Pregnancy: Week by Week

The phases of pregnancy are described as trimesters:

• First trimester: Week 0 (first day of last menstrual period) to week 13 plus 6 days

• Second trimester: Week 14 to week 27 plus 6 days

• Third trimester: Week 28 to delivery

Events

Weeks of Pregnancy*

First Trimester

Last menstrual period before fertilization

1

13 Weeks of Pregnancy

Image credit: SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY

Fertilization of an egg by a sperm

2

Fertilized egg (zygote) divides and develops into a hollow ball of cells called the blastocyst.

Blastocyst implants in the wall of uterus.

3

Outer cells of blastocyst develop into placenta and amniotic sac.

Inner cells of blastocyst develop into embryo.

4

Neural tube, the area that will become the brain and spinal cord, begins to develop.

Areas of embryo that will become major organ systems begin to develop.

5

Heart muscle tissue (myometrium) develops and begins to beat.

Buds develop that will become the arms and legs.

6

Heart chambers (atria and ventricules) form.

Blood starts to flow through major blood vessels.

7

Brain continues to develop.

Lungs, digestive system, and kidneys begin to develop.

Hands and feet begin to form.

8

Major organ systems continue to develop.

Bones develop and begin to harden.

Elbows and toes form.

Hair follicles form.

9

Eyelids and outer ears become more developed.

Teeth begin to develop.

Intestines rotate.

Kidneys begin to function, and the embryo begins to urinate (urine is mostly amniotic fluid).

Umbilical cord is fully developed, and blood vessels pass through the cord from the embryo to the placental villi.

At the end the 10th week, the fetal stage begins.

10

Facial features become more developed.

All major organ systems have formed but will continue to grow and develop.

Liver and spleen begin making red blood cells and platelets.

Genitals begin to develop.

Nails form on fingers and toes.

Fetus starts to open and close hands and mouth.

11 to 13

Second Trimester

Fetus starts to move.

Soft hair (called lanugo) begins to cover the head and body.

14 to 15

27 Weeks of Pregnancy

Image credit: SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY

Fetus moves more vigorously, and the mother starts to be able to feel the movement.

Eyes can move, but eyelids stay closed until 27 weeks.

Fetus can start hearing sounds.

16 to 18

A white, cheese-like substance (vernix) starts to cover the fetal skin.

Fetus can suck its thumb and can swallow.

19 to 21

Eyebrows and eyelashes appear.

Intestines begin to make meconium (stool).

Blood cells begin to be made by the bone marrow.

22 to 24

The fetus is active, changing positions often.

Fetus stores fat.

Air sacs begin to form in the lungs.

25 to 27

Third Trimester

Eyelids open; fetus can blink.

Lungs may be developed enough to breathe air.

28 to 29

40 Weeks of Pregnancy

Image credit: SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY

Brain and nervous system become fully developed.

Major organ systems are mature, but continue to grow and develop.

Fetus starts to practice breathing by breathing in amniotic fluid.

Fetus begins to gain weight more rapidly.

Fetal head settles into position for birth.

30 to 36

Labor and delivery

37 to 42