The Fascinating Stages of Fetal Development
Written by: Perry Nichols
Although many people refer to the different stages of pregnancy in terms of months, the true stages of fetal development are actually measured in weeks, with 37 to 40 weeks of gestation.
Here are those fascinating stages, starting with the developing embryo and continuing on until the final weeks before delivery.
The Developing Embryo
The most delicate and crucial stage is the period referred to as embryonic development, which is from conception to about 11 weeks. During this time, the embryo undergoes amazing transformations, with cells rapidly dividing and multiplying, setting the foundation for all of the baby's systems, including the nervous, skeletal, and respiratory systems. The placenta is also forming and expanding in order to support the growing embryo.
6 Weeks
At six weeks, both the arms and legs have already begun to develop, and the heart has started its lifelong rhythmic beat. Two weeks later, the teeth have begun forming underneath the miniscule gums, as well as the intestines and the beginning development of other major organs. Near the end of the embryonic stage, the irises are starting to form along with the central nervous system. By eight weeks, the embryo/fetus will be about the size of a kidney bean with distinctly webbed fingers.
11 to 16 Weeks
After 11 weeks, the embryo is now referred to as a fetus, and from now until the 15th or 16th week, the fetus will develop distinguishable genitalia, fingernails, fingerprints, hair, and vocal chords. Both the liver and kidneys begin to function, and soon, the bones will begin to harden with cartilage forming the skeleton. The fetus now weights nearly an ounce and will be roughly three inches long.
17 to 20 Weeks
During this period, the fetus spends the majority of its time much like a newborn, sleeping periodically throughout the day and night to provide enough energy for the tremendous growth spurt that's occurring. Under a thin layer of newly formed skin, several protective layers of fat begin to form, and the heart begins pumping many quarts of blood each and every day. The fetus is usually about five inches long at this stage and weighs approximately five ounces.
21 to 24 Weeks
The development begins to slow during the 21st and 24th weeks as the eyelids, eyebrows and facial features are now more discernible. The testicles of males now descend from the body, the fetus weighs just a little over a pound and may be as long as 10 inches during this fifth month of pregnancy.
25 to 28 Weeks
The nostrils have now opened, ligaments and tendons have been formed, and the brain continues to grow. The lungs also rapidly develop during the 25th and 28th weeks in preparation for baby's first breath of air. The eyes are also able to open now, and the majority of babies born prematurely at this point will survive, although there may be some breathing difficulties.
29 to 40 Weeks
During this final trimester of pregnancy, the lungs continue to develop and a build-up of antibodies strengthens the immune system. After the 37th week a baby is considered full term, and most will now gain about one ounce of body weight a day to prepare for life outside of the protective confines of the womb. The average baby at 9 months gestation is usually over 19 inches long and weighs near seven pounds.
