What most people don't realize is how quickly the developing human takes on the characteristics that are so familiar to us in human beings outside the womb. The rate of growth is astonishing. In fact, during the several days following implantation, the embryo doubles in size every day. If such rapid growth were maintained throughout the pregnancy, the baby would be larger than the sun at birth! In just six weeks time, the human embryo goes from looking like a "bunch of cells" to looking like a baby – though only a half inch tall!
FERTILIZATION / CONCEPTION 
At the moment of fertilization, a new and unique human being comes into existence with its own distinct genetic code. There is enough information in this tiny zygote to control human growth and development for the rest of its life.
3 WEEKS from fertilization
By the end of the third week of pregnancy, approximately 21 days after fertilization and 8 days after first taking shape, the heart starts beating.
6 WEEKS from fertilization
By the sixth week, the brain emits measurable brain impulses. Small bodily movements can be observed at this time, initially affecting the entire body but gradually becoming more specific. The embryo responds reflexively to stimulus and may be able to feel pain. Bone ossification has begun, lips have appeared, all 20 teeth buds are in the gums, the diaphragm has formed, the kidneys are producing urine and the stomach is producing gastric juices. 
8 WEEKS from fertilization
By the eighth week of pregnancy, every organ is present and in place. The embryonic period is now over. Ninety percent of the structures found in an adult human being can be found in this tiny embryo (now called a fetus) which is only about an inch and a half long.
10 WEEKS from fertilization
Fingernails, toenails and unique fingerprints all appear at this time.
16 WEEKS from fertilization
Though the fetus has long been in almost perpetual motion, by 16-20 weeks from fertilization, the body is large enough for the mother to start feeling the kicks. Eyelids have now completely covered the eyes, and fine hairs have begun to cover the entire body.
22 WEEKS from fertilization 
Where modern medical services exist, twenty-two weeks from fertilization (24-weeks gestational age) is generally considered the beginning of viability outside the womb. At this point in pregnancy (barely half way through a full term) the odds of long-term survival are still relatively slim, but new medical advances continue to increase the likelihood of survival and decrease the age of viability.
BIRTH
The fetus initiates labor by stimulating the adrenal cortex to secrete a hormone that induces the mother's uterus to begin contracting. It is the fetus who determines when it's time for birth. 