Day 1: Fertilization
The sperm joins with the egg to form one cell. This single cell contains the complex genetic makeup for every detail of a new human being. After fertilization, nothing new is added but oxygen, nutrition and time.1
1st Month: 1–4 Weeks
The first cell divides in two, and cell division continues as it travels down the fallopian tube to the uterus. Foundations of the brain, spinal cord, and nervous system are already established and by day 21 the heart begins to beat in a regular fashion.2 Muscles are forming, and arms, legs, eyes, and ears have begun to show.
2nd Month: 5–8 Weeks
By six weeks, brain waves can be detected.3 The jaw forms, including teeth and taste buds.4 The baby begins to swallow amniotic fluid, and sometimes hiccups.5 Fingers and toes are developing and at seven weeks the chest and abdomen are fully formed.6 Swimming in the amniotic fluid, she now looks like a miniature human infant.7,8
3rd Month: 9–12 Weeks
Unique fingerprints are evident and never change.9 The baby now sleeps, awakens, and exercises her muscles. The baby is very active. The gender can be visually determined and family resemblances may appear.10 By the end of the month all the organs and systems of her body are functioning.11
4th Month: 13–16 Weeks
By the end of the fourth month, the baby is 8–10 inches in length and weighs about one-half pound. Her ears are functioning and she hears her mother's heartbeat, as well as external noises like music.12 Lifesaving surgery has been performed on babies at this age.
5th Month: 17–20 Weeks
If a sound is especially loud, the baby may jump in reaction to it. Thumb-sucking has been observed during the fifth month.13
Oil and sweat glands are functioning. She grows rapidly in size and strength while her lungs become more developed.14
The baby can now recognize her mother's voice. She exercises by stretching and kicking. She uses the senses of hearing, touch and taste, and she can even look around with open eyes.15
The baby swallows a gallon of amniotic fluid per day and often hiccups.16 The baby's kicks are stronger and mom may be able to feel an elbow or heel against her abdomen.17
Gaining one half pound per week, the baby is getting ready for birth. The bones in her head are soft and flexible to more easily mold for the journey down the birth canal.18 Of the 45 generations of cell divisions before adulthood, 41 have already taken place. Only four more come before adolescence. Ninety percent of a person's development happens in the womb.19