Embryo vs fetus vs baby: What's the difference and at which stage is your pregnancy in?

While you may hear individuals refer to the “baby” when they are pregnant, the various stages of pregnancy have their own nomenclature. 

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 fertilized egg moves down the fallopian tube, it splits into two cells, four cells, and eventually even more cells. A week after the sperm has fertilized the egg, the fertilized egg has traveled 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 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 won’t have her period that month, which could be the first indicator of pregnancy.

How do I know if I’m expecting?

From the moment you are 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

When two zygotes come together, a zygote is formed, which divides promptly to become an embryo. As the pregnancy progresses, the embryo develops into a fetus. The fetus 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 fetus, we can all choose to use the terms that are most familiar to us. 

Gestational age: Embryo vs fetus

An embryo’s or fetus’ gestational age is expressed in weeks. When someone says they are 15 weeks pregnant, they are referring to the gestational age of their fetus.

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 fertilization defines gestational age.

The precise timing of fertilization is frequently more difficult to determine (except in cases of assisted reproductive technology, like in vitro fertilization or IVF).

In most cases, fertilization 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 fetus’s true age (which starts with fertilization) 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 Embryo?

Doctors utilize 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 fertilized 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 grows into the placenta and embryo. When the amniotic sac develops (about day 10 to 12 following fertilization, 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 tenth 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 fertilization, 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 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, 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 center to unite and create a single diploid cell at that location (center).
• 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.

When is a fetus a baby legally?

When the embryonic period ends in the 10th week of pregnancy, the embryo is referred to as a fetus. A fetus is a developing infant beginning in the 11th week of pregnancy.

The fetal 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 fetus will largely be growing and maturing tissues.

Although a fetus who is at least 24 weeks old may be viable if given sufficient care after birth, there is no set time for fetal “viability” (the ability to survive outside the uterus). A fetus is less likely to survive than an older fetus because its lungs and brains are immature before 30 weeks of gestation.

Major developmental milestones in the fetal stage

• Weeks 10-13: the fetus continues to grow fast, and the kidneys begin to generate urine.
• In weeks 14-17, external genitalia has developed, coordinated limb motions have begun, bones have hardened, and ocular movement has commenced.
• Weeks 18-21: visible brows and head hair, fetal uterus, and formation of vaginal canal
• In weeks 22–26: the fetus has fingernails and is gaining weight.
• Weeks 27-30: The lungs and brain have matured. To the point where the fetus would almost probably survive if born and given intensive care at this stage. The eyelids are open, toenails are visible, and the fetus is gaining weight.
• In weeks 31-35, pupils respond to light.
• Weeks 36-40: Firm grip and daily fat gain of 14 grams (12 ounces).

What is a newborn?

The transition from fetus to newborn (also known as neonate) at birth is complicated and must happen quickly for the infant to be able to function independently.

The fetus 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 fetal 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 mostly 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.

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