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The embryo has the beginning of a circulatory system and the developing heart begins to beat. The neural tube (which later becomes the spinal cord and brain) has formed. The locations of the ears, eyes, and nose are just becoming evident.

Length of the embryo: less than 1/8 inch (3 millimeters) from crown (head) to rump (bottom).

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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.

Length of the embryo: 5/8 inch (15 millimeters) from crown (head) to rump (bottom).

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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. Reflex activity begins but movements are too slight to be felt.

Length of the embryo: 1 1/4 - 1 inch (30 millimeters) from crown (head) to rump (bottom).

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

Length of the fetus: 2.4 inches (61 millimeters) from crown (head) to rump (bottom).

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Some bones are formed sufficiently to be visible on an x-ray of the mother. The spinal cord is continuing to develop. The bone marrow is making blood cells. External genitalia are forming.

Length of the fetus: 3.4 inches (87 millimeters) from crown (head) to rump (bottom).

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The first trimester is complete and the second has begun. The joints are now functioning and limb movements become more coordinated. Fingerprints are forming.

Length of the fetus: 4.7 inches (120 millimeters) from crown (head) to rump (bottom).

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Swallowing and chest movements are clearly present. The ears stand out from the head now. Muscles are spontaneously active. The pregnant woman may start to feel movements now.

Length of the fetus: 5.5 inches (140 millimeters) from crown (head) to rump (bottom).

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20 weeks

The nervous system and lungs continue to mature. Body fat continues to accumulate.

Length of the fetus: 6.3 inches (160 millimeters) from crown (head) to heel.

22 weeks

A downy hair covers the body. The fetus can swallow and the digestive system begins to fill with meconium (the dark green substance forming the first feces of a newborn infant). The lungs are still very immature.

Length of the fetus: 10 inches (250 millimeters) from crown (head) to heel.

Survival outside the womb - before 22 weeks:

National research shows that before 22 weeks’ gestational age, there is no possibility that a fetus can survive outside the womb. After this time, survival depends on a number of factors, most importantly gestational age at the time of birth.

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

Length of the fetus: 11.8 inches (300 millimeters) from crown (head) to heel.

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The fetus has fingernails and clearly demonstrates reflexes such as sucking and gripping. The skin is still thin and translucent.

Length of the fetus: 14 inches (350 millimeters) from crown (head) to heel.

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28 weeks

The pregnancy enters its third trimester. The eyes of the fetus are partially open and eyelashes are present.

Length of the fetus: 14.8 inches (375 millimeters) from crown (head) to heel.

30 weeks

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

Length of the fetus: 15.7 inches (400 millimeters) from crown (head) to heel.

32 weeks

The lungs are continuing to mature. The head is growing to make room for the developing brain.

Length of the fetus: 16.7 inches (425 millimeters) from crown (head) to heel.

34 weeks

The fetus can turn its head from side to side. A protective layer of fat is accumulating under the skin, filling out the arms and the legs.

Length of the fetus: 17.7 inches (450 millimeters) from crown (head) to heel.

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36 weeks

The nervous system and lungs continue to mature. Body fat continues to accumulate.

Length of the fetus: 18.7 inches (475 millimeters) from crown (head) to heel.

38 weeks

The fetus is gaining weight of about one ounce per day and losing the fine down and vernix caseosa that covered the body.

Length of the fetus: 19.6 inch (500 millimeters) from crown (head) to heel.

40 weeks

The due date has arrived. The fetus can grasp firmly. The head continues to be the largest body part. The body usually appears plump. Toenails reach the toe tips.

Length of the fetus: 20.2 inch (510 millimeters) from crown (head) to heel.