Fetal Development
If you’re pregnant (or think you might be pregnant), you may be wondering what exactly is going on in there! If you know how far along you are, you may want to keep track of exactly what stage of development your embryo, fetus, or baby is in. Even in the early stages of pregnancy, fetal development is a complex and fascinating process.
Did you know?
Fertilization, the union of a female egg and a male sperm, is the first step in a complex series of events that leads to pregnancy. At the moment the sperm meets the ovum (egg), the union results in a fertilized ovum (zygote).[i] At this moment, a distinct and unique set of DNA is created, containing all the instructions needed to create a human life.
Fertilization can happen within hours after a sexual encounter, long before you miss your period or suspect you might be pregnant.
[i] [Considine, Douglas (ed.). Van Nostrand’s Scientific Encyclopedia. 5th edition. New York: Van Nostrand Reinhold Company, 1976, p. 943]
Fertilization has taken place. Then what? Within 24 hours after fertilization, the zygote (fertilized egg) rapidly divides into many cells. By the eighth week of pregnancy, the embryo develops into a fetus. The weeks of pregnancy are divided into three trimesters. Each trimester has its own milestones. See the chart below for some highlights! (Please note: pregnancy “months” are 28 days (4 weeks), measured from the first day of your last menstrual period. There are about 40 weeks to a typical pregnancy.)
The First Trimester (weeks 1-12)
During the first three months of pregnancy, the most amount of growth and change will occur. The fertilized egg will change from a small grouping of cells into a fetus exhibiting human features.
Month 1 | All that is needed to sustain the pregnancy is developing, such as the amniotic sac and the yolk sac, which will supply nutrients as the fetus grows. In these first few weeks, the mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin. The tiny "heart" tube will beat 65 times a minute by the end of the fourth week. By the end of this month, the baby is smaller than a grain of rice. |
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Month 2 |
Facial features begin to develop. All systems are developing rapidly, including the neural system (brain, spinal cord, nerves), the digestive system, and the skeletal system. By week 8, the embryo is officially called a fetus and is approximately 1 inch long. |
Month 3 |
The arms, hands, fingers, feet, and toes are fully formed. At this stage, the fetus is able to open and close its fists and mouth. The reproductive organs are starting to form. By the end of the month, all organ systems and features are in place. Over the next few months, the fetus will continue to grow and mature. The fetus is approximately 3-4 inches long. |
Month 1 |
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All that is needed to sustain the pregnancy is developing, such as the amniotic sac and the yolk sac, which will supply nutrients as the fetus grows. In these first few weeks, the mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin. The tiny "heart" tube will beat 65 times a minute by the end of the fourth week. By the end of this month, the baby is smaller than a grain of rice. |
Month 2 |
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Facial features begin to develop. All systems are developing rapidly, including the neural system (brain, spinal cord, nerves), the digestive system, and the skeletal system. By week 8, the embryo is officially called a fetus and is approximately 1 inch long. |
Month 3 |
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The arms, hands, fingers, feet, and toes are fully formed. At this stage, the fetus is able to open and close its fists and mouth. The reproductive organs are starting to form. By the end of the month, all organ systems and features are in place. Over the next few months, the fetus will continue to grow and mature. The fetus is approximately 3-4 inches long. |
The Second Trimester (weeks 13-28)
Month 4 | The fetus can suck their thumb, stretch, yawn, and make faces. The nervous system is starting to function. You can now find out the gender via ultrasound! By the end of the month, the fetus is approximately 6 inches long. |
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Month 5 |
The fetus is developing muscles, and you may even start to feel movement. Hair begins to grow. By the end of the month, the fetus is approximately 10 inches long. |
Month 6 |
Finger and toe prints are now visible, and the fetus begins to open and close their eyes. The fetus can hear and respond to sounds. You might even feel when they have the hiccups! By the end of the month, the fetus is approximately 12 inches long. |
Month 7 |
The fetus continues to mature and starts to put on more body fat. Hearing is fully developed, and they respond to sound, pain and light. The fetus is now approximately 14 inches long. |
Month 4 |
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The fetus can suck their thumb, stretch, yawn, and make faces. The nervous system is starting to function. You can now find out the gender via ultrasound! By the end of the month, the fetus is approximately 6 inches long. |
Month 5 |
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The fetus is developing muscles, and you may even start to feel movement. Hair begins to grow. By the end of the month, the fetus is approximately 10 inches long. |
Month 6 |
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Finger and toe prints are now visible, and the fetus begins to open and close their eyes. The fetus can hear and respond to sounds. You might even feel when they have the hiccups! By the end of the month, the fetus is approximately 12 inches long. |
Month 7 |
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The fetus continues to mature and starts to put on more body fat. Hearing is fully developed, and they respond to sound, pain and light. The fetus is now approximately 14 inches long. |
The Third Trimester (weeks 29-40)
Month 8 | The fetus is still putting on more fat. The brain is developing rapidly, and they can hear and see. Most of the internal systems are now well-developed, and they are approximately 16-18 inches long. |
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Month 9 |
The lungs are almost fully developed and functional, and the fetus continues to develop and mature. The fetus is between 17-19 inches long. |
Month 10 |
The baby is ready to go! They may change positions now to get ready for birth as well as move less, since their space is reduced. They can arrive into the world any day now, approximately 18-20 inches long. |
Month 8 |
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The fetus is still putting on more fat. The brain is developing rapidly, and they can hear and see. Most of the internal systems are now well-developed, and they are approximately 16-18 inches long. |
Month 9 |
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The lungs are almost fully developed and functional, and the fetus continues to develop and mature. The fetus is between 17-19 inches long. |
Month 10 |
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The baby is ready to go! They may change positions now to get ready for birth as well as move less, since their space is reduced. They can arrive into the world any day now, approximately 18-20 inches long. |
If you think you may be pregnant, the first thing to do is to have the pregnancy medically confirmed. This is done though an ultrasound exam. A positive pregnancy test alone cannot confirm if the pregnancy is developing in the right place or if it is viable (developing healthily, not miscarrying). Schedule an ultrasound to determine that you have a viable, intrauterine pregnancy.
If you are considering abortion, you need to know how far along you are to know what options are available to you. You can contact us today at 508-671-7146 to book a pregnancy confirmation appointment or contact a pregnancy medical clinic in your area.
(Please note: we do not provide, prescribe, or refer for abortion.)
Resources
[1] [Considine, Douglas (ed.). Van Nostrand’s Scientific Encyclopedia. 5th edition. New York: Van Nostrand Reinhold Company, 1976, p. 943]
[1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3211703#maincontent
[1] https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth#:~:text=Within%2024%20hours%20after%20fertilization,are%20divided%20into%20three%20trimesters.
[1] https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302#:~:text=Just%20four%20weeks%20after%20conception,the%20eyes%20and%20ears%20developr area.
