        She said unassisted. A fetus of 21 weeks 6 days still needs much medical assistance to survive, they do not survive on their own. And only a few of them have survived. Medicine can create an artificial uterus, so to speak, with ventilators, fluids, heat, 24 attention… an incredible amount of support is required. They are not independently viable.

        • usuario
          Scott Thorstad

          Even if the pregnancy was picture perfect and the LIFE FORM goes to full term & born healthy… They are not independently viable either.

    • usuario
      whatacrock

      And you have psychological and ethical potential until the day you die… developmental stages are ongoing not to mention form and substance are posited terminology no different to any other nomenclature ascribed whether it be biological or ethical but it doesn’t negate its inherent value.

  4. usuario
    Laura

    This was really helpful, thanks.

  5. usuario
    Laxman Aucharmal

    It is useful and Scientific information. It should be known by every Male and Female that this is what happens after sex, within the body. This is the real knowledge.

  6. usuario
    Rawan

    Which cell in our body is the zygote? I mean, everyone is actually the result of divisions of the zygote, but when we grow up which cell in our body is the zygote one that brings life to us?

    • avatar
      Sandra F.Fertility counselor

      Dear Rawan,

      A zygote is a stage of embryo development that ends before being born. Therefore, there is no cell in an adult human body that is a “zygote”.

      • usuario
        Rawan

        Ah, I got it!
        The zygote cell within his divisions turned into different cells, so in the end , there’s no zygote one.
        Thanks a lot 🙂

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