1. Intra uterine fetal development
G&D of the fetus is typically divided into 3 stages:
Preembryonic (ovum) stage: fertilization to 2-3 weeks.
Embryonic stage : 4-8 weeks of gestation.
Fetal stage : 9 weeks after conception until birth.

2. Preembryonic (ovum) stage:
fertilization to 2-3 weeks.
The conceptus: an ovum during the period from conception until primary villi appear days after fertilization or 4 weeks since LMP.
Rapid cell division and differentiation was occurred.
Develop embryonic membrane.
The inner cell mass of blastocyst forming fetus that the cells differentiate into three primary germs layers:

3. A- the ectoderm (ecto =outside )
Gives skin ,nails ,tooth and nervous system.
B- The endoderm (endo = inner )
Gives the epithelial inner lining of GIT, respiratory system , endocrine glands and auditory canal
C- The mesoderm (meso = middle )
Gives the connective tissue ,muscle, blood and vascular system.

5. Embryonic stage:4-8 weeks of gestation.
Starts from the end of ovum stage until 8 weeks of gestation (or 10 weeks since LMP ) when the ovum measure approximately 3 cm.
The most critical stage of physical development as highly vulnerable to teratogenes like: virus ,radiation ,infection and drugs.
Developmental interference during this stage lead to congenital anomalies.

6. Organogenesis take place in this stage as the next:
The first system to function is the cardiovascular, the blood vessels begin early in the third week, heart starts to pump blood at 5th week, fetal heart rate is
Fetal circulation: the single umbilical vein carries oxygenated blood from the placenta.
Two umbilical arteries return most of the mixed blood from fetus to placenta.

7. The lung begins to grow as apart from upper digestive tract at the 4th week not functional until birth. A substance called surfactant forms in the lung at 27th week.
Alimentary tract (GI) formed at 4th week but function not accomplished before delivery.
The kidneys develop rapidly as 5th week and begin to function and passing urine at 8th week.

8. The fetus is covered with a white, creamy substance (skin) at 18th week and the fetus is covered by lanugo hair at 20th week (5th month).
CNS begins to develop at 3rd week, the neural tube complete at 4th week.
External genital are well enough developed and can easily distinguished at 12 week.

9. At the end of 22 weeks the fetus is 210 mm length and 630 gram weight .
At the end of 26 weeks the fetus is 250 mm and 1000 gram weight.
At the end of 30 weeks the fetus is 280 mm and 1700 gram weight.
At the end of 34 weeks the fetus is 320 mm and 2500 gram weight.
At the end of 38 weeks the fetus is 360 mm and 3400 gram weight.

10. Fetal stage : 9 weeks after conception until birth
It starts from the embryonic stage until delivery of fetus .
There is a change but not dramatic as in the preceding stages.
Every organ system and external structure is presented during this stage.

11. The fetus can be detected by trans vaginal ultrasound as early as 4 weeks after LMP and by abdominal ultrasound in the second trimester.
Fetal heart sound can be heard with fetoscope by weeks of gestation and by electronic Doppler can detect heart motion at 10 week of gestation.
Fetus is less vulnerable to teratogenes but may interrupt normal functional development of organs particularly the brain.