Guide to Fetal Development in Each Trimester

Image of a fetus in utero

Every expectant mom wants to know all she can about prenatal development. It is fun and educational to know what is going on with your baby during each trimester during your pregnancy.

First Trimester Prenatal Development

The pregnancy begins, of course, with conception, the fertilization of the egg with sperm. From there, the cells begin rapidly dividing to create a baby.

DNA and Sexual Differentiation

Instructions for the baby's development are contained in DNA from both the mother and the father. Every person's genes are collected in 46 bundles of DNA called chromosomes. The chromosomes contain all the instructions needed for a baby to form, for a child to grow, and for an adult's body to function. A woman's egg contains 23 chromosomes, and a man's sperm contains 23. When they combine, a full set of instructions is created.

The baby's sex is determined right away. It's the man's sperm that makes the difference; it can contain instructions for either a boy or a girl. However, during these early stages of the first trimester, the fetus (actually called an "embryo" from week four until about week eight) doesn't look like a baby at all.

The First Month: Laying the Groundwork

As this fertilized egg develops into an embryo, several different kinds of cells are formed within the egg:

  • First, the ectoderm cells will develop into sense organs and the nervous system.
  • The mesoderm cells then make the circulatory, skeletal, and muscular systems.
  • Finally, the digestive system and some glands are developed from the endoderm cells.

At this time, a layer of cells is also forming in the uterus, which will become the placenta. The placenta supplies nutrition to the baby throughout the pregnancy.

During the first month, the heart begins to beat, and the first parts of the nervous and digestive systems begin to form. The embryo is about a quarter of an inch long at the end of the first month and has little bumps where his or her arms and legs will later develop.

The Second Month: Brain Growth

By the end of the second month, the embryo has grown to about one and a half inches. Bones, muscles, and the face and neck begin to develop during this time. A lot of focus in the second month of prenatal development is on the brain, so if you saw an embryo during this stage, it would have a very big forehead compared to the size of the rest of the embryo.

The Third Month: Beginning of Independent Movement

The third month is when things really start to happen. The baby's genitals begin to form and the buds for the baby's future teeth start to develop. The organs start to show signs of activity. By the end of the first trimester, independent movement of the body may occur. You may not feel this movement, but it is happening!

Prenatal Development in the Second Trimester

There's a lot of prenatal development going on in the second trimester. By the end of this trimester, your baby has a chance of surviving outside the womb.

The Fourth Month: Obvious Gender Changes

This is when the arms and legs fully form and the head becomes a smaller part of the fetus in proportion to the rest of the body. The baby's bones are beginning to form. Your baby's sex will become apparent during this time -- but an ultrasound may not be able to differentiate between the two yet. The skin appears red because of the blood coursing through the baby's veins, but as the trimester progresses the skin develops glands, hair, and finger and toenails.

Fifth and Sixth Months: Viability Outside the Womb

Don't be surprised if the fetus reacts to loud noises beginning around week 19. That's when the ears begin to work. During this week, girls develop a uterus and the prostate forms in boys as well.

By the end of the fifth month, your baby weighs about a pound. From week 23 on, your baby could live outside of the womb. Although it would be dangerous for the baby to be born at this time, with expert medical care, it's possible for a baby this young to survive. The baby's lungs aren't quite ready to breathe air yet, the skin isn't fully developed, and the eyes aren't ready to open but they can survive in the NICU.

At the end of the second trimester, the eyes have completely formed and the eyelids are fully open. By this time, you should be feeling the baby move. An ultrasound exam around this time should be able to tell if it's a boy or girl.

Third Trimester Development

The fetus grows a lot during the last trimester. This is largely because it is finally putting on body fat and preparing to regulate its own temperature at birth.

Seventh and Eighth Month: Maturing Organs

During this time, the internal organs mature so that the baby's intestines can handle food. The lungs can breathe air, the eyes can open and handle sunlight, and the brain can interpret what's happening and begin to learn. The nervous system has developed enough at this time so that the baby can control its own body temperature. The bones are fully developed by week 29 but still remain somewhat soft. Because the space inside your uterus is getting tight, your baby won't be able to move as freely as before, but the baby still should be able to give you a good kick or two.

Healthy babies born after week 28 have a 90% chance of survival without physical or neurological damage.

Full Term: Finishing Touches

The fetus is considered "full term" at 37 weeks, but babies born in week 36 usually survive without too many problems. The fingernails have reached the tips of the fingers and the baby has developed a firm grasp. In preparation for birth, the baby often settles into a head-down positon in the uterus. Your placenta continues to supply antibodies to your baby to help him or her fight off infections after birth. In boys, the testes continue to descend into the scotum during these last weeks.

The Big Day

When it's time to give birth, you're about to make the transition from being pregnant to being a mom-and it's time for your little one to make the transition from fetus to baby. Instead of getting oxygen from inside your body, the baby will breathe on his/her own for the first time. Soon, he/she will be ready to eat, ready for diaper changes, and ready to begin growing up.

It can be a lot of fun to track your baby's prenatal development, but remember that these markers are merely estimates. Every baby and every pregnancy is different, so if you're not having the exact week by week events that other people have, there's not necessarily a reason to worry. Your doctor or midwife will keep you up to date with all of the changes that your baby is going through and if there is a problem. Just enjoy your pregnancy and get ready for your baby's arrival.

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Guide to Fetal Development in Each Trimester