What is a Fetus? - Growth & Position

Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

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.

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

Week fourteen marks the beginning of the second trimester. During this three-month period, the fetus continues important development. Bones develop in the skeleton, and genitals become more obvious. By week sixteen, parents can end the suspense and have an ultrasound to see if it's a boy or a girl. Organs continue to develop during this time, and the baby grows about an inch every two weeks. At this point, the mother will most likely trade in her jeans for stretchy pants.

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.

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