What’s the Difference Between an Embryo and a Fetus?

  Photo by  Autumn Mott  on  Unsplash
Photo by Autumn Mott  on Unsplash

What is an Embryo?

In biological terms, an embryo is a growing or unhatched offspring that is in the process of development. Typically, the word “embryo” refers to a human offspring during the first eight weeks after fertilization. In everyday conversation, an embryo may be referred to as a fertilized egg, a fetus, an unborn child, a zygote or a human embryo. While these terms communicate the same basic idea, there is actually a scientific difference between them.

What’s the difference between an Embryo and a Fetus?

Those terms mark different phases of development. As noted above, an embryo is the term for a fertilized egg during the first eight weeks. After the eight week mark, the embryo becomes a fetus. According to Dr. O’Brien from Women & Infants Hospital of Rhode Island,

“The embryo is defined as the developing pregnancy from the time of fertilization until the end of the eighth week of gestation, when it becomes known as a fetus.”

An embryo is in its earliest stage of development. A fetus is in the later stages of development. The embryo develops during the embryonic period. The fetus develops during the fetal period.

What happens during the Embryonic Period?

During the embryonic period, cells begin to learn their roles–what cells will be heart cells, what cells will be brain cells, what cells will become skin…

The embryonic period begins at fertilization at lasts through the 10th week of gestation (8th week by embryonic age). The two weeks after fertilization are sometimes referred to as the Germinal Stage or Germinal Period. This stage is also part of the embryonic period–the very first phase of growth.

During this phase, all of the major organs begin to form, but the embryo is incredibly fragile. Common occurrences like viruses, medications, radiation, and infections can be detrimental to an embryo. These dangers are sometimes called teratogens in medical journals. These teratogens can cause deformities or lead to miscarriage.

Because the developing embryo is so fragile and so small (about one inch long at two months’ gestation), most women will not announce their pregnancy until this period is over. In fact, some women may have miscarriages and be unaware (mistaking it for a period) during this time. All embryos are incredibly fragile during this phase.

What happens during the Fetal Period?

About two months after conception, we enter the Fetal Period. This is the third and final phase of development for a fetus. This phase lasts until birth (or the end of the pregnancy).

Once an embryo has become a fetus, the development focuses more on growth and preparation for life outside of the womb. Just over a month into this phase (about three months’ gestation) sex organs begin to form. At this time, you can most likely find out the sex of your baby via ultrasound, if desired.

As bones and muscles grow, the fetus itself begins to grow at an astounding pace. You may begin to feel fetal movement during this phase. Your baby is beginning to move around inside the uterus–which is his home for now. Organ systems continue to develop, allowing them to actually start working. For example, Baby’s lungs will “practice breathing” for life outside the womb.

During the last three months of pregnancy, the brain begins to develop at a rapid speed, continuously growing in size. A layer of fat forms under the skin to serve as insulation. The respiratory and digestive systems begin to function independently for the first time.

When is pregnancy considered viable?

After 26 weeks of gestation, a fetus is considered viable. However, as many devastated parents know–this is a guideline and not a rule. A healthy, low-risk pregnancy can unexpectedly end with a stillbirth or fetal death. And yet, as many trilled parents know–a baby born at only 24 weeks can sometimes survive life outside the womb.

When a baby is born at a fetal age of 26 weeks, he has an 80% chance of surviving to one year of age. If a he’s born at 27 weeks, his chances of survival are even higher–at 90%. The youngest baby to ever survive premature birth was born at only 21 weeks and 6 days gestation. These miracle children amaze their parents, their communities, and the world at large.

Family Life with a Preemie

If you’ve been blessed with a survivor preemie, you may notice she will not follow the developmental milestones of her peers during infancy and toddler-hood. Most experts agree that premature children (excluding those with other medical issues) catch up to their peers around 5 years of age. Once your little fighter reaches kindergarten, she’ll be indistinguishable from her peers.

On a personal note–my family experienced a premature birth nearly one year ago. My sister and I were both pregnant. I was due on March 30, 2016, and she was due near the middle of June. I had checked into the hospital, preparing for an induction for the birth of my son when I started receiving frantic phone calls and texts from my sister, my other sister, and my Mom.

While we live on opposite ends of the country, motherhood has brought us closer together and we keep in better touch now that we lived closer.

In the morning, I got a call from my sister who had said she’d been bleeding. She’d been bleeding throughout the pregnancy, but her doctors couldn’t find any particular reason for it. Then, she felt as though her water had broken. This was her fourth pregnancy–so she knew what was happening.

She checked in to the local hospital, but being a small, rural outfit that’s more like a triage center than a hospital, the doctors there admitted they were unequipped to provide care. She was transferred via ambulance to a major hospital an hour away. The doctors there confirmed her water had broken and they would need to perform an emergency c-section. My sister had never before had a c-section. Did I mention she’s also terrified of needles, surgery and just about any medical procedure except birth? She was scared, but like any mother, she knew what she had to do for her child.

They began prepping for surgery. My sister, despite having given birth many times, being nearly 30, and my constant complaints, is tall and slim. She has a flat stomach and at 26 weeks pregnant–even the surgeons had a hard time this was the patient they were performing a c-section on.

My sister overheard the anesthesiologist asking, “Her? Are you sure? She doesn’t even look pregnant!” 

It was true. She didn’t look pregnant. She hadn’t even started talking to her two-year-old about his new little sister and yet my niece was successfully delivered via C-section that day. She stayed in the NICU for two months after birth, getting stronger and healthier every day. Unfortunately, that NICU was an hour away from home and my sister couldn’t visit as often as she wanted since she had to tend to her other children.

As for my son, after pointlessly laboring for what felt like a million years, I also had an emergency c-section. His cousin, who was due two months after him is actually one day older than him! While my son is walking and talking, she has just started crawling. With her fighting spirit, I know she’ll catch up to him, probably soon that we all think!

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