Pregnancy: Fetal Development

Fertilization

Day 1 - Sperm joins the ovum (egg) to form one cell. The fertilized egg immediately upon conception has 23 chromosomes from each parent, 46 in total. These 46 chromosomes contain all of the genetic information for this individual's human development - gender, hair and eye color, height, skin tone.

First Trimester

Day 3 to 4 - The fertilized egg travels down the fallopian tube into the mother's uterus.

Day 7 to 9 - Implantation: The fertilized egg implants or embeds itself in the lining of the mother's uterus where it will get nourishment to continue growing. It is now approximately a few hundred cells.

Day 10 to 14 - The fertilized egg is now called an embryo. It is developing rapidly and is now about the size of a pin head. It is nestled in a protective shell in the uterus wall. The cells begin to group together to form parts of what will be a fetus.

Day 21 - The embryo's body is approximately 1/10 of an inch long. The brain has two lobes. At some point during this week, before the mother's period is one week late, the embryo's heart will begin to beat.

Day 28 - The embryo is now ¼ inch long, 10,000 times larger than the fertilized egg. The blood vessel which will later develop into the heart and circulatory system begins to pump blood. A ridge of tissue forms down the length of the embryo. This will later become the brain and spinal cord. Arm and leg buds are present. By day 31 the outlines of fingers appear.

Day 31 - By the end of first month the heart is now beating 65 times per minute. The eyes, mouth, and brain have begun development. The embryo's simple kidneys, liver, digestive tract, blood stream, and heart, and an early umbilical cord are in the early stages of formation.

Day 40 - Brain waves can be detected and recorded.

Week 6 - It is now ¾ inch long (23 millimeters). The liver is producing blood cells, and the brain begins to control movement of muscles and organs. The mother is about to miss her second period.

Week 7 - The embryo begins to move. The jaw forms and teeth buds are in the gums.

Week 8 - Now about 1 ½ inches long (40 millimeters), the embryo is now called a fetus - Latin for "young one" or "offspring." Everything is now present that will be found in the fully developed adult. The heart has been beating for a month. The stomach and kidneys have begun to function, and there are approximately 40 muscle sets are beginning to function through the direction of the nervous system.

Week 10 - All major body features have appeared. The fetus is approximately 2 ½ inches long (60 millimeters). It weighs about ½ ounce.

Week 12 - All external body features have appeared: fingers and toes can be seen and genitalia are becoming distinguishable. The brain is fully formed, which some think, allows the fetus to feel pain. The fetus now has sleep and wake cycles. It exercises its muscles energetically, turning its head, curling its toes, and opening and closing its mouth, sucking its thumb.

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Second Trimester

Week 14 - The fetus is now approximately 5 inches long. Its limbs are developed. Fine hair grows on its head and genitals are visible. The fetus can turn its head, stretch, raise its eyebrow, wrinkle its forehead, and hear.

Week 16 - By the fourth month, the fetus is 8 to 10 inches in length by the end of this month. Many pregnant women will begin to "show" at this point. The ears are functioning and there is increasing evidence that the fetus can hear many things: the mother's voice and her heartbeat as well as noises outside of the mother. The umbilical cord conveys 300 quarts of fluid per day.

Week 18 - By 4 ½ months, the respiratory circulation occurs, and the woman can feel the fetus moving.

Week 23 - At 5 ½ months, the kidneys are starting to work and the air sacs of the lungs are developing. The fetus is proportioned like a newborn. The lips are distinct and the eyes are formed.

Weeks 24-26 - During this period the eyes begin to open and the fetus can detect light filtering through the mother's abdominal wall. Some fetuses born at this time are able to survive with special care.

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Third Trimester

Weeks 27-28 - At seven months development still occurs and the chances of survival are even greater for a baby born early. It is common for a fetus at this stage to get the hiccups, which may even be felt by the expectant mother.

Week 29-30 - The fetus weighs nearly three pounds and measures nearly 10 ½ inches from crown to rump -- almost 17 inches when you include the legs. The head is getting bigger, and brain growth is very rapid at this time. The fetus may be covered with a waxy substance called lanugo, which protects the delicate skin.

Week 31 - The lungs and digestive tract are almost fully developed. The fetus weighs about 4 pounds and looks more like a newborn. The fetus sleeps 90 to 95% of the time and sometimes experiences REM sleep, an indication of dreaming.

Week 32-38 - The fetus grows rapidly, now weighing up to 6 ½ pounds. At week 34, the fetus continues to develop through weight gain and the development of antibodies. By the end of the pregnancy, the fetus will reach a weight anywhere from 6-9 pounds, being fully ready to make its entrance into the world.

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