What is an embryo? Is there a difference between a zygote and an embryo? Which comes first: a zygote, an embryo or a fetus? Is an embryo considered a pregnancy?
Have you ever found yourself curious about the intricate journey of how babies come into existence? We’re not referring to the intimate act itself, but rather the remarkable sequence of events that unfolds once a sperm cell encounters an egg cell. You might notice that when people are expecting, they often casually call it the “baby,” but it’s good to know that each phase of pregnancy has its own special name.
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When Does a Pregnancy Start
The commencement of your pregnancy is the first day of your last menstrual cycle. This is referred to as gestational age or menstrual age.
It occurs approximately two weeks prior to pregnancy. Though it may seem strange, the first day of your last period will be significant in determining your due date. Your healthcare provider will bring up this date, which will be used to assess how far along you are in your pregnancy.
How Does Conception Work
A viable man’s sperm cell interacts with a woman’s egg cell as it travels down one of the fallopian tubes from the ovary to the uterus.
As the fertilised egg moves down the fallopian tube, it splits into two cells, four cells, and eventually even more cells. A week after the sperm has fertilised the egg, the fertilised egg has travelled to the uterus and has developed into a developing cluster of roughly 100 cells known as a blastocyst.
The blastocyst then attaches to the uterine lining (the endometrium). This procedure of connecting is known as implantation. As estrogen and progesterone are produced, the endometrium thickens, providing the nutrients needed for the blastocyst to grow and eventually mature into a baby.
Hormones are generated as cells divide, some of which develop into the baby and others which form the placenta, which gives the infant nutrition and oxygen, telling the body that a baby is forming inside the uterus.
These hormones also instruct the uterus to retain its lining rather than lose it. This means a woman will not have her period that month, which could be the first indicator of pregnancy.
How Do I Know if I’m Pregnant
From the moment you have conceived, the hormone human chorionic gonadotrophin (hCG) is present in your bloodstream. This hormone is produced by the placental cells (food source for the growing fetus).
A pregnancy test identifies this hormone as well. Although this hormone is present from the beginning, it takes time for it to accumulate in your body. After the first day of your last period, it takes three to four weeks for hCG levels to rise sufficiently to be detected by pregnancy tests.
Embryo vs Fetus: Stages of Baby’s Development
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When two gametes come together, a zygote is formed, which divides promptly to become an embryo. As the pregnancy progresses, the embryo develops into a foetus. The foetus is known as a neonate or newborn after it is born.
Although it is unusual to hear a pregnant woman use those terminologies to describe her embryo or foetus, we can all choose to use the terms that are most familiar to us.
Is there a difference between a zygote and an embryo?
Yes, there is a significant difference between a zygote and an embryo. A zygote is the initial cell formed when a sperm cell fertilizes an egg cell during conception. It is a single-cell structure with a full set of genetic material from both parents.
On the other hand, an embryo is a later developmental stage that follows the zygote. It begins when the zygote undergoes multiple cell divisions and organises into a multicellular structure. During this stage, various specialised cell types start to form, setting the foundation for the development of organs and body systems.
Which comes first: zygote or embryo or fetus?
The chronological order is as follows: A zygote comes first, formed when sperm fertilises an egg. The zygote then undergoes multiple cell divisions and development, becoming an embryo. After the embryonic stage, the term “fetus” is used to describe the developing organism from approximately the ninth week of gestation until birth.
Gestational age: Embryo vs foetus
An embryo’s or foetus’ gestational age is expressed in weeks. When someone says they are 15 weeks pregnant, they are referring to the gestational age of their foetus.
Gestational age is determined in a professional setting, such as when you visit your doctor, using the first day of the last menstrual period (LMP), which is a convenient date for the pregnant woman to remember. For embryologists (those who study the development of embryos), the time of fertilisation defines gestational age.
The precise timing of fertilisation is frequently more difficult to determine (except in cases of assisted reproductive technology, like in vitro fertilisation or IVF).
In most cases, fertilisation happens within 12 hours of ovulation. Ovulation can occur at any point throughout the menstrual cycle, but the clinical method for determining gestational age assumes it occurs on the 14th day of a 28-day cycle.
Of course, because menstrual cycles differ in length and the day of ovulation varies from person to person, this assumption could be wrong.
As a result, there’s a two-week(ish) difference between an embryo’s or foetus’s true age (which starts with fertilisation) and how we estimate it based on someone’s last period.
Unless otherwise specified, we’ll use gestational age calculated from the last menstrual cycle, which means you’re two weeks pregnant on the day you find out you’re expecting.
What is an Embryo?
Doctors utilise the date of a woman’s last menstrual period to establish a pregnancy.
- The first two weeks of pregnancy are defined as the time leading up to ovulation when the body prepares to release an egg.
- Week 3 marks the start of ovulation or the release of an egg. An egg that has been fertilised by a sperm cell is called a zygote. The zygote develops into a blastocyst, which is a cell cluster.
- The blastocyst implants in the uterine wall in week 4 of pregnancy and grow into the placenta and embryo. When the amniotic sac develops (about day 10 to 12 following fertilisation, or around the start of week 5 of pregnancy), the blastocyst has designated an embryo.
- An embryo is a stage of human development that occurs between the fifth and 10th weeks of pregnancy.
Major developmental milestones in the embryonic stage
- Week 3: The implantation procedure begins.
- Week 4: The Forming of the Central Nervous System
- In Week 5: the eyes, ears, and upper limb buds (arms) start to grow, and cardiac activity in the heart begins.
- Week 6: Lower limb buds (legs) and hands and feet begin to form.
- In Week 7: The appearance of fingers
- Week 8: Eyelids begin to develop External genitalia begins to distinguish in week 9
What is an embryo sac?
An embryo sac is believed to form when the haploid megaspore nucleus divides. It has six haploid cells without cell walls and two haploid nuclei.
Sometimes the haploid nuclei unite to produce a single cell (endosperm motor cell). During fertilisation, one male nucleus and one egg nucleus unite to produce a zygote, which leads to the embryo’s growth. The endosperm nucleus is formed when the primary endosperm nucleus and the second male nucleus merge.
There are two stages to the embryo sac formation:
Stage 1:
- The first stage of megasporogenesis occurs when a single diploid mother cell goes through meiosis and forms a haploid megaspore tetrad.
- Three of them disintegrate, but one survives to become the embryo sac.
Stage 2:
- Megagametogenesis is the second stage, in which an 8-nucleate, the 7-celled gametophyte is produced by mitosis of a functional haploid megaspore, resulting in an embryo sac.
- The polar nuclei, out of all eight, migrate to the centre to unite and create a single diploid cell at that location (centre).
- When this single diploid cell joins the sperm, the triploid endosperm is created.
- The other three nuclei differentiate into antipodal cells, whereas the other two differentiate into synergid cells that degenerate with time.
Is an embryo a pregnancy?
No, an embryo is not considered a pregnancy on its own. Pregnancy encompasses the entire process of developing a baby from conception to birth, which includes the stages of zygote, embryo, and fetus.
The term “pregnancy” is used to describe the entire duration from fertilisation to the delivery of the baby. An embryo is just one of the early stages of pregnancy, followed by the fetal stage as the pregnancy progresses.
When Is a Foetus a Baby Legally
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When the embryonic period ends in the 10th week of pregnancy, the embryo is referred to as a foetus. A foetus is a developing infant beginning in the 11th week of pregnancy.
The foetal stage starts 10 weeks after the last period and lasts until the baby is born. By the beginning of this stage, all of the major organ systems have formed, but they are still immature. From this point on, the foetus will largely be growing and maturing tissues.
Although a foetus who is at least 24 weeks old may be viable if given sufficient care after birth, there is no set time for foetal “viability” (the ability to survive outside the uterus). A foetus is less likely to survive than an older foetus because its lungs and brains are immature before 30 weeks of gestation.
In Singapore, a pregnancy can be terminated or aborted before 24 weeks.
Major Developmental Milestones in the Foetal Stage
- Weeks 10 to 13: the foetus continues to grow fast, and the kidneys begin to generate urine.
- In weeks 14 to 17, external genitalia has developed, coordinated limb motions have begun, bones have hardened, and ocular movement has commenced.
- Weeks 18 to 21: visible brows and head hair, fetal uterus, and formation of the vaginal canal
- In weeks 22 to 26: the foetus has fingernails and is gaining weight.
- Weeks 27 to 30: The lungs and brain have matured. To the point where the foetus would almost probably survive if born and given intensive care at this stage. The eyelids are open, toenails are visible, and the foetus is gaining weight.
- In weeks 31 to 35, pupils respond to light.
- Weeks 36 to 40: Firm grip and daily fat gain of 14 grams (12 ounces).
What Is a Newborn
The transition from foetus to newborn (also known as neonate) at birth is complicated and must happen quickly for the infant to be able to function independently.
The foetus prepares for the shift by producing hormones (cortisol, adrenaline, and thyroid hormones) that will increase upon delivery. This will assist the child in resuming normal blood sugar, body temperature, and blood pressure levels quickly.
Newborns’ circulatory and respiratory systems go through a lot of changes. The foetal circulatory system has additional connections that allow more oxygenated blood from the placenta to reach the developing brain and heart through the umbilical cord while avoiding the lungs.
Once the child learns to breathe on their own after delivery, these extra cardiovascular connections are severed. Blood flow to the lungs increases as the newborn or baby must breathe to receive its own oxygen.
Understanding the distinctions between embryo, fetus, and baby is like decoding the chapters of the incredible story of pregnancy. It’s a journey from a tiny, single-cell zygote to a fully-formed human being, and each stage is a marvel in itself. While these terms may sound technical, they encapsulate the magic of life’s progression.
Embrace and cherish each phase of your pregnancy, knowing that it’s a unique chapter in the beautiful story of parenthood.
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This article was written by Margaux Dolores and republished with permission from theAsianparent Philippines.
Updates from Camille Eusebio
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