Fetal Development by Trimester

From the time of conception, your baby will grow and change inside your uterus until he or she is born. Each trimester has a unique set of developmental milestones. In the first trimester, your baby will grow from a fertilized egg into a moving fetus with eyes, ears, and functioning organs. In the second trimester, your baby’s features develop and you may be able to feel your baby move. In the third trimester, your baby will grow rapidly in preparation for birth.

During your first prenatal care visit, your doctor will calculate your estimated due date, which will be approximately 40 weeks from the first day of your last menstrual period. Even though your baby is not conceived until about two weeks after you start your period, week one of your pregnancy is considered to be the week of your menstrual period.

Your first trimester lasts until the end of week 13, the second trimester lasts from week 14 to the end of week 26, and your third trimester is from week 27 until birth. Most babies are born between weeks 38 and 42.

]]>First Trimester]]> (Weeks 0-13)

]]>Second Trimester]]> (Weeks 14-26)

]]>Third Trimester]]> (Weeks 27-40)

First Trimester Development

Your baby is conceived when a sperm cell penetrates the egg, around the end of week two of your pregnancy. It is at this moment that your baby’s sex is determined, depending on whether the sperm cell was carrying an X chromosome (a baby girl) or a Y chromosome (a baby boy).

Fertilization

The fertilized egg cell begins to divide soon after conception, and is made up of hundreds of cells by week three, when it implants itself into the lining of your uterus. As your baby continues to grow, it becomes an embryo, which is made up of three layers. The inner layer will be your baby’s lungs, liver, and digestive system; the middle layer will become your baby’s bones, kidneys, sex organs, and heart; and the outer layer will make up your baby’s skin, hair, eyes, and nervous system.

The spinal cord, brain, heart, and lungs grow and develop rapidly during the first trimester. In addition, the mouth, nose, eyes, ears, toes, and fingers begin to form. Your baby’s heart will begin to beat around week six, even though it sometimes cannot be heard until around week 12. The umbilical cord, which serves to nourish your baby and dispose of his or her wastes throughout the pregnancy, is formed during the first trimester.

Nine (9) Week Old Fetus in Utero

Your baby’s digestive system and reproductive system develops during the first trimester. And as his or her muscles continue to form in week nine, your baby may begin to move (but you probably cannot feel these movements yet).

By the end of your first trimester, your baby will be about three inches long and weigh ½ an ounce.

Second Trimester Development

In the second trimester, your baby’s hair, including eyebrows and eyelashes, begins to grow. Muscles and bones continue to develop, allowing more movement. If your baby is a girl, her eggs will develop in her ovaries early in the second trimester. By about week 18, your baby can hear your heartbeat and may even be startled by loud noises.

Sixteen (16) Week Old Fetus in Utero

Your baby will grow quickly in the second trimester, and you will be able to feel this, because it will put pressure on your lungs, stomach, bladder, and kidneys. You may begin to feel your baby move during the fifth month, and by the end of the second trimester, you will be able to feel that your baby has distinct resting and alert periods. As a result of the development of taste buds and sensory neurons, your baby will be able to taste and touch during the second trimester.

By the end of week 23, your baby probably weighs about a pound. Babies born this early may be able to survive with the help of expert medical care, but they will have mental and physical disabilities.

Your baby’s eyes are sealed shut until the end of the second trimester, when your baby starts to blink. By the end of the second trimester, your baby will weigh almost two pounds and will be about one foot long. By now, all of the essential organs have formed.

Twenty-four (24) Week Old Fetus in Utero

Third Trimester Development

Early in your third trimester, your baby may begin to recognize the sound of your and your partner’s voices. There will be a lot of moving during the third trimester; you should be able to feel about 10 movements per hour. Since babies begin “practicing” breathing during the third trimester by moving their diaphragm, you may find that your baby gets the hiccups from time to time.

Fetus in Utero, Third Trimester

During the final few weeks before birth, your baby will gain a good deal of his or her weight. By week 33, your baby is in position for delivery, ideally with his or her head resting on your cervix. The bones harden, the skin becomes thicker, and by 34 weeks, your baby would be able to survive outside of your womb without extensive medical intervention, although oxygen supplementation may be needed.

Beginning at week 35, your baby will grow rapidly, gaining 1/2 -3/4 pound per week. Babies are considered to have grown to “full-term” by week 37. But your baby will continue to grow and gain weight, and will most often be delivered between weeks 38 and 42, weighing an average of 7.5 pounds at birth and usually measuring about 20-22 inches long.

References:

The 1st trimester of pregnancy. National Women’s Health Information Center website. Available at: http://womenshealth.gov/Pregnancy/first.cfm . Accessed August 8, 2005.

The 2nd trimester of pregnancy. National Women’s Health Information Center website. Available at: http://womenshealth.gov/Pregnancy/second.cfm . Accessed August 8, 2005.

The 3rd trimester of pregnancy. National Women’s Health Information Center website. Available at: http://womenshealth.gov/Pregnancy/third.cfm . Accessed August 8, 2005.

Pregnancy calendar. Nemours Foundation website. Available at: http://kidshealth.org/parent/pregnancy_newborn/calendar/pregnancy_calendar_intro.html . Accessed December 8, 2005.

RESOURCES:

American College of Obstetricians and Gynecologists
http://www.acog.org

American Pregnancy Association
http://www.americanpregnancy.org

Last reviewed May 2007 by ]]>Jeff Andrews, MD, FRCSC, FACOG]]>

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.