What Is the Difference Between an Embryo and a Fetus?

Both embryo and fetus are terms used to describe the stages of a baby’s development during pregnancy. Gestational age or the length of time a person is pregnant is the main difference between an embryo and a fetus. This article will explain more about an embryo and a fetus.

Human embryo in the womb during pregnancy

Vladimir Zotov / Getty Images

What Is an Embryo? 

An embryo is the early stage of a baby’s development during pregnancy. The first stage is the fertilized egg (zygote). It divides to become a blastocyst and travels down the fallopian tube to the uterus, where it may implant to establish the pregnancy.

The developing baby is termed an embryo from week five until the end of the 10th week of pregnancy (counted from the last menstrual period).

What Is a Fetus? 

A fetus is the later stage of a baby’s development during pregnancy. From the end of week 10 until you give birth, the baby is called a fetus.

Embryonic Stage 

During the embryonic stage, important systems and organs begin to form. Cells multiply to create these systems. 

Major Milestones During the Embryonic Stage

From week five to week 10, your baby begins to develop the:

  • Brain 
  • Spine
  • Heart tissue
  • Fingers and toes
  • Lungs 
  • Inner ear 
  • Muscles for the mouth, nose, and eyes 

Fetal Stage 

During the fetal stage, the baby goes through rapid growth and change. The systems and organs that formed during the embryonic stage develop more. The fetus also begins to look more like a human.

Major Milestones During the Fetal Stage

During weeks 10 through 12, your baby begins to develop the: 

  • Eyelids 
  • Fingernails 
  • Cartilage 
  • Liver 
  • Genitals 
  • Urine in the kidneys 
  • Insulin in the pancreas

During weeks 13 through 28, your baby begins to develop: 

  • Skin
  • Hair 
  • Fat 
  • A more complete skeleton 
  • Blood cells in the bone marrow 
  • Taste buds 
  • Fingerprints and footprints 
  • Hearing 
  • Sucking reflex 
  • Nervous system 

During weeks 29 through 40, your baby begins to develop: 

  • More body fat 
  • Fully functional organs and systems 
  • Ability to store minerals in the body 
  • Ability to sense light changes 
  • Ability to turn into a head-down position to prepare for birth 

Embryo vs. Fetus: Which Stage Is More High-Risk?

The first trimester of your pregnancy (week one through week 12) is considered to be the most important for a baby’s development and is also the time when most miscarriages and problems happen. Many structural and birth defects can happen during the first trimester. This means that the embryonic stage is more high-risk.

What to Expect at Your First Prenatal Appointment

You should schedule the first prenatal appointment as soon as you learn that you are pregnant. During the first prenatal appointment, you will discuss your: 

  • Medical history
  • Any preexisting conditions 
  • Prescription and over-the-counter medications you are taking 
  • Lifestyle and diet changes 
  • Additional prenatal supplements or vitamins to take 
  • Due date 
  • Other questions about the pregnancy 

Your healthcare provider may do the following: 

  • Physical exam
  • Order laboratory tests to check for blood type, infections, and other conditions 
  • Order other blood and urine tests 
  • Order tests to check for genetic or inherited conditions 
  • Ultrasound scan using sound waves to check your baby, depending on how far along you are in the pregnancy 

Summary 

An embryo and a fetus are different stages of your baby’s development during pregnancy. The embryonic stage happens first and lasts through week 10. The fetal stage comes next and lasts from the end of week 10 until you give birth.  

A Word From Verywell

Each stage of your baby’s development is important. This is why you should schedule a prenatal appointment with your healthcare provider as soon as possible. Talking to your healthcare provider and getting the right tests can help you have a healthier pregnancy. 

Frequently Asked Questions

  • Is an embryo a fertilized egg?

    An embryo grows from a fertilized egg. The egg has to be fertilized by sperm for a baby to develop.

  • What is the difference between an embryo and zygote?

    The zygote is the development stage before the embryo forms. A zygote is the fertilized egg.

  • What are the stages in pregnancy?

    In general, pregnancy is divided into three trimesters. The first trimester lasts from week one through week 12. The second trimester is week 13 through week 28. The third trimester is week 29 through 40.

     

    During each trimester, your body and the baby go through many changes. The first trimester allows your baby’s major organs to begin developing. The second trimester is a time of rapid growth and change for your baby.

    In the third trimester, your baby gains weight and their organs mature.

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3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Department of Health and Human Services Office on Women's Health. Stages of pregnancy.

  2. MedlinePlus. Fetal development.

  3. American College of Obstetricians and Gynecologists. Prenatal development: how your baby grows during pregnancy.