A fingernails grow. In boys, testes descend and

A fetus is the term for a developing baby in the uterus from week 11 until the time of birth.

Read on to learn more about the growth and development of a fetus, then quiz yourself.

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Bun in the Oven

When you hear the expression, ”bun in the oven,” your mind most likely goes to a very specific place. No, not a loaf of bread in a kitchen appliance. In our culture, this phrase clearly refers to a pregnant woman with a growing baby inside her.

From start to finish, this ”bun” undergoes many different phases and is actually known by different names depending on the stage of development. During the majority of development in the uterus, it is known as a fetus.

Woman with child
Pregnant woman

Specifically, the fetal stage begins around week eleven of development and continues until birth. Prior to becoming a fetus, this developing baby is called an embryo. During the embryonic phase, weeks one through ten, the initial stages of growth occur. This includes a crucial stage called organogenesis, in which internal organs begin development.

By the time the embryo becomes a fetus, it already has many of the same bodily features as you and I. In this lesson, we will follow a fetus and its growth from week eleven on up to birth.

Fetal Development in the Second Trimester

Let’s take a look at exactly where the fetus is in terms of development at week eleven. About the size of a grape tomato, or one and a half inches long, the fetus is small but recognizable as a baby. Its head is extremely large and disproportionate, but eyes, ears, and a nose are all present.

The tiny being has stubby arms and legs, as well as rudimentary internal organs. Excited parents listening in at the doctor’s office can hear a beating heart, bringing life to the mother’s growing belly.

Fetus at twelve weeks of development
Ultrasound of fetus at sixteen weeks
Ultrasound of fetus

During weeks nineteen through twenty-two, the outer covering of the fetus changes dramatically. A cheesy coating called vernix caseosa appears, protecting the baby’s skin. Also becoming apparent is a fine covering of hair on the fetus known as lanugo.

By the end of this three-week period, the fetus finally tips the scale at around one pound. Lengthwise, it compares to a small banana, measuring about seven inches.From week twenty-three to twenty-seven, finer details appear on the fetus. Fingerprints and footprints form, and fingernails grow. In boys, testes descend and girls now have a uterus and ovaries complete with a lifetime supply of eggs. During this time, the lungs undergo a very important phase.

Surfactant, the liquid lining the inside of the lungs, is produced. This crucial substance will allow lungs to work properly after birth. Week twenty-seven marks the end of the second trimester.

The Third and Final Trimester

Week twenty-eight marks the beginning of the third trimester. During the next three months, the fetus will transform from what seems like a miniature model of a person to a chubby baby.

Consequently, this may mark the mother’s largest amount of discomfort as she does not have an infinite amount of space to accommodate this growing person.At week twenty-eight, the fetus is around ten inches long and weighs close to two pounds. To reach roughly eight pounds at forty weeks, growth in this trimester will be tremendous. It is also at twenty-eight weeks that the fetus has a 90% chance of survival without impairment if born this early.So this is the home stretch: the fetus now has twelve more weeks in which to complete important in-utero development. By week thirty, the fetus is opening its eyes and may have plenty of hair on its head.

It is able to practice breathing now, as this will be a crucial skill once out of the mother’s uterus. At this point, red blood cells have formed in the bone marrow.By thirty-seven weeks, the position of the fetus becomes an important issue. Around this time, the fetus is most likely descending into the mother’s pelvis in preparation for birth. Its position should be head down at this point. If it is not, this is known as a breech position and dropping into the pelvis may not occur.There are several different types of breech positions, and sometimes a physician can turn the baby out of this position.

This can be accomplished by a procedure called an external cephalic version. If the baby is still in the breech position when labor begins, surgery in the form of a Cesarean section may be needed.

Example of fetus in breech position
fetus in breech position

During the next three weeks, the fetus will increase in size from around six pounds to an average of eight pounds.

Length is typically about eighteen to twenty inches. By week forty, the pregnancy is at term, and labor and delivery are imminent.

Lesson Summary

A fetus is a developing baby inside its mother from weeks eleven to birth.

At eleven weeks, organogenesis has already begun, which is the formation of internal organs. The heart is beating, and arms and legs are already present. The fetal stage takes up the second and third trimester, marking significant growth and development.

Organs continue to grow, and features such as hair and fingernails are formed. Lung surfactant is being produced, and the fetus is practicing breathing. By thirty-seven weeks, the fetus should be head down and descending into the mother’s pelvis in preparation for birth.


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