Prenatal Development

The embryo develops into a miniature human being. This stage begins on the 15th day after conception and continues until about the 8th week, or until the embryo is 1.2 inches in length. During this period the cells of the embryo are not only multiplying but are taking on specific functions. This process is called tissue differentiation It is during this critical period of differentiation (most of the first trimester or three-month period) that the growing fetus is most susceptible to damage from external sources (teratogens) including viral infections such as rubella. x-rays and other radiation, and poor nutrition.

Formation of the heart begins in 3 weeks, the beginning development of the brain and spinal cord, and the beginning of the gastrointestinal tract. Beginnings of the vertebra, the lower jaw, the larynx (voice box), and the rudiments of the car and eye develop at weeks 4 and 5 (1/4 inch long). The heart, which is still outside the body, now beats at a regular rhythm Although arm and leg "buds are visible with hand and foot "pads", the embryo still has a tail and cannot be distinguished from pig, rabbit, elephant, or chick embryo by an untrained eye.

At week 6 (1/2 inch 1/1000 of an ounce), the formation of the nose, jaw, palate, and lung buds takes place.

      In the 7-week (7/8 inch, 1/30 ounce) eyes move forward on the face, and the eyelids and tongue begin to form.

At the 8-week (1 inch, 1/15 ounce) embryo now resembles a human being. The facial features continue to develop and the external ear appears. Also, we see the beginnings of external genitalia. By now, the circulation through the umbilical cord is well developed. The long bones begin to form and the muscles are able to contract. Teratogens may still cause heart problems and stunting of the fingers and toes.

• Period of Fetus (end of the second lunar month to birth)

At this point, the embryo is developed enough to call a fetus. All organs and structures found in a full-term newborn are present. The head comprises nearly half of the fetus' size and the face is well formed at weeks 9 to 12 (3 inches, 1 ounce). The eyelids close now and will not reopen until about the 28th week. The tooth buds for the baby teeth appear. The genitalia is now clearly male or female.

At the 8-week (1 inch, 1/15 ounce) embryo now resembles a human being. The facial features continue to develop and the external ear appears. Also, we see the beginnings of external genitalia. By now, the circulation through the umbilical cord is well developed. The long bones begin to form and the muscles are able to contract. Teratogens may still cause heart problems and stunting of the fingers and toes.

Weeks 13 to 16 (16 inches) mark the beginning of the second trimester. Although the skin of the fetus is almost transparent, fine hair develops on the head called lanugo. The fetus makes active movements, including sucking, which leads to some swallowing of the amniotic fluid. A thin dark substance called meconium is made in the intestinal tract. The heart beats 120-150 beats per minute and brain waves are detectable.

Eyebrows and lashes appear and nails appear on fingers and toes at weeks 17 to 20 (8 inches). This is an exciting time for the parents. The mother can feel the fetus moving (quickening) and also hear the heartbeat with the help of a stethoscope.

All the eye components are developed, footprints and fingerprints are formed at weeks 21 to 24 (11.2 inches, 1 lb 10 oz) and the entire body is covered in cream cheese-like vernix caseosa. The fetus now has startle reflex action. Many reflexes, which are automatic and unlearned responses to specific stimuli, appear: swallowing. coughing, and sucking.

During 25 to 28 weeks (15 inches, 2 lbs 11 oz) we can observe the rapid brain development of the fetus. The nervous system is developed enough to control some body functions, and the eyelids open and close. A baby born at this time may survive, but the chances of complications and death are high during this period.

The 29 to 32 weeks (15 to 17 inches, 4 lbs 6 oz) development occurs towards an independent life. For example, respiration movements are predicted even though oxygen is being provided through the placenta. There is a rapid increase in the amount of body fat and the fetus begins storing its own iron, calcium, and phosphorus. The bones are fully developed but still soft and pliable. There are rhythmic breathing movements present, the fetal body temperature is partially self-controlled, and there is increased central nervous system control over body functions. 

The body hair begins to disappear at the week of 33 to 36 (16 to 19 inches, 5 lbs. 12 oz to 6 lbs. 12 oz.:). A baby born at 36 weeks has a high chance of survival.

 At 38 weeks (19 to 21 inches 7 or 8 pounds) the fetus is considered full term. It fills the entire uterus, and its head is the same size around as its shoulders. The mother supplies the fetus with the antibodies it needs to protect it against disease.