First trimester: Your growing baby

Conception usually occurs about two weeks after the beginning of your last menstrual cycle. The fertilized egg divides, and the cells multiply and form a ball.

After traveling down the fallopian tube, this cluster of cells implants in the wall of your uterus. Then, nourished by the blood supply to your uterus, the cells begin to form the baby and the placenta.

By week 13, your baby is completely formed, is 3 1/2 inches long, and weighs about one ounce.

Baby's development: Weeks 1 to 13

Week 1

Date of last period.

Week 2

The ovary releases an egg, which is fertilized by a sperm and then begins to divide into a cluster of cells.

Week 3

The cluster of cells attaches to the uterus.

Week 4

Cells multiply and group together to make different body structures.

Week 5

Nervous system, spine and brain are developing. Baby is one tenth of an inch long.

Week 6

Heart is developing in the chest cavity and now has two chambers.

Week 7

Eyes and ears are developing; brain and spinal cord are almost fully developed. Heart now has four chambers and is beating strongly enough to circulate blood.

Week 8

Illustration of baby at week 8Eyes and inner ears become further developed. Baby can open his or her mouth and suck. All major internal organs are now in place but are not yet fully developed. Baby is 1 inch long and weighs less than 1 ounce.

Week 9

Chest cavity is separated from abdominal cavity by a muscle that later becomes the diaphragm. Baby is starting to kick, although mother can't feel it yet.

Week 10

Umbilical cord is formed, and blood is circulating in it. Head is still large in proportion to the body.

Week 11

All essential internal organs are formed and most begin to function. Head is nearly half the size of the baby and external genitalia are developing.

Week 12

Illustration of baby at week 12Baby is "practicing" breathing movements.

Week 13

Vocal cords are beginning to form. Baby is completely formed. Baby is 3 inches long and weighs 1 ounce.

Source: Allina Health's Patient Education Department, Beginnings: Pregnancy, Birth and Beyond, seventh edition, ob-ah-90026
First Published: 10/04/2002
Last Reviewed: 12/02/2015


There might be some bleeding when the cluster of cells implants. It can be normal to have a little bit of spotting.