How Does the Baby Develop?

The time between fertilization and birth is about 38 weeks for a normal pregnancy.It is divided into three periods called trimesters.

The egg divides into cells that continue to divide to form all the tissues of the body until they become a fully formed human being that can live outside the mother's body.

Here is a look at the major changes that take place during each trimester.

First trimester

Week 1 to 4

At this stage in development, the developing baby is known as an embryo.
The eyes and ears are developing.
The mouth, head, and legs are growing.
The heart is beating and lungs are forming.
At the end of this month the embryo is about half an inch long and weighs less than one ounce.

Week 5 to 9

All organs are present.
Brain waves can be detected and recorded.
The jaw forms, including teeth buds in the gums.
By week eight the developing baby is called a fetus.
The developing baby is about an inch long and weighs less than one ounce.

Week 9 to 13

Fingerprints can be seen in the skin.
Fingers, toes, and soft nails have formed.
The fetus can respond to touch.
The heartbeat can be heard through a stethoscope.
Movement includes making a fist and moving the head.
Reproductive organs are formed. But it is still hard to tell if the fetus will be a boy or girl.
At the end of the month the fetus is four inches long and weighs a little more than an ounce.

Second trimester

Week 14 to 18

The fetus can hear the mother's voice, heartbeat, and outside noises.
At the end of the month, the fetus is about seven inches long and weighs about five ounces.

Week 19 to 23

Long, white eyelashes appear.
The mother may feel the fetus move.
An ultrasound may show hand movements and, possibly, if the baby will be a boy or a girl.
At the end of this month, the fetus is about 10 inches long and weighs about a pound.

Week 23 to 27

The developing baby sucks his or her thumb.
The developing baby becomes more active and turns.
At the end of the sixth month the developing baby is about 12 inches long and weighs about a pound-and-a-half. It is possible for a developing baby born during this month to survive outside the womb as an infant.

Third trimester

Week 28 to 32

All five senses are active.
The developing baby responds to light and sound.
The developing baby may hiccup or cry and can taste sweet and sour.
Rapid growth and significant weight gain occur and there is a good deal of kicking.

Week 32 to 36

Significant growth of the brain occurs.
At the end of this month the developing baby is about 16 inches long and weighs about four pounds. If born premature, the baby has a very good chance of surviving.

Week 36 to 40

Length is usually 19 to 21 inches.
The developing baby will gain about two more pounds.
The heart is pumping about 250 gallons of blood a day.

For more info on Pregnancy, check out What physical changes happen in the mother?, or What happens at birth?

Last revised: April 21, 2008