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The Stages of Fetal Development in Humans

By Andrea Coventry

If your science teacher has the class studying fetal development and you're looking for more information, this study guide has it all. Learn all about how the fetus develops, what happens in each trimester and when development is considered over and baby growth begins.

Duration of Pregnancy

Model of a Human Fetus Looking for a study guide on fetal development? If so, here you'll find all you need to know!

Each trimester is approximately one-third of the entire term of pregnancy. Pregnancy is counted in 40 weeks and babies are often born within 38-42 weeks. So, the first trimester lasts from weeks 1-12. The second is from weeks 13-26. The third trimester is from week 27-birth.

Growth and development is measured at different weeks and months within each trimester. The mother's doctor will carefully monitor the growth of the fetus during pregnancy and advise the mother accordingly.


Conception occurs when the sperm cell from the father meets up with the egg cell from the mother. Another name for this is fertilization. When the egg cell becomes fertilized, it becomes a zygote. The zygote starts as a single cell and then begins a period of rapid cell division. It will divide in half and then each half will divide in half and so on. The fancy name for a bunch of divided cells is blastocyst. Half of the blastocyst will become the placenta, which attaches to the mother's uterus and provides nourishment for the developing fetus. The other half develops into the embryo. The embryo will eventually turn into the fetus. The fetus is the term given to the developing baby prior to birth.

First Trimester

The first trimester is weeks one through twelve, or the first three months. By the end of the first month, the embryo is the size of a grain of rice. It resembles a tadpole, with a little tail that eventually disappears. The heart, lungs, kidneys and the liver are beginning to form.

In the second month, the embryo's arms and legs start to grow, along with a defined torso and head. Eyelids, tongue, and teeth also start to form. The heart also starts to beat. At the end of this month, the embryo is now known as a fetus.

During the third month, that heartbeat can be detected, particularly around the 10-week mark. The tail disappears and fingers and toes start to form. Earlobes and eyelids continue to develop. Reproductive organs that determine whether the baby will be male or female are starting to form, but they cannot yet be detected on an ultrasound. The fetus has also developed a sense of touch, primarily in the cheeks, genitals, palms and soles of the feet. The fetus weighs about one ounce and is three inches in length.

The third month is also when the fetus can begin to suck its thumb, roll its fingers into a fist, and starts nodding its head.

Second Trimester

The second trimester consists of the fourth through sixth months, or weeks thirteen through twenty-six.

In the fourth month the major organs have mostly finished forming. Ears have developed to where the fetus can hear its mother's heartbeat and other external sounds. A nose is formed. Bones and muscles are getting stronger, allowing the fetus to move around more and to kick. This is the first time that it actually looks like a real baby. Doctors can also determine the sex of the fetus, or whether it is a boy or a girl. It weighs about six ounces and measures about seven inches in length.

Quickening occurs during the fifth month. This is when the mother is able to finally feel all of that kicking and moving around. Eyelashes and eyebrows are starting to form, as well as a few hairs on the head. The body is covered with a soft white hair known as lanugo and a soft white waxy substance known as vernix caseosa. She can now taste and will respond to what her mother eats. The fetus now weighs one pound and measures eight to ten inches in length.

Hiccuping starts during the sixth month. The fetus may also respond to outside stimuli, such as loud sounds or talking, by blinking, kicking, or otherwise moving around. The heart, kidneys and lungs are fully formed. Facial features are becoming more apparent. It weighs about two pounds and is twelve to thirteen inches in length.

Third Trimester

The third trimester includes the seventh, eighth, and ninth months. It includes weeks twenty-seven through the actual birth.

During the seventh month, blinking occurs more frequently and eyes may actually stay open for periods of time. Hearing has sharpened to full development and taste buds have formed. Protective fatty tissues are also starting to develop. Weight is usually 2 1/2 to 3 1/2 pounds and length is usually around fourteen to sixteen inches.

In the eighth month, the fetus's organs are mature, except for the lungs and the brain is rapidly developing. Fingernails have now grown to the fingertips. Skin is smoothing out from its wrinkly state. His sense of touch is well-developed. Because the baby is much larger, weighing in at four to six pounds and measuring sixteen to eighteen inches in length, he has slowed down in his somersaults.

During the ninth and final month, the vernix caseosa and lanugo start to disappear as protective fat layers thicken. Lungs have fully developed. Skin looks pink and has finished smoothing out. Toenails have come in. The fetus starts to turn with her head pointing toward the mother's birth canal. Reflexes have kicked in and she will start grasping and turning her head. She has a strong sense of taste. Average height and weight is 7 1/2 pounds and twenty to twenty-two inches.

When birth occurs, fetal development has ceased and the new stages of infant development have begun.