Stages of Fetal Development


  • Formation of a viable zygote by the union of the male sperm and the female ovum; fertilization.
  • Normal hormonal balance, normal cycle, healthy pregnancy.
  • Normal gametes.
  • Healthy fallopian tubes - normal conduction of ovum, no ectopic pregnancy.
  • Healthy, normally positioned uterus, normal implantation.


Embryonic Development: 4-8 weeks of embryo development.

1.5 Weeks
Completely developed embryo.
2nd Week
Primitive streak.
3rd Week
Central nervous system begins to develop.
Heart development initiated - beating begins.
4th Week
Complete mass (baby and chorion, etc.) - about 1" long and the size of a pigeon's egg.
Embryo inside is about 3/8" and weighs less than 1 gram.
Outpouching from anterior brain - early eyes - limb buds of arms and legs.
5th Week
Nose and lips formation begins.
Brain is developed into 5 components and lumen of spinal cord is continuous with brain vesicles allowing free cerebral spinal fluid flow.
8th Week
Major organs begin development.
Now about size of hen's egg.
Embryo is 1" long and about 4 grams.
Hands and feet are seen.
Baby is extremely reactive to its environment.
Male sex hormone (testosterone) produced by testes
Masculine development in males - no change in females.
12th Week
Embryo is about the size of a goose egg.
Placenta is well established and weighs more than the baby.
Baby is approximately 3 1/2 "long and weights about 60 grams (2 oz.). Fingers and toes can be seen.
This is the end of the embryonic stage.


Fetal Development

14-16th Weeks
Brain developed to the point that baby can suck, swallow and make irregular breathing movements.
16th Week
6", 6 oz. (180 grams)
Complete closure of nasal septum and palate.
Fetal heart beat heard with amplification.
Fetal movement is recognized.
Sex is distinguishable now.
Meconium formation in intestines.
20th Week
8", 10 oz. (300 grams).
Lanugo - fine hair covering over entire body, probably for protection of skin.
Vernix Caseosa - secreted by fetus, made of sebaceous material, and other materials and is probably protective.
Has some antimicrobial substances.
Fetal heart beat heard: 120-160 beats per minute.
24th Week
12", 1 1/2 lbs. (720 grams).
Baby is maturing, not considered viable (until 28 week).
28th Week
Baby can survive outside uterus if lungs capable of breathing.
10-20% survival if born at this time.
14", 2 1/2 lbs.
Viable (legally).
Fetus in breech position until 32nd week.
32nd Week
Maturing. 50% survival if born at this time.
Should turn to head down position.
16", 3 1/2 lbs. (1680 grams).
Skin is red and wrinkly.
The downy type hair is less plentiful and has more vernix.
30-34th Week
Baby is the same size as placenta.
36th Week
94% survival rate if born at this age.
18", 5 1/2 lbs.
Has some subcutaneous fat.
The baby stores things during this last trimester.
Mother should watch her weight at this time.
Fingernails are now at the tips of the fingers.
40th Week
Full term. 20", 7 lbs. (3360 grams).
Length is better indication of maturity of fetus.
Very little lanugo, lots of vernix.

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