The Development of the Baby Week by Week

The First Trimester

Zygote Development

Ovualtion to 12 - 24 hours - single sperm penetrates the egg and a joining of gentic information occurs. A single cell is formed, call a zygote
Day 2-3 - the zygote travels down the fallopian tube, cells divide into a mulberry-like mass which is 1/100 inch wide, this is called a morula
Day 5 - with additional cell division, the morula becomes a blastocyte, with an inner core and an outer shell of cells. The outer group of cells become the membranes that nourish and protect the inner group of cells, which becomes the fetus.
Day 7 - 9 -the blastocyte implants in the uterus. At this point the endometrium (the lining of the uterus) has grown and is ready to support a fetus. The blastocyte burrows into the endometrium where it receives nourishment. It is barely visible, but doubles every 24 hours. The placenta and supporting infrastructure for pregnancy develop at this time as well. It is estimated that up to 5 5% of zygotes never reach this phase of growth

Embryo Development

Week 2 - cells of the embryo are multiplying, and taking on specific functions. This process is called tissue differentiation.
Week 3 - formation of the heart, the beginning development of the brain and spinal cord, and the beginning of the gastrointestinal tract.
Weeks 4 and 5 - 1/4 inch long: the beginnings of the vertebra, the lower jaw, the larynx (voice box), and the rudiments of the ear and eye. The heart, which is still outside 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.
Week 6 --1/2 inch, 1/1000 of an ounce: the formation of the nose, jaw, palate, lung buds. The fingers and toes form, but may still be webbed. The tail is receding, and the heart is almost fully developed.
Week 7 - 7/8 inch, 1/30 ounce (less than an aspirin): the eyes move forward on the face, and the eyelids and tongue begin to form. All essential organs have begun to form.
Week 8 -1 inch, 1/15 ounce: The embryo now resembles a human being. The facial features continue to develop and the external genitalia and the external ear appears. The circulation through the umbilical cord is well developed. The long bones begin to form and the muscles are able to contract.

Fetus Development

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

The Second Trimester

Weeks 13 to 16 -- 6 inches: the skin of the fetus is almost transparent, fine hair develops on the head. 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 beats120-150 beats per minute and brain waves detectable.
Weeks 17 to 20 -- 8 inches: eyebrows and lashes appear and nails appear on fingers and toes. The can mother feel the fetus moving and the fetal heartbeat can be heard with a stethoscope.
Weeks 21 to 24 -- 11.2 inches, 1 lb. 10 oz.: all the eye components are developed, footprints and fingerprints are forming, and the entire body covered in cream-cheese-like vernix caseosa. The fetus now has a startle reflex.

The Third Trimeseter

Weeks 25 to 28 -- 15 inches, 2 lbs. 11 oz.: rapid brain development. 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.
Weeks 29 to 32 -- 15 -17 inches, 4 lbs. 6 oz.: 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.
Weeks 33 to 36 -- 16 -19 inches, 5 lbs. 12 oz. to 6 lbs. 12 oz.: The body hair begins to disappear. A baby born at 36 weeks has a high chance of survival.
Weeks 37 to 40 -- 19 - 21 inches 7 or 8 pounds: At 38 weeks, 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.