Cygote, Embryo and Fetus: What Are the Differences?

By (embryologist) and (babygest staff).
Last Update: 10/25/2019

The definitions of zygote, embryo and fetus tend to be confused because they all refer to the future baby, albeit at different stages of prenatal development. In this article we will clarify the differences between these concepts so that the ART-Patients can understand the explanations of the specialists.

What's a zygote?

To understand the definition of cigote, often written as zigote, it is important to know the concept of gamet.

Gametes are the reproductive cells produced in the gonads or sexual organs. In humans, we distinguish between female gametes, eggs, and male gametes, sperm.

The main characteristic of these cells is that they are haploid, that is, they possess half the genetic information that the rest of the cells of the human body, which are diploid. Thus, human somatic cells have 46 chromosomes, while sexual cells have 23. The process that results in this reduction of information is known as meiosis.

When both sexual cells, egg and sperm, join together in fertilization, their nuclei fuse to form a single cell: the zygote. Therefore, we can define the zygote concept as the cell that results from the fertilization of the ovum by the sperm.

The size of the zygote is equal to that of the ovum, but its genetic endowment is 46 chromosomes (23 of each gamete). From it, the rest of the cells that will form the new living being will develop.

Although the zygote stage is short compared to full embryonic development, it is a very important period, as it is the first stage of life.

Embryo Development

It is considered that the development of an embryo goes from day 1 after fertilization (zygote stage) until the 8th week of gestation. During this period, the embryo will develop and more and more cells will be produced, as well as cell specialization.

Within the embryonic stage there are some specific stages. We are going to discuss them below:

Morula

After fertilization, the zygote has two cells, then four, eight, sixteen... Approximately on day 4 of its development, the embryo is a large group of identical cells, difficult to distinguish. They form a set that reminds us of a blackberry and, therefore, the embryo at this time is known as a morula.

Blastocyst

Around day 5-6 of development, cell differentiation begins, which will result in two cell groups. On the one hand, the cells of the trofoectoderm (external part), which will give rise to the placenta and other structures necessary to maintain pregnancy. On the other hand, the cells of the internal cell mass (ICM), which will form the fetus. In the embryonic interior, a kind of cavity is formed: the blastocele.

This embryonic structure is what we know as the blastocyst.

The next step in embryonic development is for the blastocyst to be stripped of the zona pellucida (surface layer that protects it) and implanted in the endometrium. From this moment on, it can already be said that the woman is pregnant.

From now on, the embryo grows at a very fast rate and ceases to have a rounded shape to have a more elongated appearance and similar to the fetal structure, although without any details.

Fetal Stage

It is in the 8th week of gestation when the embryonic stage ends and the fetal stage begins. We can therefore say that an 8-week-old embryo is already a fetus.

In the fetus, more and more cellular differentiations will take place, and the different organs will be created in this way. As development progresses, the fetus will acquire the appearance of a baby.

This period is the longest of all prenatal evolution. Throughout this stage of gestation, the woman will undergo various controls that will determine if the rate of growth and fetal development is adequate.

It is considered that the birth must take place between the 38th and 42nd weeks of pregnancy, considering the beginning of the same as the date of the last menstrual period. Once the birth has taken place, that is to say, after the birth, we will be able to use the qualifying baby to refer to the new being.

Embryonic development in in vitro fertilization

When in vitro fertilization (IVF) treatment is applied, either on its own or for surrogacy, part of the embryo-fetal development occurs in the laboratory.

The cygote stage and part of the embryonic stage will take place in culture plates, until the embryo transfer occurs.

Generally, embryos are transferred on day 3 or day 5-6. Although in some cases it is done, it is not common to transfer embryos on day 2, day 4 or beyond day 6 of development.

FAQs from users

Can an embryo outside the uterus reach fetal stage?

By Andrea Rodrigo (embryologist).

No. An ectopic pregnancy or extrauterine pregnancy should be operated on as soon as possible to avoid major problems in the woman's body. Most commonly, it occurs in the fallopian tubes, although it can also occur in other areas of the body such as the ovary, the cervical canal, the pelvic area, and so on.

Is it correct to speak of 'fetus' at 2 months?

By Andrea Rodrigo (embryologist).

In principle, yes, because 2 months is about 8 weeks and it is at this time when the embryo gets called fetus.

How can one observe whether embryo implantation has taken place and therefore its development continues up to the fetal stage?

By Andrea Rodrigo (embryologist).

Most women feel symptoms after embryo implantation, caused mainly by the hormonal imbalance inherent in the beginning of pregnancy. The hormone responsible for most of these symptoms is beta-hCG.

The most common symptoms are nausea, vomiting, heaviness, tiredness and slight dizziness. On the other hand, there are women who have a little bleeding due to implantation.

In any case, ultrasound is the main tool for studying embryo-fetal evolution.

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References

Authors and contributors

 Andrea Rodrigo
Andrea Rodrigo
Embryologist
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia along with the Valencian Infertility Institute (IVI). Postgraduate course in Medical Genetics. More information about Andrea Rodrigo
Adapted into english by:
 Romina Packan
Romina Packan
Babygest Staff
Editor and translator for the Babygest magazine in English and German. More information about Romina Packan

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