Fetal Development: From conception to birth

So when is a baby a human? When is it wrong, regardless of what is legal, to take the life of a developing baby? At what point in this timeline is the new life not a baby? By week five, a baby has eyes, legs, and hands; by week six, brain waves can be detected. What was a baby just a few minutes before week 5, if not a baby? You see, it is unavoidable: The fertilized egg is human in every way.

Day 1 Fertilization: all human chromosomes are present; a unique human person begins.
Click photo to enlarge.
Illustration by R.K. O'Bannon
Day 6 The very small child, sometimes called an "embryo", begins implantation in the uterus.  
Day 22 The heart begins to beat with the child’s own blood, often a different type than the mother's.
Week 3 By the end of third week, the child’s backbone spinal column and nervous system are forming. The liver, kidneys, and intestines begin to take shape.
Week 4 By the end of week four, the child is 10,000 times larger than the fertilized egg.
Week 5 Eyes, legs, and hands begin to develop.
Week 6 Brain waves are detectable; mouth and lips are present; fingernails are forming. When
Week 7 Eyelids and toes form; the nose is distinct. The baby is kicking and swimming.
Week 8 Every organ is in place, bones begin to replace cartilage, and fingerprints begin to form. By week eight, the baby can begin to hear.
Weeks 9 and 10 Teeth begin to form, fingernails develop. The baby can turn her or his head, even frown. The baby can hiccup. When
Weeks 10 and 11 The baby can “breathe” amniotic fluid and urinate. Week 11 the baby can grasp objects placed in her or his hand; all organ systems are functioning. The baby has a skeletal structure, nerves, and circulation.
Week 12 The baby has all of the parts necessary to experience pain, including nerves, spinal cord, and thalamus. Vocal cords are complete. The baby can suck her or his thumb. When
Week 14 At this age, the heart pumps several quarts of  blood through the body every day.
Week 15 The baby has an adult’s taste buds.  
Month 4 Bone marrow is now beginning to form. The heart is pumping 25 quarts of blood a day. By the end of the fourth month, the baby will be 8-10 inches in length and will be one half of its birth weight.
Week 17 The baby can have dream (REM) sleep. When
Week 19 Babies can routinely be saved at 21 to 22 weeks after fertilization, and sometimes they can be saved even younger.
Week 20 The earliest stage at which partial-birth abortions are performed. At 20 weeks, the baby recognizes her or his mother's voice. When
Months 5 and 6 The baby practices breathing by inhaling amniotic fluid into its developing lungs. The baby will grasp at the umbilical cord when she or he feels it.

Most mothers feel an increase in movement, kicking, and hiccups from the baby. Oil and sweat glands are now functioning. The baby is now twelve inches long or more, and weighs up to one-and-a-half pounds.

Months 7
through 9
Eyeteeth are present. The baby opens and closes her or his eyes. The baby is using four of the five senses (vision, hearing, taste, and touch.) Baby knows the difference between waking and sleeping, and can relate to the moods of her or his mother.

The baby’s skin begins to thicken, and a layer of fat is produced and stored beneath the skin. Antibodies are built up, and the baby’s heart begins to pump 300 gallons of blood per day. Approximately one week before birth, the baby stops growing and “drops”, usually head down, into the pelvic cavity.

Birth At between 38 and 40 weeks of a normal pregnancy, a little girl or boy who has been alive for nine months is born.  

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