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Fetal Development

Your Growing Baby
A baby goes through a lot of changes over the course of nine months inside the womb. Here is a brief look at just some of the development landmarks that occur during this time.

Obstetricians count weeks of pregnancy from the first day of a woman's last period because there is often no way to determine exactly when conception occurred. To determine the age of the unborn child since fertilization, just subtract two weeks from this table.

Week 2
At fertilization, also called conception, the sperm joins with ovum (egg) to form a single cell that contains the complex genetic blueprint for every detail of human development. At this point the child's sex, hair and eye color, height, and skin tone are already determined. The unborn child at this stage is called a zygote until it reaches the uterus 3-4 days later, at which point it is called an embryo. On the seventh day after fertilization, the embryo finds a suitable place to rest and implants into the uterus.

Week 4
The embryo is very tiny at this stage, and cannot be seen easily on an ultrasound by the untrained eye. However, the heart can be seen beating by ultrasound, and in fact has been beating since week 3. The beginning of the spinal cord, muscles, and nerves develop. Arms, legs, eyes and ears start to form. The amniotic sac and placenta are in place and providing nourishment to the embryo.

Week 6
The embryo is about a fifth of an inch in length. This is a critical period of growth for the embryo. Head, mouth, intestines, and liver begin to take shape. The eyes begin to form and the extremities can be seen in much more detail, including all ten fingers. The embryo is now moving and responds to touch.

Week 10
At the ninth week the embryo is now called a fetus and is about one inch in length. Facial features, limbs, hands, feet, fingers, and toes become apparent. The nervous system is responsive and many of the internal organs begin to function. The fetus is able to grasp and can suck his/her thumb. Fingerprints are already visible in the skin. In addition to being able to sense touch, the fetus can now feel pain. At week 12, the fetus sleeps and awakens.

Week 14
The fetus is growing very rapidly at this point, and it is now three inches long and weighs almost an ounce. The mother can feel energetic movement, and hair is visible on the scalp. Eyelids, fingernails, and toenails also form. The child's sex becomes apparent.

Week 18
The fetus is now about five inches long. The child blinks, grasps, and moves her mouth. Hair grows on the head and body. The ears are now functioning and the baby can hear his mother's voice. By the end of the 4th month, the fetus weighs a half pound or more and is eight to ten inches in length.

Week 22
The head now accounts for a third of the fetal length. The child now weighs approximately half a pound and spans about ten inches from head to toe. Sweat glands develop, and the external skin has turned from transparent to opaque.

Week 26
The tongue has formed taste buds and the eyes have fully developed. The baby's lungs are completely functioning so the child can now inhale, exhale and even cry. If this baby were to be born prematurely, the child would have a good chance of survival with sufficient medical care.

Week 30
The fetus is usually capable of living outside the womb and would be considered premature at birth. Vision, hearing, taste, and touch are all being used now. The baby opens and closes her eyes and looks around. Research shows that the child can recognize her mother's voice.

Week 40
This marks the end of the normal gestational period. The baby is now ready for life outside of his mother's womb. By this time, the infant usually weighs six to nine pounds. At birth the placenta will detach from the side of the uterus and the umbilical cord will cease working as the child takes his first breaths of air.

Fetal Developmental Milestones

Learn more by watching this short video (available in English and Spanish).

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