Merck Manual

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Stages of Pregnancy

Stages of Pregnancy

Although pregnancy involves a continuous process, it is divided into three 3-month periods called trimesters:

  • First trimester: Weeks 0 to 12

  • Second trimester: Weeks 13 to 24

  • Third trimester: Weeks 25 to delivery


Weeks of Pregnancy

1st Trimester

The woman’s last period before fertilization occurs.


Stages of Pregnancy

Fertilization occurs.

The fertilized egg (zygote) begins to develop into a hollow ball of cells called the blastocyst.


The blastocyst implants in the wall of uterus.

The amniotic sac begins to form.


The area that will become the brain and spinal cord (neural tube) begins to develop.


The heart and major blood vessels are developing. The beating heart can be seen during ultrasonography.


The beginnings of arms and legs appear.


Bones and muscles form. The face and neck develop.

Brain waves can be detected.

The skeleton is formed. Fingers and toes are fully defined.


The kidneys begin to function.

Almost all organs are completely formed.

The fetus can move and respond to touch (when prodded through the woman’s abdomen).

The woman has gained some weight, and her abdomen may be slightly enlarged.


2nd Trimester

The fetus’s sex can be identified.

The fetus can hear.


Stages of Pregnancy

The fetus’s fingers can grasp. The fetus moves more vigorously, so that the mother can feel it.

The fetus’s body begins to fill out as fat is deposited beneath the skin. Hair appears on the head and skin. Eyebrows and eyelashes are present.


The placenta is fully formed.


The fetus has a chance of survival outside the uterus.

The woman begins to gain weight more rapidly.


3rd Trimester

The fetus is active, changing positions often.

The lungs continue to mature.

The fetus’s head moves into position for delivery.

On average, the fetus is about 20 inches long and weighs about 7 pounds. The woman’s enlarged abdomen causes the navel to bulge.


Stages of Pregnancy