Ever wonder what process takes place during the development of your body when it is still in your womb? The parents always wonder. They can’t wait to see their baby. That is why the 3D and 4D ultrasound business in malls are selling well.
After fertilization, the zygote is formed. It divides further to form blastomeres till it becomes a solid ball like morula. Sumpaico wrote that it is in this 12-16 cell stage that the product of conception enters the uterine cavity three days after conception. A fluid-filled cavity forms in between the cells, thus dividing the embryo into a trophoblast and inner cell mass. The structure is now called the blastocyst, which is the stage at which the zygote implants a week after fertilization. The embryonic pole that bears the inner cell mass attaches first to the spongy layer of the maternal womb. The primitive chorionic villi, which are akin to the roots of the trees that anchors plants to the ground, form from the trophoblastic layer. It heralds the end of the zygote stage and the beginning of the embryonic stage.
On the first month of pregnancy, the inner cell mass becomes two-layered disk, thus forming amniotic cavity and yolk sac . The ectodermal cavity surrounds the amniotic cavity and the yolk sac is surrounded by the endodermal layer. It becomes well-defined on the 5th week of pregnancy. The body stalk becomes differentiated. A so-called primitive streak forms to give rise to mesoblast. It eventually results to a trilaminar embryo with notochord process, which is a primordium of the spinal column. The neural tube ,embryonic heart and the three branchial arches develop. The arm and leg buds appear within the sixth week of gestation. The characteristic curvature of the embryo is noted. The body stalk contains umbilical vein and two umbilical arteries when the amnion covers the body stalk. On the 8th week of pregnancy, the head is larger than the body. The length of the baby on sitting position is usually measured in the ultrasound and is known as crown-rump length (CRL). It is about 22 to 24 cms at this stage.
During the 10th week of gestation, CRL increases to 30 cm. The head becomes more rounded. The eyelids fused, which heralds the arbitrary end of the embryonic period. The urine appears as the urinary bladder develops. The ears, fingers and the toes are fully formed. According to Chua, each organ now has definite time sequence for its appearance and differentiation. By this time, baby starts to move and respond to stimuli by squinting, opening the mouth, finger closure or toe movements. Any event that could alter the morphology or appearance of any organ of the embryo could not cause any effect if the event happens before the organ appears or after the organ has already differentiated.
The 11th week of gestation is the fetal stage. The head constitute a half of its CRL. The eyes are closed. The intestines are within the abdominal end of the umbilical cord and are capable of peristalsis. On the 12th week(third month), the CRL is 6-7 cm already and the external genitalia are distinguishable .The intestinal loops are now inside the abdomen.Gross movements are now noticeable by ultrasound. The bones develops centers of ossifications,which are places where the bone cells are produced and eliminated.
When the fetus reach the 16th week,the 4th month of pregnancy, the baby’˜s CRL can reach 12 cm. The baby can swallow amniotic fluid and absorbs its water. Gross examination could identify the sex of the fetus. The forearms and lower legs are crossed. Fingers are flexed. On the 20th week or 5th month, the baby weighs about 320 grams. The CRL is about 16 cm. The fetal breathing becomes regular. Fine hair called lanugo appears and covers the entire body. Scalp hair starts to become visible.
On the 6th month, the baby weighs about 630 grams. The CRL is about 21 cms. The pneumocytes start to appear. The baby shows ability to suck. It also could now hear some sounds. So, we could start allowing it to listen to good music.
The baby weighs about 1000 grams at 28th week or 7th month of pregnancy. The vernix caseosa, the white waxy material that covers the baby, start to appear. The papillary membrane are lost so the eyes are partially opened. Eyelashes also appear. Skin is reddish.
The CRL is about 28cm on the 8th month of pregnancy. The baby could weigh 1800 grams. The eyes are sensitive to light,but the form and color perception is complete only after birth. Taste can be possible. It now has toenails.The skin is wrinkled.
On the 36th week or the 9th month , the baby weighs about 2500 grams. The CRL is about 32 cm. Subcutaneous fat make the wrinkling disappear. Fingernails reach the fingertips. Then, on the 40th week or the 10th month, the fully developed fetus weigh about 3400 grams and the CRL is about 36 cms. Testes descends to the scrotum among male fetuses. External ear has a full cartilage. Fingernails are long . Breasts protrude,and chest is prominent. The baby is ready for delivery.
List of Sources:
Sumpaico WW. Reproductive Immunology. In: Sumpaico WW, Ocampo-Andres IS, Capito LRB, Diamante AN, Carnero S, Gamilla Z. Textbook of Obstetrics, 3rd ed. 2002 . Association of Writers of Philippine Textbook of OBGYN,Inc. Quezon City.
Chua, L. Fetal Morphological and Physiological Development. In: Sumpaico WW, Ocampo-Andres IS, Capito LRB, Diamante AN, Carnero S, Gamilla Z. Textbook of Obstetrics, 3rd ed. 2002 . Association of Writers of Philippine Textbook of OBGYN,Inc. Quezon City.