How A Fetus Grows


The fetus grows from an embryo into a baby ready for birth. See more pregnancy pictures.
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The journey of the fetus through pregnancy is a complex and fascinating miracle that scientists have broken into three phases -- the three trimesters that represent a nine-month pregnancy. The fetus metamorphoses from an embryo of multiplying cells into a being with organs and muscles that is ready for life outside the womb. Read about the journey of the fetus here:

  • How a Fetus Develops in the First Trimester During the first trimester the initial cell multiplies, and organs grow visible even before the embryo becomes a fetus. The head grows disproportionately large and the tiny heart begins to beat. The fetus is two to three inches long and weighs less than one ounce by the end of the third month, at which point it begins to make breathing movements, and the intestinal and skeletal muscles begin to work and respond to local pressure.
  • How a Fetus Develops in the Second Trimester The second trimester is a time for the maturation of the organs, which are already present, but the baby is not yet able to live outside the mother's body. By the sixth month fine hair covers the body and the mother can feel the baby's movement.
  • How a Fetus Develops in the Third Trimester The final three months are devoted to growth, and the ninth month to independent existence. The longer the baby remains in the womb during the third trimester, the greater their chance of surviving birth. By this time the fetus is usually 20 to 22 inches long and weighs seven to eight pounds.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.

How A Fetus Develops in the First Trimester

By the end of the first trimester the organs of the fetus are visible.
By the end of the first trimester the organs of the fetus are visible.
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In the first weeks after conception, the single cell rapidly divides into many cells. A hollow ball of cells is formed and becomes attached to the wall of the uterus (womb). Some of the cells become the placenta; the rest become the embryo. The latter group of cells develops into a four-layered disk. Each layer is converted into different areas of the body. The outer layer, or ectoderm, for example, develops into skin, hair, nails, and the nervous system. The inner layer, or endoderm, develops into the intestines and lungs. The middle layers develop into the heart, bones, and muscles.

By three weeks after fertilization, or about one week after the first period is missed, the embryo is 1/10 inch long and has an oval shape. In the next few weeks, it becomes more curved, and a head and tail become discernible. The beginnings of the spinal cord and brain take shape. A heart begins to form. Tiny eyes can be seen. Arms and legs begin to bud.

By the fourth week after fertilization, traces of all the organs of the body are present. Bulges that will become the ears and nose appear. The gut is formed from blind pouches within the embryo; these pouches push forward, creating an opening in the head that will become the mouth. A crude face begins to take shape. At this point, the embryo is only 1/4 inch long.

The embryo is called a fetus at the seventh to eighth week. It has grown to a length of one inch. The head is disproportionately large because of the size of the developing brain, while the abdomen seems large because of the growing liver. Fingers and toes appear. The rudiments of all the major hormone-producing glands -- the pituitary, thyroid, and adrenal glands -- are present. Amazingly, the tiny heart begins to beat.

By the end of the third month, the fetus is two to three inches long and weighs less than one ounce. Nails form on the fingers and toes. The bones begin to calcify. The sex organs begin to develop. The tooth buds form in the mouth. The fetus begins to make breathing movements and starts to swallow amniotic fluid. The muscles of the intestines contract and relax, as if digesting food. Skeletal muscles begin to work as well, so the fetus can move in response to local pressure.

After three months some women might not even look noticeably pregnant. However, during the second trimester the baby really starts to grow and develop. Learn about what changes to expect in the fourth through sixth months in the next section.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.

 

How A Fetus Develops in the Second Trimester

Although all the organs are present by the end of the first trimester, the fetus is not yet able to live outside the mother's body. The second trimester is devoted primarily to maturation of the organs. By the fourth month, the fetus moves spontaneously but is too small for the mother to feel this movement. The fetus is four to five inches long and weighs three ounces.

By the fifth month, however, the fetus is six inches long and weighs 1/2 pound. The mother's growing perception of the baby's movement is known as quickening. In the fifth to sixth month, the body becomes covered with fine hair, or lanugo, and coarser hair appears on the head.

After the sixth month you are ready for the final push before the baby is born. Some mothers find this to be the most uncomfortable time. In the next section, we will show you what to expect during the third trimester.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.

How A Fetus Develops in the Third Trimester

The fetus is fully developed by the beginning of the third trimester. The last three months, therefore, are devoted to growth. By the seventh month, the fetus is about 14 to 17 inches long and weighs about two pounds. The skin is red, wrinkled, and thin. It becomes covered with vernix, a thick, white, sticky material composed of skin cells, lanugo, and oily skin secretions.

A baby born at this time has at least a 60 percent chance of survival with appropriate medical care, and the survival rate increases with each passing day as the baby develops. Babies born this early can respond to taste, light, and sound.

The chance of survival for a baby born during the eighth month of gestation increases to 90 percent. By this time, the fetus is 16 to 18 inches long and weighs three to four pounds.

The final preparations for independent existence occur during the ninth month. Surfactant, a substance that lines the lungs and allows them to expand easily, develops. Fat is stored, and its deposition under the skin smoothes out the wrinkles. Much of the lanugo disappears.

By the final month of pregnancy, the fetus is usually 20 to 22 inches long and weighs seven to eight pounds. The baby is large and strong enough for the next step -- birth and independent life. That one cell has come a long way, from embryo to fetus to newborn.

Though it is only nine months, some new mothers feel that their pregnancy will last forever. During this time of both slow and rapid change, it's important to have a sense of the complete process to understand what is happening to you and your baby.

ABOUT THE CONSULTANT:

Dr. Elizabeth Eden, M.D. is a practicing obstetrician with her own private practice in New York City. She serves as an attending physician at the Tisch Hospital of the New York University Medical Center, as well as a Clinical Assistant Professor at the New York University School of Medicine.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.

 

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