Stages of Embryonic and Fetal Development
Fetal Development
3 Months
Fetal stage begins, human aspect develops, primary ossification centers appear in long bones and skull, and external genitalia develop.
11 Weeks
Initial development of nails and fingers occurs. The neck is set, and sex can be distinguished. Between 9 and 11 weeks, urine starts to be released into the amniotic fluid.
12 Weeks
The fetus measures 8cm and begins to exhibit fetal movements. It has thin skin and primitive ears. The liver is the primary producer of red blood cells.
4 Months
Fast increase in length occurs, with low weight gain.
5 Months
The fetus measures 15cm and weighs 500g. It is covered in lanugo (hair) and has eyebrows and hair on the head.
23 Weeks
Hair and eyebrows are present. Heat-producing brown fat develops. The fetus has eye movements, blinking, and can experience fright.
7 Months
The fetus measures 25cm and weighs 1.100g. Subcutaneous fat appears, and the fetus can survive if born prematurely. The mature CNS controls breathing movements, rhythm, and temperature. The spleen begins to form red blood cells (Hematopoiesis).
9 Months
The fetus measures 50cm and weighs 3kg.
- Intrauterine growth retardation can lead to neurological deficits, congenital malformations, meconium aspiration, hypoglycemia, and hypocalcemia.
Role of the Placenta
- Exchange of metabolic products and gases occurs.
- Maternal antibodies are transmitted (passive immunity).
- The placenta stimulates the formation of hormones such as progesterone, estrogen, estriol, and somatotropin (for the fetus and breast).
Mesonephros (Provisional Kidney)
- Large and elongated excretory organs develop by the end of the 9th week. These interim kidneys function for 4 weeks.
- By the 3rd month, the mesonephros degenerates, and the mesonephric tubules become conduits with different histology. They form a connection between the gonad and the reproductive tract.
Metanephros (Permanent Kidney)
- Appears in the 5th week and remains as the permanent kidney.
- Urine is excreted into the amniotic cavity from the 5th week.
Gametogenesis
A process in which meiosis occurs to reduce the number of chromosomes and cytodifferentiation occurs to complete maturation.
Neuropolos Defect Closing
Meningocele
Occurs if the caudal pole fails to close properly, exposing the spinal column to the outside.
Anencephaly
Absence of the cranial vault and brain. Skull formation is poor in some stretches at the cervical level.
Hydrocephalus
Pathology related to the development of spina bifida with meningomyelocele. It can be caused by blockage of the ventricular system or diseases of the subarachnoid space.
Somites
The number of somites indicates the age of the embryo (Soma = body).
- Begin to form on day 20 from the paraxial mesoderm.
- By the end of the 5th week, there are 44 pairs on each side of the neural tube.
2 Months: Embryonic Development in the Organogenic Period
Growth
Cell divisions occur rapidly.
Morphogenesis
Development of the shape, size, and other characteristics of organs.
Differentiation
Maturation of organs to enable them to perform specific functions.
42 Days
Nasal fovea appears, measuring 11mm. Hand fingerings are also present.
48 Days
The embryo measures 16mm and has a longish body with limb buds.
52 Days
Elbow and knee appear. A small tail is still present.
56 Days
The tail disappears. The embryo measures 30mm. Facial features are present, but genitalia are not yet distinguishable. The gut protrudes into the umbilical cord.
Organogenesis
Malformations can occur during this period.
Teratogens
Substances that can cause birth defects, such as alcohol and cigarettes.
Birth Defects
Defects present at birth, which can be inherited genetically or caused by environmental factors in utero.
Germ Layers
Ectoderm
Gives rise to the CNS, PNS, sensory epithelia of the eye (retina), ear, nose, skin, hair, nails, mammary glands, pituitary gland, subcutaneous glands, and dental enamel.
Mesoderm
Gives rise to connective tissue, cartilage, bone, striated and smooth muscle, heart, blood vessels, kidneys, gonads, genital tract, pericardium, pleura, peritoneum, spleen, and adrenal cortex.
Endoderm
Gives rise to the epithelial lining of the gastrointestinal and respiratory tracts, tonsils, thyroid gland, and parathyroid glands.
Placenta
Formed by chorionic villi.
Chorionic Villi
After forty weeks, the villi degenerate and oxygen supply to the fetus decreases.
- Exchange of CO2, O2, and waste products occurs. Hormones cannot cross the placenta.
- Viruses can cross the placenta, such as rubella, smallpox, chickenpox, measles, and poliovirus (can cause death or birth defects).
- The majority of drugs can cross the placenta.
Estimation of Embryo Age
Gestational Age
Counted from the first day of the last normal menstrual period (LMP).
Embryonic Age
Begins after fertilization, two weeks after LMP.
Conceptional Age
Used when the actual date of conception is known (chorionic cavity size can be used to estimate the date of conception).
Fetal Age Estimation
- LVC: Length vertex-cortex
- Fetal head circumference – Biparietal diameter
- Femoral length / abdominal circumference / length of foot