Zygote, Embryo and Fetus all refer to the Future Baby
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.
The different sections of this article have been assembled into the following table of contents.
1. 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.
2. 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.
3. 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.
4. 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.
From BabyGest article written by By Andrea Rodrigo (embryologist) and Romina Packan (babygest staff)
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