Counseling for the 2012 Clinical Assistant Physician Examination: Fetal Development
The ovum is referred to as a fertilized egg or embryo within two weeks after fertilization; from the third to eighth week after fertilization, it is called an embryo; starting from the end of the eighth week, it is called a fetus. The characteristics of fetal development at different stages of pregnancy are as follows:
8 weeks: The embryo is about 3 cm long and begins to take on a human shape. The head is exceptionally large, with eyes, ears, nose, and mouth that can be identified. The early heart has formed and is beating. This can be detected through ultrasound examination.
12 weeks: The fetus is 7-9 cm long and weighs approximately 20g. External genitalia have developed, and there is subtle movement in the limbs. Most bones have begun to ossify.
16 weeks: The fetus is approximately 10-17 cm long and weighs 100-120g. The skin is red, smooth, and transparent with some lanugo (fine hair). Bones continue to develop, and skeletal shadows can be seen through X-ray examination. External genitalia can distinguish between male and female. During abdominal examination, fetal heartbeat can be heard, and pregnant women may feel fetal movements.
20 weeks: The fetus is 18-27 cm long and weighs 280-300g. The skin is dark red with reduced transparency. There is a layer of vernix caseosa covering the body. The fetal head accounts for one-third of the entire body, and hair growth begins. Swallowing activity starts to appear.
24 weeks: The fetus is 28-34 cm long and weighs 600-700g. Subcutaneous fat begins to accumulate, and the skin has wrinkles.
28 weeks: The fetus is 35-38 cm long and weighs 1000-1200g. The body is slim, the skin is red with vernix caseosa, and fingernails/toenails have not yet reached the tips of the fingers/toes. Female labia have developed, with the labia majora covering the labia minora and clitoris. Male testicles have descended into the scrotum. Due to the lack of subcutaneous fat, the face has many wrinkles and looks like an elderly person. If born, the baby can cry, swallow, and move its limbs but has weak vitality and requires special care to survive.
32 weeks: The fetus is 40 cm long and weighs 1500-1700g. The skin is deep red, and facial lanugo has fallen off. With proper care after birth, the baby can survive.
36 weeks: The fetus is 45-46 cm long and weighs approximately 2500g. There is plenty of subcutaneous fat, and facial wrinkles have disappeared. Fingernails/toenails have reached the tips of the fingers/toes. After birth, the baby can cry and suckle, and the chances of survival are very high.
40 weeks: The fetus is fully developed, about 50 cm long, and weighs approximately 3000-3300g. The skin is pink, and subcutaneous fat is well-developed. Most fetal lanugo has disappeared, and the hair is 2-3 cm long. Fingernails/toenails extend beyond the tips of the fingers/toes. Limb movements are active, the baby can cry loudly, and there is a strong sucking reflex.
The length and weight of the fetus increase gradually with the progression of pregnancy. For easy memorization, the following formulas are generally used:
Before 20 weeks of pregnancy:
Length = square of the month of pregnancy (cm)
Weight = cube of the month of pregnancy * 2 (g)
After 20 weeks of pregnancy:
Length = month of pregnancy * 5 (cm)
Weight = cube of the month of pregnancy * 3 (g)