Fetal Development by Trimester
by Krisha McCoy, MS
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 milestones. In the first trimester, your baby will grow from a fertilized egg into a moving fetus with eyes, ears, and working 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 to get ready for birth.
Your doctor will calculate your estimated due date during your first prenatal care visit. It will be about 40 weeks from the first day of your last menstrual period. Even though your baby is not conceived until about 2 weeks after you start your period, week 1 of your pregnancy is considered to be the week of your menstrual period.
Your first trimester lasts until the end of week 12. The second trimester lasts from week 13 to the end of week 26. Your third trimester is from week 27 until birth. Most babies are born between weeks 38 and 42.
First Trimester (Weeks 0-12)
Second Trimester (Weeks 13-26)
Third Trimester (Weeks 27-40)
Your baby is conceived when a sperm cell penetrates the egg. This happens around the end of week 2 of your pregnancy. It is at this moment that your baby’s sex is determined. This depends on whether the sperm cell was carrying an X chromosome (a baby girl) or a Y chromosome (a baby boy).
The fertilized egg cell begins to divide soon after conception. The egg is made up of hundreds of cells by week 3. It has now moved down the fallopian tube and implanted itself into the lining of your uterus. As your baby continues to grow, it becomes an embryo. An embryo is made up of 3 tissue 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. 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 6. It sometimes cannot be heard until around week 10-12. 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.
Your baby’s digestive system and reproductive system develops during the first trimester. As his or her muscles continue to form in week nine, your baby may begin to move, but you 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.
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.
Your baby will grow quickly in the second trimester. You will be able to feel this. It will put pressure on your lungs, stomach, bladder, and kidneys. You may begin to feel your baby move during the fifth month. By the end of the second trimester, you will be able to feel that your baby has 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 usually 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 2 pounds and will be about 1 foot long. By now, all of the essential organs have formed.
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. 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.
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 and the skin becomes thicker. By 34 weeks, your baby would be able to survive outside of your womb without extensive medical intervention, although oxygen therapy 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. Your baby will weigh an average of 7.5 pounds at birth and measure about 20-22 inches long.
A pregnancy that goes beyond 42 weeks is considered past the due date. At this time, the doctor may induce labor. If your pregnancy has gone beyond 40 weeks, be sure to talk to your doctor.
Pregnancy: Stages of pregnancy. Office on Women's Health website. Available at:https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/stages-pregnancy. Updated September 27, 2010. Accessed January 30, 2017.
Pregnancy calendar. Nemours Foundation website. Available at:
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Accessed January 30, 2017.
Office on Women's Health
The American Congress of Obstetricians and Gynecologists
The Society of Obstetricians and Gynaecologists of Canada
Women's Health Matters
Last reviewed January 2017 by Andrea Chisholm, MD
Last Updated: 3/15/2015
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