Fetal Terminology

The terminology used to describe the developing structure changes based on the gestational age:

• Ovum: The term used from the $1^{st}$ three weeks for the entire structure, including the sac.

• Embryo: The term used from the $3^{rd}$ week up to the $8^{th}$ week.

• Fetus: The term used from the $9^{th}$ week until term (birth).

It is not always easy to estimate the exact age, so length, weight, degree of development, and period of gestation are all taken into consideration.

Development at 3 to 8 Weeks

• 3rd Week: The ovum is the size of a grape and is covered with fine, shaggy-looking chorionic villi.

• 4th Week: The sac is $2\text{cm}$ long; the embryo measures $1\text{cm}$ and weighs $1\text{gram}$. It develops head/tail folds and limb buds.

• 8th Week: The sac is the size of a hen’s egg, and the chorionic villi have disappeared. The embryo is $3\text{cm}$ long and weighs $4\text{grams}$. Amniotic fluid is $5-10\text{m}\ell\text{s}$. Centers of ossification are apparent in some bones. Hands and feet are recognizable, and the head is large in proportion to the body.

Development at 10 to 20 Weeks

• 10th Week: Urine is secreted.

• 12th Week: The sac is the size of a goose egg. The placenta is fully formed and weighs more than the fetus. The cord, fingers, and toes are evident. Length is $10\text{cm}$, weight is about $50\text{grams}$. Fetal head size can be measured by ultrasound.

• 16th Week: The fetus measures $15\text{cm}$ and weighs $170\text{grams}$. The nasal septum and palate fuse (failure results in cleft palate). A good heartbeat is present, and sex can be distinguished. Meconium is present in the intestine. The head is covered with hair. Fetal movements are present but not yet felt by the mother.

• 18th Week: The fetus is about $20\text{cm}$ long and weighs $185\text{grams}$. The first fetal movement (quickening) is experienced by a mother who has been pregnant before (multigravida).

• 20th Week: Fetus is $25\text{cm}$ long and weighs $400\text{grams}$. Vernix (waxy protective coating) and Lanugo (fine hair) are present on the skin. Fetal movement is now felt by a first-time mother (primegravida). The fetal heart can be heard on auscultation.

Development at 24 Weeks to Term

• 24th Week: Fetus measures approximately $30\text{cm}$ long and weighs $700\text{grams}$. The fetus can move legs and arms. Eyelids separate, and eyebrows and eyelashes are present.

• 28th Week: Measures $36\text{cm}$ long and weighs $1360\text{grams}$. The fetus is legally viable, meaning it can survive if born at this age (termed a premature baby).

• 32nd Week: About $46\text{cm}$ long and weighs $2.5\text{kgs}$. There is little subcutaneous fat. The planter creases are visible, nails reach the fingertips, and the ear cartilage is soft.

• 40th Week (Term): Measures $50\text{cm}$ long and weighs $3.2\text{kgs}$. The skin is firm and pink, not wrinkled, due to the presence of subcutaneous fat. The nails are longer than the fingertips. At birth, the baby cries well and moves limbs actively. In male babies, both testicles are in the scrotum (descended).

SUMMARY OF FETAL DEVELOPMENT

Derivatives from the Zygote

The developmental pathway flows from the fertilized ovum to the morula, which then develops into the blastocyst (composed of the trophoblasts and the inner cell mass).

• From the Trophoblasts: Develops the chorion frondosum, the placenta, and the chorion membrane.

• From the Inner Cell Mass: Develops the fetus, umbilical cord, the amnion, and some liquor Amnii (amniotic fluid).

Role of Human Gonadotrophic Hormone (H.C.G.)

This hormone is produced from the cells of the trophoblasts and is carried in maternal blood. H.C.G. maintains the corpus luteum by enhancing the action of the Luteinizing Hormone (LH) from the Anterior Pituitary Gland (APG), causing the corpus luteum to grow and produce Progesterone, which maintains the pregnancy. H.C.G. is excreted in the urine, which is the basis for pregnancy tests.

POST-NATAL STRUCTURAL CHANGES

After birth, the circulatory and supporting structures that served the fetus transition into adult ligaments and closed vessels:

• The Foramen Ovale (opening between atria) becomes the Fossa Ovalis.

• The Umbilical Vein (intra-abdominal part) becomes the Ligamentum Teres.

• The Ductus Venosus (shunts blood past the liver) becomes the Ligamentum Venosum.

• The Hypogastric Arteries (proximal parts) become the Obliterated Hypogastric Arteries.

• The Ductus Arteriosus (shunts blood past the lungs) becomes the Ligamentum Arteriosum.