If You Are Pregnant - Fetal Development

FETAL DEVELOPMENT

How old is the fetus?

A pregnant woman may notice her first missed menstrual period at the end of the second week after conception, or about four weeks after the first day of her last normal period. There are different kinds of tests for pregnancy. Some may not be accurate for up to three weeks after conception, or five weeks after the first day of the last normal period.

The Week in the blue block beside each picture is the age from conception. The gestational age (in parenthesis) is usually figured from the first day of the last menstrual period.

Week 2 (4 weeks after the first day of the last normal menstrual period)
Implantation begins the first week and the embryo continues to grow. The embryo is about 1/100 of an inch long at this time.

Week 4 (6 weeks after the first day of the last normal menstrual period)
The embryo is about 1/6 inch long and has developed a head and a trunk. Structures that will become arms and legs, called limb buds, begin to appear. Blood is beginning to be pumped through fetal circulation. Heartbeat is visible by ultrasound.

Week 6 (8 weeks after the first day of the last normal menstrual period)
The embryo is about 1/2 inch and has a four chambered heart and nostrils. Fingers and toes begin to form. Reflex activity begins with the development of the brain and nervous system.

Week 8 (10 weeks after the first day of the last normal menstrual period)
The fetus, until now called an embryo, is about 1-1/4 inches long (with the head making up about half this size) and weighs less than 1/2 ounce. The beginnings of all key body parts are present, although they are not completely positioned in their final locations. Structures that will form eyes, ears, arms and legs are identifiable.

Week 10 (12 weeks after the first day of the last normal menstrual period)
The fetus is about 2-1/2 inches from head to rump, weighing about 1-1/2 ounces. Fingers and toes are distinct and have nails. The fetus begins small, random movements, too slight to be felt. The fetal heartbeat can be detected with a doppler or heart monitor.

Week 12 (14 weeks after the first day of the last normal menstrual period)
The fetus is about 3-1/2 inches from head to rump and weighs about 2 ounces. The fetus begins to swallow, the kidneys make urine, and blood begins to form in the bone marrow. Joints and muscles allow full body movement.

Week 14 (16 weeks after the first day of the last normal menstrual period)
The fetus is about 4-3/4 inches from head to rump and weighs 4 ounces. The head is erect and the arms and legs are developed.

Week 16 (18 weeks after the first day of the last normal menstrual period)
The fetus is about 5 inches from head to rump and weighs about 6 ounces. The skin is pink and transparent and the ears stick out from the head.

Week 18 (20 weeks after the first day of the last normal menstrual period)
The fetus is about 6-1/4 inches from head to rump, weighing about 10 ounces. All organs and structures have been formed, and a period of simple growth begins. Respiratory movements occur, but the lungs have not developed enough to permit survival outside the uterus. By this time the woman may feel the fetus moving.

Week 20 (22 weeks after the first day of the last normal menstrual period)
The fetus is about 7-1/2 inches from head to rump, has fingerprints and perhaps some head and body hair, weighing about one pound (16 ounces). There is little chance before this time that a baby could survive outside the woman's body. Fetal heartbeat can be heard with a stethoscope.

Week 22 (24 weeks after the first day of the last normal menstrual period)
The fetus is about 8-1/4 inches from head to rump and weighs about 1-1/4 pounds. Changes are occurring in lung development so that some babies are able to survive (with intensive care services). Surviving babies may have disabilities and require long-term intensive care.

Week 24 (26 weeks after the first day of the last normal menstrual period)
The fetus is about 9 inches from head to rump and weighs about 2 pounds. The fetus can respond to sound. About 4 out of 10 babies born now may survive (with intensive care services).

Week 26 (28 weeks after the first day of the last normal menstrual period)
The fetus is about10 inches from head to rump and weighs about 2-1/2 pounds. The eyes are partially open and can perceive light. About 9 out of 10 babies born now will survive (with intensive care services).

Week 28 (30 weeks after the first day of the last normal menstrual period)
The fetus is about 10-1/2 inches from head to rump and weighs almost 3 pounds. The fetus has lungs that are capable of breathing air, although medical help may be needed. The fetus can open and close its eyes, suck its thumb and respond to sound. Nearly all babies born now will survive (with intensive care services).

Week 30 (32 weeks after the first day of the last normal menstrual period)
The fetus is about 11 inches from head to rump and weighs more than 3 pounds. Skin is thicker and more pink. Almost all babies born now will live (with intensive care services).

Week 32 (34 weeks after the first day of the last normal menstrual period)
The fetus is about 11-3/4 inches from head to rump and weighs about 4-1/2 pounds. Ears begin to hold shape. Almost all babies born now will live (with intensive care services).

Week 34 (36 weeks after the first day of the last normal menstrual period)
The fetus is about 12-1/2 inches from head to rump and weighs about 5-1/2 pounds. Scalp hair is silky and lays against the head. Almost all babies born now will live.

Week 36 (38 weeks after the first day of the last normal menstrual period)
The fetus is about 13-1/2 inches from head to rump and weighs about 6-1/2 pounds. Lungs are usually mature. The fetus can grasp firmly. Almost all babies born now will live.

Week 38 (40 weeks after the first day of the last normal menstrual period)
The fetus is about 14 inches from head to rump, may be more than 20 inches overall, and may weigh from 6-1/2 to 10 pounds. The baby is full-term and ready to be born.