Many people disagree one the time when an embryo becomes a fetus, with some adamant it happens at the time of conception, and others stating the change occurs when a fetal heartbeat can be heard. The truth is that a baby within a uterus is continually growing from the time of conception, and consistently changing every hour, day, week, and month. So, when does the change actually happen?

When Does an Embryo Become a Fetus?

An embryo officially becomes a fetus at the end of the embryonic period, which is the 8^th week after conception, the 10^th week of pregnancy. After that, the fetal period begins, which is the latter stages of pregnancy. During the 5-10^th weeks of pregnancy (the embryonic period), the brain, lungs, heart, and other internal organsas well as the arms and legs all begin to form. Generally, your date of expected delivery is devised 40 weeks from the first day of your last period, meaning that your pregnancy essentially starts before the date of conception when calculating your menstrual/gestational age. Once the embryonic period has finishedand the fetal period begins, the fetus continues to grow and developto get ready for life outside of the uterus.

The First Trimester: Baby Development Week by Week

Week 1-2

• Week one begins the first week of a mothers last period, before conception.
• During the end of week 2, an ovary releases an egg, making it the most likely time of conception if one was to have unprotected sex.

Week 3

• During sexual intercourse, sperm enters the vagina via penile ejaculation. The strongest sperm journeys through the cervix to the fallopian tubes where it meets and combines with the egg released from the ovary, forming what is known as a zygote (combination of sperm and egg).
• Shortly after conception/fertilization, the zygote moves down the fallopian tube and travels toward the uterus.
• As the zygote moves toward the uterus, it begins splitting to form a morula, a cluster of cells which looks aesthetically similar to a petit raspberry.
• This cluster contains an inner assemblage of cells as well as an external shell. The inner cells develop into the embryo, which develops into the fetus/baby, the outer shell develops into membranes, which are structures that protect and nourish the embryo. This is known as a blastocyst.

Week 4

• The blastocyst continues toward the uterus and submerges itself within the uterine wall (called implantation). The lining of the mother’s uterus has now become thick with blood, ready to support a baby’s growth.
• The mother’s blood works to nourish the embedded blastocyst, and the placenta begins to form.
• When does an embryo become a fetus? Not yet, and although the embryo is still rather small around the size of a sesame seed, you may begin to experience pregnancy symptoms such as frequent urination, food cravings, morning sickness, breast tenderness, etc. Some women may also gain weight at this time.
• By this time, the mother-to-be should have had her first Ob/Gyn appointment.

Week 5

• The embryonic period officially begins at week 5. This is when internal organs and limbs begin to develop.
• The embryo now consists of 3 layers, the ectoderm (top layer), is where the child’s outer layer of skin, nervous system, inner ears, eyes, and various connective tissues develop.
• The middle layer, known as the mesoderm, will lay the foundation for the child’s muscles, bones, kidneys, heart, and primary circulatory system, as well as the majority of the reproductive system.
• The inner layer, called the endoderm, is where the child’s bladder, intestines and lungs develop. Now it is simply a tube coated with mucous membranes.
• For those who ask “When does an embryo become a fetus?” now a heartbeat may be discovered via an ultrasound.
• Also, at this time, the developing embryo is at most risk from external toxins, making it important that pregnant women avoid the consumption of alcohol, caffeine, cigarette smoke, and certain medications to avoid harming the developing child at this crucial time.
• The baby is around the size of a pen tip now. 

Week 6

• By this time, the neural tube begins to close, and the child’s heart is now pumping blood.
• Facial features begin to appear, and the baby’s body will start to look C-shaped.
• Leg and arm buds begin to grow, ears and eyes begin to form, and there is tissue growth that will eventually develop into the spinal chord and other bones.

Week 7

• At this time, the child’s facial features are rapidly developing, so is the brain.
• Small nostrils are now visible and the lenses of the eyes start developing.
• Arm buds develop further to become paddle-shaped.
• The baby is around the size of the top of a pencil eraser, or slightly bigger.

Week 8

• The arms and legs of the growing child become longer, and hands and feet begin to form.
• The brain continues to steadily grow and develop, and the lungs start to form.
• The baby’s ears begin to develop a shell-shaped structure, and the eyes are now visible. 
• The nose and upper lip have formed, and the baby’s body is beginning to straighten.
• Now the baby is around half an inch long.

Week 9

• Elbows develop as the arms grow.
• Toes begin to develop and become visible.
• All essential organs have started to grow.
• The baby is around ¾ of an inch long.
• Between this week and the 12^th, womenover the age of 35, or those with a family history of chromosomal abnormalitieswill likely to undergo a test called chorionic villus sampling (CVS) to detect such abnormalities of the baby (such as Down syndrome). This involves a doctor removing a small piece of tissue from the placenta as test sample.

Week 10

• When answering “When does an embryo become a fetus?”the official time is considered to be at the end of this week.
• Facial features become distinctive, eyelids develop further and start opening and closing, the outer ears begin to shape, and rotation of the intestines occurs. The head becomes rounder, and the neck starts development.

Week 11

• The baby is now officially called a fetus. The head makes up around half of the fetus’s length, but the rest of the body soon catches up.
• The child’s liver begins to develop red blood cells, and external genitalia begin to develop around the end of the week.
• The baby is around 2 inches in length and weighs around 8 grams (1/3 ounce).

Week 12

• The child begins to develop fingernails and what can be considered a distinctive human face.
• The baby measures around 2 ½ inches, weighing around 14 grams (1/2 ounce).
• By this time, many mothers-to-be notice that their old cloths are becoming tight.

For a detailed illustration of how the embryo becomes a fetus, please watch:

What Happens Next?

Now that you have an answer to the question “When des an embryo become a fetus?” you may wish to know what happens next.

During the second trimester, the baby begins to move and develop hearing. By week 22, the baby’s heartbeat can be heard using a stethoscope. At week 26, rapid brain development occurs. In the third trimester, the respiratory and nervous systems develop, as do bones, fingernails, and hair.