Pictures and Descriptions of the Developing Embryo and Fetus

How do medical professionals describe a pregnancy at different stages?

Healthcare providers call an unborn child either an “embryo” or a “fetus” depending on the age. When a pregnancy is eight weeks or less (measured from the time of conception), they
describe it as an “embryo.” When the age is more than eight weeks (measured from the time of
conception), they use the term “fetus.”

How do healthcare providers describe the age of an embryo or fetus?

Healthcare providers describe the age of a fetus in two ways. The first is based on the number of weeks since the first day of the woman’s last menstrual period. This is referred to as “LMP” age, because “LMP” is short for “last menstrual period.” It is also known as “gestational age.”

The second way to describe the age of a fetus is based on the number of weeks since the time of conception. This is referred to as “conceptional age” or “age from the time of conception.”

What is the difference between LMP age and conceptional age?

The LMP age of an embryo or fetus is approximately two weeks more than the conceptional age.

For example, if your healthcare provider says that the fetus is 12 weeks LMP age (again, LMP stands for last menstrual period), it also means that the fetus is 10 weeks conceptional age, or 10 weeks of age measured from the time of conception.

On the other hand, if your healthcare provider says the fetus is 12 weeks conceptional age, it also means that the fetus is 14 weeks LMP age, or 14 weeks measured from the last menstrual
period.

The pictures of embryos and fetuses in this booklet will give the conceptional age and the LMP age. The LMP age will be in parentheses.

How do healthcare providers measure the length of an embryo or fetus?

Healthcare providers usually measure the length of an embryo or fetus from the crown of the head to the rump. This is known as “crown-rump” length, or “CR” length for short.

The photo below shows how the crown-rump length of an embryo is measured. This way you can see what is meant by “crown” and what is meant by “rump.”

The pictures in this booklet will give the size of the embryo or fetus according to CR or crown-rump length.

cr

crown

 

 

 

 

 

rump

 

Image Source: A Colour Atlas of Life Before Birth
by: Marjorie A. England, 1990.
Mosby-Wolfe Europe Limited, London, UK.

Each pregnancy is unique. The development of each embryo and fetus is based on a number of variable factors.

The following materials describe the typical, normal development of an embryo and fetus, giving typical sizes.


Typical Embryo at
Four Weeks from the Time of Conception
(Six Weeks LMP)

Length from crown to rump: Less than 1/8 inch (3 millimeters)

 

The embryo has the beginning of a circulatory system. The cells that will form the heart have begun to move and blood flow begins. The neural tube (which later becomes the spinal cord and brain) forms by week five. The locations of the ears, eyes, and nose are just
becoming evident.

 

 

 

Image Source: Lennart Nilsson/Bonnier Alba AB

The size of this photograph is larger than the actual typical size of an embryo at this stage


Typical Embryo at
Six Weeks from the Time of Conception
(Eight Weeks LMP)

Length from crown to rump: 5/8 inch  (15 millimeters)

 

The heart continues to develop. The umbilical cord is formed. Structures that will form eyes, ears, arms and legs begin to form. The spinal cord is also beginning to take shape. Toward the end of this period is the first time the embryo can be seen with the naked eye.

 

 

 

 

Image Source: Lennart Nilsson/Bonnier Alba AB

The size of this photograph is larger than the actual typical size of an embryo at this stage.


Typical Fetus at
Eight Weeks from the Time of Conception
(10 Weeks LMP)

Length from crown to rump: 1 ¼ – 1 ½ inches (30 millimeters)

 

 

The beginnings of organs are present, although not fully developed or in their final locations. The shape of the lungs is nearly formed. The intestines are beginning to coil. The diaphragm is completed. The digits of the hand are separated but webbed. Eyes and ears are more obvious. Tongue and taste buds are present. The fetus begins reflex activity but movements are too slight to be felt.

 

 

Image Source: Lennart Nilsson/Sonnier Alba AB.

The size of this photograph is larger than the actual typical size of an embryo at this stage.


Typically Fetus at
Ten Weeks from the Time of Conception
(12 Weeks LMP)

Length from crown to rump: 2 3/8 inches (61 millimeters)

 

At this age, the embryo is now called a fetus. The heartbeat can usually be heard using ultrasound or fetal doppler. The kidneys are present but are not fully functional. The nose is clearly visible, and mouth or finger movement may occur spontaneously. Anatomy noted in photo:

1. Arm              5. Knee
2. Brain           6. Mouth
3. Ear               7. Toes
4. Eye               8. Umbilical cord

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is larger than the actual typical size of a fetus at this stage.


Typical Fetus at
12 Weeks from the Time of Conception
(14 Weeks LMP)

Length from crown to rump: 3 3/8 inches (87 millimeters)

 

 

Some bones are formed sufficiently to be visible on an x-ray. The spinal cord is continuing to develop. The bone marrow is making blood cells. External genitalia are forming and the sex of the fetus can occasionally be correctly determined by experienced observers looking at ultrasound images.

 

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is about the actual typical size of a fetus at this stage.


Typical Fetus at
14 Weeks from the Time of Conception
(16 Weeks LMP)

Length from crown to rump: 4 3/4 inches (120 millimeters)

 

 

The joints are now functioning and limb movements become more coordinated.

 

 

 

 

 

Image Source: Lennart Nilsson/Sonnier Alba AB.
The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
16 Weeks from the Time of Conception
(18 Weeks LMP)

Length from crown to rump: 5 1/2 inches (140 millimeters)

 

 

Swallowing and chest movements are clearly present. The ears stand out from the head now. Muscles are spontaneously active. The mother may feel movements now. Anatomy noted in photo:

1. Ear
2. Eye
3. Umbilical cord

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typically Fetus at
18 Weeks from the Time of Conception
(20 Weeks LMP)

Length from crown to rump: 6 1/4 inches (160 millimeters)

 

 

 

A protective waxy coating on the skin, called “vernix caseosa” is present. The spinal cord is continuing to develop. The bone marrow is making blood cells. The sex of the fetus can be more easily identified by ultrasound.

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
20 Weeks from the Time of Conception
(22 Weeks LMP)

Length from crown to rump: 7 1/2 inches (190 millimeters)

 

 

A downy hair covers the body. The lungs are still very immature and survival rates at this age are rare, even with intensive care.

1. Abdomen
3. Eye
6. Umbilical cord

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
22 Weeks from the Time of Conception
(24 Weeks LMP)

Length from crown to rump: 8 1/4 inch (210 millimeters)

 

 

Most of the organs are now formed and the fetus enters now a period of growth and further organ development.

 

 

 

 

Image Source: Lennart Nilsson/Bonnier Alba AB.
The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
24 Weeks from the Time of Conception
(26 Weeks LMP)

Length from crown to rump: 9 inches (230 millimeters)

 

 

The fetus has fingernails and clearly demonstrates reflexes such as sucking and gripping.

 

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
26 Weeks from the Time of Conception
(28 Weeks LMP)

Length from crown to rump: 9 13/16 inches (250 millimeters)

 

 

The eyes are partially open and eyelashes are present.
Anatomy noted in photo:
1. Abdomen
2. Eye
3. Hand

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
28 Weeks from the Time of Conception
(30 Weeks LMP)

Length from crown to rump: 10 5/8 inches (270 millimeters)

 

 

 

At this time, the eyes are fully open. There may be hair on the fetus’ head and body and the skin is smooth.

 

 

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A England, 1990. Mosby-Wolfe Europe Limited, London, UK.

The size of this photograph is smaller than the actual typical size of a fetus at this stage.


Typical Fetus at
38 Weeks from the Time of Conception
(40 Weeks LMP)

Length from crown to rump: 14 3/16 inch (360 Millimeters)

 

 

 

At this time, the fetus is fully developed or considered to be full-term.

 

 

 

 

 

Image Source: A Colour Atlas of Life Before Birth by Marjorie A. England, 1990. Mosby-Wolfe Europe Limited, London, UK.

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