Fetal Development

Who is this Life Inside Me?

The following are key milestones in the development of the unborn fetus. More detailed information is available at our center.

Day 1
Sperm joins with ovum (egg) to form one cell—smaller than a grain of salt. The new life has inherited 23 chromosomes from each parent, 46 in all. This one cell contains the complex genetic blueprint for every detail of human development—the child’s sex, hair and eye color, height, skin tone.

Day 21
The heart begins to beat.

Day 28
The backbone and muscles are forming. Arms, legs, eyes, and ears have begun to show.

Day 35
Five fingers can be discerned in the hand. The eyes darken as pigment is produced.

Day 40
Brain waves can be detected and recorded.

Week 7
The embryo begins to move spontaneously. The jaw forms, including teeth buds in the gums. Soon the eyelids will seal to protect the embryo’s developing light-sensitive eyes and will reopen at about the 7th month.

Week 8
At a little more than an inch long, the developing life is now called a fetus—Latin for “young one” or “offspring.” Everything is now present that will be found in a fully developed adult. The heart has been beating for more than a month, the stomach produces digestive juices and the kidneys have begun to function. Forty muscle sets begin to operate in conjunction with the nervous system. The fetus’ body responds to touch, although the mother will not be able to feel movement until the 4th or 5th month.

Week 12
The fetus now sleeps, awakens and exercises its muscles energetically—turning its head, curling its toes, and opening and closing its mouth. The palm, when stroked, will make a tight fist. The fetus breathes amniotic fluid to help develop its respiratory system.

Month 5
Half the pregnancy has now passed, and the fetus is about 12 inches long. The mother has definitely begun to feel movement by now. If a sound is especially loud or startling, the fetus may jump in reaction to it.

Month 6
Oil and sweat glands are functioning. The delicate skin of the growing baby is protected from the fetal waters by a special ointment called “vernix.” If the baby were born in this month and given the proper care, he would survive.

Month 7
The baby now uses four of his senses—vision, hearing, taste, and touch. He can recognize his mother’s voice.

Month 9
Toward the end of this month, the baby is ready for birth. By this time the infant normally weighs 6 to 9 pounds, and his heart is pumping about 250 gallons of blood a day. He is fully capable of life outside the womb.

The average duration of pregnancy is 280 days from the first day of the mother’s last menstrual period, but this varies. Most babies (85 percent to 95 percent) are born somewhere between 266 and 294 days.

Source: The First Nine Months, Focus on the Family. Used by permission.