Prenatal Development

Amanda Raubach


When an egg is in the fallopian tube, it is ready to be fertilized. If one sperm makes it into the fallopian tube and burrow its way into and egg, then it has fertilized the egg, creating a zygote. If the sperm has a Y chromosome, then it will be a boy. If it has an X chromosome, then the baby will be a girl.
Within the first week of coneption, while still in the fallopian tube, the egg divides into many cells, forming an embryo, and keeps dividing as it travels to the uterus. When the cells have divided into 32 cells, it is called a morula. After multiple rounds of more cell division, cleavage, has produced 100 cells, the embryo is called a blastula. When the blastula reaches the uterus and implants itself into the uterine lining, it is now called a blastocyst. This is moment, pregnancy has officially taken place. Placental tissue secretes a hormone called hCG. This hormone supports the endometrial lining, which will support pregnancy.


The single-layered blastocyst is transformed into a gastrula after the blastula folds inward. First, the embryo becomes asymetrical. This creates an "egg cylinder" which allows the placenta to form. Next, the formation of a primitive streak. This will establish bilateral symmetry, determine the site of gastrulation, and initiate germ layer formation. Lastly, the epithelial to mesenchymal transition and ingression (EMT). Cells must lose their epithelial characteristics, cell-cell adhesion, so they can move from the epithelium of the epiblast through the primitive streak to form a new layer. Gastrulation creates three layers; the ectoderm, mesoderm, and endoderm. These layers will form the ouside (hair,skin), middle (muscle, connective tissue), and inside (epithelium, digestive organs) of our bodies.
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First Trimester

The first trimester is the first three months of pregnancy. During the first month, the fertilized egg becomes a blastocyst that burrows itself into the lining of the uterus. It is enveloped in an amniotic sac that protects the soon-to-be baby. The placenta, an organ that will transfer nutrients from mother to baby, begins to develop. The mouth, lower jaw, a primitive face, and throat begin to develop as well. By the end of one month, the baby will be the size of a grain of rice!

During the second month of pregnancy, the baby's facial features continue to develop. A skeleton made of cartilage is formed. Little flaps of skin that will develop into ears, little nubs which will become the arms and legs, fingers, toes, and eyes will also form and develop during this time. Around week six, the baby's first heartbeat and brain waves are made. By the end of this month, the baby becomes about a fetus that is about an inch long.

Throughout the third, and final, month of the first trimester, the baby now has arms, hands, fingers, legs, feet, toes, and can open and close its fist and mouth. Toenails and fingernails begin to develop, as well as the external ears. Teeth begin to form along with the baby's reproductive organs, although ultrasounds are not able to determine the sex of the baby just yet. The baby's circulatory and urinary systems are able to work and the liver can produce bile. Around week seven, the umbilical cord is formed. The baby is now 3-4 inches and weighs one ounce. After the first trimester is complete, the chances of miscarriage drop considerably.

Second Trimester

The second trimester begins during week 14 of pregnancy into week 27. During the fourth month, the baby has fingerprints, can hiccup, yawn, and weighs about as much as a chicken breast. The cartilage skeleton begins to turn to bone due to the higher amount of calcium the baby is absorbing. Little hairs begin to grow on the baby, the baby's brain now can control its heartbeat, and the baby can taste the food the mother eats via her amniotic fluid.

By the fifth month, the baby's arms and legs have developed more, and can move them around in the uterus. Myelin begins to cover the baby's nerves. The baby now has little eyelashes and eyebrows, and is covered in lanugo, which is a "fur coat" to keep the baby warm until she/he builds up more fat in their system.

In the sixth month, the baby's brain has developed enough to open and close the baby's eyelids. The bones continue to grow stronger. Also, capillaries begin to form to carry oxygenated blood throughout the body.

Third Trimester

The third trimester starts at week 28 of pregnancy and ends until birth, which is usually around week 40. During the seventh month of pregnancy, more fat is deposited into the baby. By now, the baby's hearing has fully developed and it can respond to stimuli like sound, touch, pain, and light. The baby can move around in the uterus.
By the eight month of pregnancy, the baby weighs about 5 lbs, and continues to develop more fat. The brain is swiftly developing at this point, so the baby can now see and hear. Most of the internal systems have developed, although the lungs may still be immature.
During the ninth, and final, month of pregnancy, the baby is still growing and maturing. The lungs are fully developed, his/her reflexes are now coordinated so the baby can turn their head, respond to stimuli, and they have a firm grip. The baby will position itself towards the birth canal so he/she is ready for delivery. By the end of this month, the baby is ready to come into the world!