First trimester: What's happening to your baby

The exact moment your baby was conceived cannot be determined. However, it usually happens about 2 weeks after the beginning of your last menstrual cycle.

Within hours of fertilization, the egg begins to divide. It takes 3 to 5 days for the cluster of cells to travel down the Fallopian tube to the uterus. In 10 days, the egg is firmly implanted in the uterine wall, where the cell cluster is nourished by the blood supply of the uterine lining.

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	Eyes 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	Baby 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.

When will I feel my baby move?

Your baby is moving inside you long before you can feel tiny movements. You might feel your baby move during the end of your first trimester, but it's also completely normal not to feel movement until you are 4 to 5 months pregnant.

The moment when you first feel life is different for each woman.

• It's sometimes called "quickening."
• Some women describe the feeling as "butterfly kicks" (Just wait until the "butterfly" is 8 months old and kicking pretty hard).

After you become aware of what you are feeling, you may realize that you have been feeling this movement
for a few weeks.

Related Links

What's happening to you: Your body, Weight gain

Pregnancy Condition Center: First trimester

Pregnancy & Parenting eMagazine: Registration

First published: 10/04/2002
Last updated: 06/01/2003

Reviewed by: Allina Patient Education experts