Rapid development occurs inside the womb as a single cell forms into an embryo, fetus and then a baby. You may find yourself wondering about your baby's fetal development as she grows and forms external and internal features.
Weeks 1 to 2
When sperm fertilizes the egg, the cell that is formed is called a zygote. At that moment, the zygote has all of the DNA it needs to form a baby. The zygote travels down the fallopian tube and divides to form a ball of cells. This ball of cells, the blastocyst, has an outer shell. The inner cells become the embryo and the outer cell will become the membranes that protect it. The blastocyst reaches the womb and digs into the uterine wall. Here, the blastocyst develops into an embryo, and the external features begin to form.
Weeks 3 to 8
By the time the embryo is three weeks gestation, his brain, spinal cord, heart and gastrointestinal tract begin to form. He has arm and leg buds and eyes and ears structures by five weeks. According to the American Pregnancy Association, he is a half-inch long at this stage. By week six, he will have webbed fingers and toes and will soon develop nipples, hair follicles and elbows. All his essential organs have begun to form, and by week eight he has eyelids and facial features.
Weeks 9 to 18
According to Medline Plus, a service of the U.S. National Library of Medicine, week nine marks the beginning of the fetal period. At this stage, the eyelids will close and the limbs will grow longer. Genitals are noticeably different between 11 and 14 weeks, and she has developed tooth buds. Her body begins to form urine and she develops fine hair on her head. Her bones are hardening and she is beginning to make coordinated movements. By 18 weeks, she is about five and a half inches long and weighs about 7 oz.
Weels 19 to 26
By 19 weeks, your baby's hearing is improving and he may be able to pick up the sound of your voice from inside the womb. His skin is wrinkled and by 20 weeks, fine hair covers his whole body. He has eyebrows and eyelashes, and his bone marrow is beginning to make blood cells. Medline Plus states that his body begins to store fat at this stage, which helps him to stay warm. He has fingerprints and has formed air sacs in his lungs. By week 26, he has fingerprints and footprints.
Weeks 27 to 42
According to the Mayo Clinic, week 27 is the end of the second trimester. By now her eyelids can open and close, and she is about 10 inches long. Her body fat continues to increase and her bones are fully developed between weeks 29 and 32. Her body is storing up iron and calcium and she can detect light through the womb. The fine hair covering her body disappears around week 36, and small breast buds appear. By week 37, she is full term and her organs can function on their own. As she approaches weeks 40 to 42, she continues to gain weight and her brain function improves.
