Health Topics for Pregnancy and Childbirth

Department of Obstetrics and Gynecology

Department of Pediatrics

UI Family Care Center

Iowa Registry for Congenital and Inherited Disorders

Fetal development

Pregnancy begins with conception. Two cells, a sperm and an egg, come together inside the uterus to make one cell. This single cell will divide over and over to make a cluster of cells. The cluster of cells then attaches to the lining of the uterus and becomes a baby. This process takes approximately 2 weeks. Even before you know you are pregnant, the baby's nervous system, heart, and lungs have begun to develop.

Pregnancy is measured in weeks or months: 40 weeks or 9 months. The months are divided into three parts called "trimesters." The first trimester is the first 3 months of pregnancy. The second trimester is the fourth to sixth months and the third trimester is the seventh to ninth months of pregnancy.

The first trimester is the most important in regard to the baby's development. That is why healthcare providers recommend that you eat well, exercise, and avoid alcohol, smoking, and drugs. By the end of the first trimester, your baby's brain, spinal cord, heart, and all the major organs are formed. Your baby looks like a miniature infant. The hands are complete and the fingernails are growing. The face is well formed and tooth buds are appearing. The baby has begun to swallow and kick, even though you cannot feel it. The baby is about 2 to 3 inches long, but weighs less than 1 ounce.

During the second trimester, the baby's bones and muscles develop. You will begin to feel the baby move. The baby kicks, sleeps, and wakes. The baby can hear sounds, although they will be slightly muffled like sounds heard underwater. Your baby will respond to music and speech. The eyes are developed and eyebrows and eyelashes can be seen. By the end of the sixth month, the baby is 11 to 14 inches long and weighs 1 to 1-1/2 pounds.

The third trimester is when the baby grows the most in weight and length. The baby will add another 5 to 8 pounds and grow another 7 to 11 inches in just this 3-month period. The baby begins to practice breathing motions and open its eyes. At the end of 9 months, the average baby weighs 6 to 9 pounds and is 18 to 22 inches long.

It is amazing to think that each new baby begins with two tiny cells. Take care of your baby by taking care of yourself. If you have questions about your pregnancy or about your baby's development, talk with your provider.

Last Reviewed 2005

Disclaimer: This content is reviewed periodically and is subject to change as new health information becomes available. The information provided is intended to be informative and educational and is not a replacement for professional medical evaluation, advice, diagnosis or treatment by a healthcare professional.

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