The Difference Between an Embryo and a Fetus
Experts explain the difference between an embryo and a fetus and why it matters.
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Medically reviewed by Alyssa Dweck, MS, MD, FACOG
On the outside, you’ll watch your little one grow from newborn to older infant to toddler, preschooler, kindergartener, and so on. It might feel like it goes by in the blink of an eye. But a little one-to-be rapidly goes through phases on the inside, too.
Expectant caregivers will catch glimpses of this development on ultrasounds, though some of it begins before an ultrasound can even detect a pregnancy. Knowing these phases can make pregnancy more fun.
“Your baby is going to go through the stages of development whether you pay attention or not, but it will make it extra special if you realize that maybe you are so tired because you’re working hard to grow your baby's kidneys or that your baby is bulking up gaining weight for delivery,” says Ashley Parr, DO, OB-GYN at the Women’s Hospital at MemorialCare Saddleback Medical Center in Laguna Hills, California.
Much of that organ development unfolds while a developing baby is still an embryo before transitioning into a fetus. What’s the difference? Experts answer that question and others, including milestone moments that happen throughout a pregnancy.
How a Pregnancy Starts
What comes first, the embryo or the fetus? The embryo. But before that, a developing baby is a zygote, explains Alexandra Kiefer, MD, an obstetrician-gynecologist at Stony Brook Medicine in Long Island, New York.
After conception, when a sperm fertilizes an egg, the short germinal stage begins. The zygote travels through the fallopian tubes, dividing rapidly as it heads toward the uterus. Eventually, the zygote splits. One part will ultimately become the embryo, and later the fetus. The other becomes the placenta, the organ that will provide vital nutrients to a developing human.
Dr. Kiefer says the zygote turns into a morula and then a blastocyst, which implants in the uterus. This implantation happens shortly before a missed period. Around the third or fourth week of a menstrual cycle—when a person is due for their period—the embryonic phase begins.
A Note on the Terms 'Embryo' and 'Fetus'
The terms "embryo" and "fetus" are used here in a medical context and, in no way, do they suggest or define viability.
The Embryonic Stage
A person may just be learning they are pregnant through a test—or may not even know at all—but an embryo is hard at work. “It’s developing all the important organ systems,” Dr. Kiefer says.
Despite the critical development, the embryo is tiny. “The moment a pregnancy test turns positive, the embryo is about the size of a pinhead,” Dr. Parr says. “By the end of the embryonic stage, the embryo is about 1.22 inches long.”
The small size is one reason health care providers will have you wait for a first ultrasound, typically around eight weeks. So much will develop in that time.
Milestones during the embryonic stage
Week four: The cardiovascular and neural plates begin to form. “The neural plate will become the neural tube, which will eventually become the spinal cord,” Dr. Kiefer says.
Week five: You may be able to detect an embryo on an ultrasound. “It’s about 3 millimeters long,” Dr. Kiefer says. “At this point, arm and leg buds have developed.”
Week six: “The neural tube closes, and you typically start to see a heartbeat,” Dr. Kiefer says.
Week seven and week eight: Organ systems are taking shape by this time.
The embryonic phase is when most pregnancy losses occur, though giving precise numbers is challenging. Research from 2015 found it’s been estimated that up to 50% of zygotes don’t become blastocysts, though a person likely hasn’t even taken a pregnancy test at this point.
“Many miscarriages occur without the mother realizing they were pregnant, so it is difficult to determine what percentage of pregnancies,” Dr. Parr says.
One 2013 study indicated the odds of pregnancy loss at week 5 were 21.3%, and the odds dropped to 5% after the sixth week, although these risks vary by factors including race.
“These miscarriages are typically due to abnormal chromosomes about half the time,” Dr. Kiefer says. “Many times, we never find out the reason for the loss.”
The Fetal Stage
By week nine, the embryo is a fetus, a phase that ends with the birth of a newborn. A fetus will continue to rapidly develop early in the fetal stage, says Monte Swarup, MD, FACOG, board-certified in OB-GYN in Chandler, Arizona., and founder of Vaginal Health Hub.
Milestones during the early fetal stage
Dr. Swarup says this development includes:
Week 9: The teeth, tastebuds, and body muscles form.
Week 10: Arms, hands, fingers, feet and toes are formed.
Week 11: Knees and elbows begin to work, bones harden, and the fetus develops more facial features.
Week 12: Circulatory and digestive systems begin, and limbs and muscles are functional.
Milestones during the second trimester
The second trimester begins at 13 weeks, explains Kecia Gaither, MD, MPH, MS, MBA, FACOG, a double board-certified OB-GYN and maternal fetal medicine specialist, and director of perinatal services/maternal fetal medicine at NYC Health + Hospitals/Lincoln in the Bronx. The odds of miscarriage drop to about 1-5%.
During the second trimester, Dr. Gaither says the fetus’s kidneys start to function by making amniotic fluid, or fetal urine. Other milestones include:
Week 18: “Mothers generally feel movement around week 18,” Dr. Gaither says. However, that can vary depending on the location of the placenta and whether it’s a first pregnancy.
Week 19: Dr. Parr says the face begins to develop its unique appearance.
Week 20: The anatomy scan occurs, where parents can see the organs develop and may learn the sex.
Week 24: Clinical viability is reached. “This is when we would expect the fetus has any chance of surviving once born,” Dr. Kiefer says, adding that it depends on lung development and a newborn would still need breathing assistance.
Week 26: A fetus might begin responding to sounds inside and can blink eyelids, Dr. Parr says.
Dr. Parr says after the 20th week, a pregnancy loss would be considered a stillbirth. March of Dimes reports stillbirths happen in 1 in 160 pregnancies.
Milestones during the third trimester
The third trimester kicks off at 28 weeks.
“At this point, about 94% of babies born are expected to survive,” Dr. Parr says. “During the final trimester, the bones are becoming harder, and the baby is putting on fat for delivery."
Embryo vs. Fetus: What To Know About the Miscarriage Risk
The risk of miscarriage, or pregnancy loss before 20 weeks of pregnancy, is highest in the first trimester with 80% of miscarriages happening during this time. The risk is particularly highest during the embryonic stage from week 0 to week 6. The risk continues to decrease after week 6 and more significantly after week 12. Keep in mind, genetic abnormalities or other health factors cause most miscarriages.
What To Expect at Your First Prenatal Appointment
Most prenatal appointments will occur around seven weeks, which can be overwhelming. Dr. Parr says health care providers will do and ask about the following:
The pregnant person’s medical history
Family medical history
Perform an ultrasound
Order bloodwork
Discuss appointment frequency and milestones
You’re encouraged to ask questions, too. Dr. Swarup suggests asking about:
How often you’ll see your health care provider
What scans you will need
Ways to cope with fatigue and morning sickness
Sound like a lot? A paper and pen can help.
“Write down our questions beforehand,” Dr. Kiefer says. “In the minute, you might forge, but of course, If you forget to ask them, you can always reach out to a doctor or midwife later.”
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