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 1 ounce.

Baby's development: Weeks 1 to 13

Week 1

Conception (when the sperm fertilizes the egg and you become pregnant) usually happens about two weeks after the beginning of your last period.

Week 2

You're ovulating. Your egg matures in an ovary. When it is released, it is swept up by the fallopian tube. 

Week 3

Your egg is fertilized by sperm. The fertilized egg divides, and the cells multiply and form a ball. The cluster of cells travel down the fallopian tube into your uterus.

Week 4

The cluster of cells attach to your uterus. Then, nourished by the blood supply to your uterus, the cells begin to form your baby and the placenta.

Week 5

Your baby's nervous system, spine and brain are developing. Baby is one tenth of an inch long.

Week 6

Your baby's heart is developing in the chest cavity and now has two chambers.

Week 7

Your baby's eyes and ears are developing. His or her brain and spinal cord are almost fully developed. Your baby's heart now has four chambers and is beating strongly enough to circulate blood.

Week 8

Your baby's eyes and inner ears continue to develop. Baby can open his or her mouth and suck. All major internal organs are now in place but are not yet fully developed. Your baby is about 1 inch long and weighs less than 1 ounce.

Week 9

Your baby is starting to kick, although you won't be able to feel it yet. Baby's chest cavity is separated from abdominal cavity by a muscle that later becomes the diaphragm.

Week 10

Your baby's umbilical cord is formed and blood is circulating in it. Baby's head is still large in proportion to his or her body.

Week 11

Your baby's head is nearly half the size of your baby. All essential internal organs are formed and most begin to function. Baby's external genitalia are developing.

Week 12

Your baby is practicing breathing movements. His or her vocal cords are starting to form.

Week 13

Your baby is completely formed, is 3 ½ inches long, and weighs about 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

Tip

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