Fetal Development
The following information is meant as a general overview of a pregnancy through each stage of development. Pregnancies may vary, however, depending on a variety of factors. Information is presented in months as well as weeks. Since medical providers most often refer to pregnancies by the gestational age, we also do that here after the initial conception information. The gestational age of a pregnancy is the length of the pregnancy since the woman’s last monthly period. A normal pregnancy can last anywhere from 38 to 42 weeks according to gestational age.
Embryoscopy and fetoscopy photos appearing on this Web site are reproduced under "fair use" terms for the purpose of public education.
First Trimester
Fertilization and Weeks 1-4
Human development begins when a woman’s egg is fertilized by a male’s sperm, usually through sexual intercourse. This is called conception. The egg and the sperm each contain 23 chromosomes and combine during fertilization for a total of 46. Chromosomes carry the genes of the mother and the father and determine physical characteristics of the zygote (fertilized egg) such as eye and skin color, and can even play a role in how a person thinks and feels. While each person retains characteristics of their parents, the new combination of genes results in each person being totally unique. |
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| 2-5 days after conception: During the first week the one-celled zygote divides into multiple cells while traveling to the uterus. At day 4 the developing human starts forming an internal cavity and is called a blastocyst. |
| 6-10 days after conception: Implantation (when the blastocyst attaches to the uterus lining) begins around the sixth day and finishes by the end of the next week. After implantation occurs, the blastocyst is called an embryo. |
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| Week 4 (gestational age): The pregnancy is producing a hormone called Human chorionic gonadotrophin (hCG) which will result in a positive pregnancy test when the levels of the hormone are high enough. A woman may not be aware that she is pregnant, but may experience symptoms. |
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Month 2: Weeks 5-8
The embryo experiences enormous growth including the beginnings of the central nervous system and the formation of the heart. (The heart starts beating around 22 days after conception.) In fact, each organ system is basically established and starting to develop during weeks 5 and 6. |
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During the seventh week the embryo’s head develops steadily along with rapid growth of the face and brain. Arms and legs continue to develop and the eyelids and tongue are beginning to form. Hands and feet begin to form. Further facial and brain development takes place in the eighth week along with the lungs; and, the embryo’s elbows, fingers and toes are developing. This is the end of the embryonic period and is one of the most important weeks for embryonic health. The embryo is maybe an inch long.
Special note: The embryo now has everything an adult human has – but needs time to develop. All major internal and external features are in development during this time. |
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Month 3: Weeks 9-13
Time for another name change! The embryo is now called a fetus – meaning “young one” or “offspring.” Often the fetus can make complex facial expressions, and will sleep and wake along the same rhythms as the mother. From this point forward development is focused on increase in size and differentiation of all that is there already: the tissues, organs, and systems. |
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The neck is distinct from the embryo’s head. Genitals are visible during week 10 but aren’t developed enough to determine the sex of the embryo. The external genitalia continue to develop throughout week 11. Most fetal waste products are passed to the mother through the placenta. The fetus is about 3 inches long.
Special note: Taste buds have formed in the fetus’ mouth, and the baby can yawn and suck. |
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Second Trimester
Month 4: Weeks 14-17
Limbs are lengthening and moving and the baby’s fingerprints and toe prints have developed. At this point the baby can even suck its thumb. |
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| Because bones are becoming more developed, the baby’s skeletal structure may be seen in an ultrasound. By week 16 the baby’s external genitals are clear. Slow eye movement is noted and hair patterns are developing. The baby is about 6 inches long.
Special note: Movement of a fetus is called “quickening” and a mother may begin to feel these movements during this month. |
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Month 5: Weeks 18-22
While the pace of growth slows some, movements by the fetus are felt by the mother at this time. On average, delivery occurs 147 days after the mother first feels movement of the fetus. Vernix is a white substance that covers the baby’s skin to protect it from amniotic fluid and the baby also has fine hair called lanugo forming on its head and body. The skin is less transparent at this point and the baby is gaining fat.
In male babies the testicles have formed and are descending into the scrotum. As early as 18 weeks the baby can feel pain. During this month the ears can begin recognizing sound. The baby is around 8-9 inches long.
Special note: At this point a woman is half way through her pregnancy! |
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Month 6: Weeks 23-27
Rapid eye movement begins to be seen at week 23, and the fetus will blink in response to a “startle” of vibratory noise applied to the abdomen at week 24 or 25. The baby can hear its mother’s heartbeat and voice. The brain is developing rapidly during these weeks. The baby is about a foot long.
A baby born at this stage may survive; statistically there is a 15% viability at 25 weeks because the respiratory system isn’t fully developed. Survival is even more likely by 27 weeks if intensive medical care is available. |
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Special note: Surgery can be preformed on a fetus as early as week 23. This amazing medical achievement allows many deformities to be corrected before birth. The most current research shows that the developing nervous system in a fetus will respond to pain. For this reason, if surgery is required, anesthesia will be administered to both mother and fetus.
This picture is of Baby Samuel who, at 21-weeks, had spina bifida surgery during which his hand emerged from the womb and grasped the surgeon’s finger. |
surgery on a 21 week old fetus |
Third Trimester
Month 7: Weeks 28-31
Now the baby is gaining a lot of weight, weighing between 3-4lbs. at the end of the month. Fingernails have grown on fingers and toes. Eyes open at week 28, and by the end of week 30 the majority of blood formation is taking place in the bone marrow. The baby responds to touch and sound. The tongue has fully functioning taste buds.
Special note: The lungs are functioning and a baby born now has an excellent chance of survival. |
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Month 8: Weeks 32-35
The skin is pink, smooth, and the limbs begin to look “chubby” as white fat deposition increases to 8% of body weight. In fact, the fetus can weigh enough at this point to be “premature by date” and not “premature by weight” if born at this time. The baby is around 18 inches long and between 5-6lbs.
Special note: Around this time the fetus descends into a position with its head down in preparation for birth. |
Month 9: Weeks 36-40
This may be called the “finishing period.” At 38 weeks the baby is considered full-term and could be born any time! The circumference of the baby’s head is nearly equal to that of the abdomen at 38 weeks, but if the pregnancy continues, the abdomen will start to exceed the size of the head slightly. The mother will feel less movement from the baby because he or she has very little room to move around in the uterus.
White fat constitutes about 16% of body fat. The fetus will continue to add about 14 gm. of fat every day during these last few weeks. The baby can be anywhere from 19-21 inches long and around 7-10lbs.
Special note: Often males are slightly longer and heavier than females. |
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