When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes. Conception usually takes place in the Fallopian tube. A single sperm penetrates the mother's egg cell, and the resulting cell is called a zygote.

The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother’s egg, and half from the father’s sperm.

The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells. Further cell division creates an inner group of cells with an outer shell. This stage is called a "blastocyst". The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.

The blastocyst reaches the uterus at roughly the fifth day, and implants into the uterine wall on about day six. At this point in the mother's menstrual cycle, the endometrium (lining of the uterus) has grown and is ready to support a fetus. The blastocyst adheres tightly to the endometrium, where it receives nourishment via the mother's bloodstream.

The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation, which produces the varied cell types that make up a human being (such as blood cells, kidney cells, and nerve cells).

There is rapid growth, and the baby's main external features begin to take form. It is during this critical period of differentiation (most of the first trimester) that the growing baby is most susceptible to damage from:

• Alcohol, certain prescription and recreational drugs, and other substances that cause birth defects
• Infection (such as rubella or cytomegalovirus)
• Radiation from x-rays or radiation therapy
• Nutritional deficiencies

The following list describes specific changes by week.

• Week 3
  • beginning development of the brain, spinal cord, and heart
  • beginning development of the gastrointestinal tract
• Weeks 4 to 5
  • formation of tissue that develops into the vertebra and some other bones
  • further development of the heart which now beats at a regular rhythm
  • movement of rudimentary blood through the main vessels
  • beginning of the structures of the eye and ears
  • the brain develops into five areas and some cranial nerves are visible
  • arm and leg buds are visible
• Week 6
  • beginning of formation of the lungs
  • further development of the brain
  • arms and legs have lengthened with foot and hand areas distinguishable
  • hands and feet have digits, but may still be webbed
• Week 7
  • nipples and hair follicles form
  • elbows and toes visible
  • all essential organs have at least begun to form
• Week 8
  • rotation of intestines
  • facial features continue to develop
  • the eyelids are more developed
  • the external features of the ear begin to take their final shape

The end of the eighth week marks the end of the "embryonic period" and the beginning of the "fetal period".

• Weeks 9 to 12
  • the fetus reaches a length of 3.2 inches
  • the head comprises nearly half of the fetus' size
  • the face is well formed
  • eyelids close and will not reopen until about the 28th week
  • tooth buds appear for the baby teeth
  • limbs are long and thin
  • the fetus can make a fist with its fingers
  • genitals appear well differentiated
  • red blood cells are produced in the liver
• Weeks 13 to 16
  • the fetus reaches a length of about 6 inches
  • a fine hair develops on the head called lanugo
  • fetal skin is almost transparent
  • more muscle tissue and bones have developed, and the bones become harder
  • the fetus makes active movements
  • sucking motions are made with the mouth
  • meconium is made in the intestinal tract
  • the liver and pancreas produce their appropriate fluid secretions
• Week 20
  • the fetus reaches a length of 8 inches
  • lanugo hair covers entire body
  • eyebrows and lashes appear
  • nails appear on fingers and toes
  • the fetus is more active with increased muscle development
  • "quickening" usually occurs (the mother can feel the fetus moving)
  • fetal heartbeat can be heard with a stethoscope
• Week 24
  • the fetus reaches a length of 11.2 inches
  • the fetus weighs about 1 lb. 10 oz.
  • eyebrows and eyelashes are well formed
  • all the eye components are developed
  • the fetus has a hand and startle reflex
  • footprints and fingerprints forming
  • alveoli (air sacs) forming in lungs
• Weeks 25 to 28
  • the fetus reaches a length of 15 inches
  • the fetus weighs about 2 lbs. 11 oz.
  • rapid brain development
  • nervous system developed enough to control some body functions
  • eyelids open and close
  • respiratory system, while immature, has developed to the point where gas exchange is possible
  • a baby born at this time may survive, but the possibilities for complications and death remain high
• Weeks 29 to 32
  • the fetus reaches a length of about 15-17 inches
  • the fetus weighs about 4 lbs. 6 oz.
  • rapid increase in the amount of body fat
  • rhythmic breathing movements occur, but lungs are not fully mature
  • bones are fully developed, but still soft and pliable
  • fetus begins storing iron, calcium, and phosphorus
• Week 36
  • the fetus reaches a length of about 16-19 inches
  • the fetus weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz.
  • lanugo begins to disappear
  • increase in body fat
  • fingernails reach the end of the fingertips
  • a baby born at 36 weeks has a high chance of survival, but may require some medical interventions
• Weeks 37 to 40
  • considered full-term at 37 weeks
  • may be 19 to 21 inches in length
  • lanugo is gone except for on the upper arms and shoulders
  • fingernails extend beyond fingertips
  • small breast buds are present on both sexes
  • head hair is now coarse and thicker