Fetal Development

2 Weeks / Conception day

The egg and sperm most often unite in the fallopian tube (tube from the ovary to the uterus) to form a single cell called a zygote. This tiny new cell, smaller than a grain of salt, contains all the genetic information for every detail of the newly created life — the color of the hair and eyes, the intricate fine lines of the fingerprint, the physical appearance, the gender, the height and the skin tone.

Days 2–5

This new life is now called an embryo, and his or her cells continually divide while traveling down the fallopian tube before arriving at the uterus, around days 3 to 4. Meanwhile, the lining of the uterus prepares to receive this new life.

3 Weeks / Days 6–10

The embryo begins to implant in the lining of the uterus on or about day 6. Once this occurs, hormones trigger the mother body to nurture the pregnancy and prevent her monthly periods. Around day 8 the baby is about the size of the “period” used in this sentence.

Four Weeks / Week 2

A pregnancy test taken at this point can measure hCG, the pregnancy hormone in the mother’s urine, and tell her if she is pregnant. By now, the embryo is completely attached to the lining of the uterus and draws nourishment from its mother.

Five Weeks

The heart, about the size of a poppy seed, is the first organ to function —it begins beating just 21 days after fertilization! The first signs of brain development are evident and the foundation for every organ system is already established and beginning to develop.

Six Weeks

Just 4 weeks after fertilization, the baby is growing rapidly and measures 1/8 of an inch long. The basic structure for the entire central nervous system (brain and spinal cord) has formed. The eyes are developing and the arm and leg buds are now visible. The heart is beating about 80 times a minute. An ultrasound can provide further medical confirmation of pregnancy.

7 Weeks

The baby is now about 1/2 of an inch long from head to bottom. The elbows and fingers can be seen. Some reports show the embryo can move it’s trunk and limbs and can respond to touch by reflex. Lungs begin to develop. Taste buds are forming on the tongue, tooth buds for “baby teeth” are taking shape in the jaw, and eyelids begin to form.

9 Weeks

The baby measures 3/4 of an inch long and weighs almost 1/8 of an punce. The developing ears and nose are visible, and there is pigment in the retina. Nipples can now be seen on the chest. The limbs and fingers are growing rapidly, and the bones in the arms begin to calcify and harden.

10 Weeks

The baby’s brain is growing rapidly. Each minute it produces almost 250,00 new neurons, and for the first time in development, the brain can make the muscles move on purpose. The upper and lower portions of the arms and legs are clearly seen, and the bony tissues of the legs begin to calcify. The fingers and toes are lengthening and are separate digits. By now the external ear is fully developed. A baby boy begins to produce the male hormone, testosterone.

11 Weeks

Because the baby has all of the major organ systems and is a distinctly recognizable human being, he or she is no longer called an embryo but is now known as a fetus, a Latin word for “young one.” The baby is about 2 inches long and can yawn and suck. The eyelids are fully formed and closed to protect the developing eyes. The intestines are developing and the kidneys begin to produce urine. During the next several weeks, his or her body will grow rapidly, increasing in weight 30 times and tripling in length in the next two months!

14 Weeks

Now 3 1/2 inches long, the “young one” is coordinated enough to find his or her thumb and suck it. You can see the beginnings of the fingernails and toenails and baby is able to urinate and swallow.

16 Weeks

The heart beats between 110 and 180 times per minute and pumps about 25 quarts of blood each day. You can see the gender of the baby on ultrasound. If she is a girl, millions of eggs are now forming in her ovaries. At almost 5 inches in length and weighing nearly 4 ounces, the baby can coordinate the movement of it’s arms and legs, though his or her mother will not likely feel it yet.

18 Weeks

In just two weeks, the fetus has almost doubled its weight to 7 ounces. The skeleton is hardening and calcifying and is visible on ultrasound. Reflexes such as blinking and frowning are now developed. The baby has its own unique fingerprints and toe prints. Some studies show that the baby can feel pain as early as 18 weeks.

20 Weeks

The fetus is now about 10 inches long from head to heel and weighs 11 ounces. Fetal movement, commonly known as “quickening,” can usually be felt by the mother. The baby has unique waking and sleeping patterns and even has a favorite position to sleep in. The pregnancy is about half over, and the mother is beginning to show.

22 Weeks

The baby is about 11 inches long and weighs about 1 pound. If the baby is male, his testicles are beginning to descend from the abdomen to the scrotum. Hair is visible on his or her head and body. From now until about 32 weeks, the baby feels pain more intensely than at any other time in development.

24 Weeks

The baby now weighs about 1 1/2 pounds and inhales amniotic fluid in preparation for breathing. The ear has developed to the point where the baby recognizes hos or her mother’s voice, breathing, and heartbeat. About a week ago, rapid eye movements began, an activity associated with dreaming. The baby may have a blink-startle response resulting from sound applied to the mother’s abdomen. Some babies born at this stage of development are able to survive.

26 Weeks

Now the baby weighs almost 2 pounds and he or she can react to sounds outside the mother’s body. Eyes can now respond to light and the permanent teeth buds are apparent in the gums. Eyelashes and eyebrows are well-formed and the hair on the baby’s head is growing longer.

28 Weeks

The baby is now about 15 inches long and weighs about 2 1/2 pounds. With the support of intensive care, a baby born at this stage is capable of breathing air. The brain is developed enough to coordinate rhythmic breathing and regulate body temperature. As the baby continues to gain weight, the skin becomes less wrinkled and more smooth.

34 Weeks

The baby is now about 17 inches long, weighs 4 1/2 pounds and continues to grow and mature. By this stage of development, the eyes are wide open, and if a light were shone into them, the pupils would constrict. The head is covered in hair, the fingernails have reached the tips of the fingers, and the toenails are close behind.

40 Weeks

The baby is now around 20 inches long and may weigh 7 to 8 pounds. He or she has a plump body and a firm grasp. Typically, the baby is head down in the mother’s pelvis and awaiting birth. Be patient—only 4 percent of babies are born on their due dates.