Fetus
A fetus is the stage of human development that occurs between the embryonic phase and birth, typically starting around ten weeks after conception. This stage is characterized by distinct human features, including arms, toes, and facial structures, which become more prominent as the pregnancy progresses. The entire pregnancy generally lasts from thirty-seven to forty-one weeks, divided into three trimesters, each marked by specific developmental milestones.
During the early weeks of pregnancy, a fertilized egg develops into a zygote and then a blastocyst, which implants in the uterine wall. By the end of the first trimester, the embryo transitions into a fetus, and the baby's organs start to form and function. As gestation continues, the fetus undergoes rapid growth, with significant developments occurring in the second and third trimesters, such as increased movement, sensory development, and fat accumulation.
Factors like maternal nutrition, stress, and environmental exposures can influence fetal growth and health. By birth, the average fetus typically weighs between six to seven pounds, although this can vary widely. Understanding fetal development is crucial for addressing maternal health and ensuring positive outcomes for both mother and baby.
Subject Terms
Fetus
A fetus is an unborn human between the embryonic stage of development and birth. A fetus normally forms within eight weeks (ten weeks gestation) of conception, which is the very beginning of a pregnancy when a sperm fertilizes an egg. Approximately three weeks after fertilization, an embryo forms inside the woman's uterus. The embryo eventually forms into a fetus, a stage distinguished by the unborn baby's visibly human features such as arms, fingers, toes, and eyes. The fetal stage lasts until birth, with the entire pregnancy usually lasting between thirty-seven to forty-one weeks.
Early Fetal Development
Pregnancy is divided into three trimesters. The first trimester usually lasts about twelve weeks; the second trimester takes place between weeks thirteen and twenty-eight; and the third trimester lasts from week twenty-nine until birth. The fetus develops near the end of the first trimester. The time between conception and birth is referred to as gestation. Determining the exact moment of conception is impossible. Doctors must measure the gestation of a pregnancy from the first day of a woman's last menstrual cycle to the present date. Gestational age is measured in weeks and goes through several phases. In the first two weeks of pregnancy, a woman has not yet gotten pregnant. Her body prepares for conception through ovulation, during which an egg is released from her ovary. This egg sits in the woman's fallopian tubes to await fertilization.
When a man and a woman engage in intercourse, the man releases sperm into the woman's vagina. The sperm travels through an opening in the woman's cervix and into the fallopian tubes. The sperm then meets the egg and enters it, creating a zygote. The zygote is a single-celled organism containing DNA from both the man and the woman. This DNA enables the zygote to form a baby. The zygote continues travelling down the fallopian tube, dividing and creating a group of cells called a blastocyst. Within a few days, this blastocyst travels to the uterus and plants itself in the uterine wall. A thick wall of blood known as the placenta has formed in the lining of the uterus; this nourishes the embedded blastocyst. The placenta provides nourishment to the baby throughout the entirety of the pregnancy in the form of essential nutrients and antibodies to help fight infection. Once the blastocyst has planted itself in the uterus, the gestational age of the pregnancy is about four weeks.
By the fifth week, the baby's major systems and structures begin to develop out of the blastocyst cells. The inner cluster of blastocyst cells becomes an embryo. At this point, the pregnancy has entered its embryonic stage. The cells within the embryo then begin to divide in a process called differentiation. During this process, each cell is assigned specific functions. Cells take form as the embryo grows larger. Soon, the baby's physical features become visible, and the brain, heart, spinal cord, and gastrointestinal tract begin to develop. The umbilical cord also forms between the embryo and placenta at this time, allowing nutrient transfer from the woman to the baby. By the sixth and seventh week, tiny arms and legs take shape, and the eyes and ears begin to form. The baby's heartbeat now has a consistently audible rhythm as the blood begins to pump through its vessels. Within nine weeks, all the essential organs of the baby have taken form. The early weeks of pregnancy require special attention to diet and other health habits to ensure the fetus does not develop any health issues or defects.
Fetal Stage of Pregnancy
The fetal stage begins at approximately the tenth week of pregnancy. Now noticeably human, the embryo is referred to as a fetus. Between weeks eleven and fourteen, many more biological distinctions occur. The baby's face becomes evident, and its genitals begin to materialize. The eyelids have formed and remain closed until about the twenty-eighth week of pregnancy. Fingernails, toenails, and tooth buds also start to appear. During weeks fifteen through eighteen, the fetus's skin begins to thicken as the muscles and bone tissue become firmer. The baby's strength increases as a result, and it begins moving and stretching its limbs. The baby can also make a fist and make sucking motions with its mouth. This activity continues to increase through weeks nineteen, twenty, and twenty-one, as the baby gains the ability to hear and swallow. A mother can usually begin to feel the fetus's movements during these weeks.
Weeks twenty-two through twenty-six see the emergence of hair on different parts of the fetus. Eyebrows, eyelashes, and fine hairs covering its whole body begin to grow. The fetal heartbeat is strong at this stage and can be heard through a stethoscope. The fetus's hearing has also improved, and it may respond to noises with movement. The baby begins to store fat, and its intestines now harbor meconium, which encompasses the baby's first feces.
During the third trimester, the fetus's brain experiences rapid growth. Its nervous system has also matured, and it can begin to control its body. The baby's bones have fully formed by week twenty-nine, and the body begins to absorb vitamins and minerals by week thirty-two. At this point, the fetus can be as long as eleven inches and usually weighs about four pounds. Rapid weight gain begins around week thirty-five, and the baby gains about half a pound a week for a month. Some women will give birth around the thirty-seventh week, which is referred to as early term birth. If this does not occur, the fetus will continue to grow in the womb for another two to four weeks. The baby's brain becomes heavier, and its overall weight is usually about six or seven pounds by the time it is born. Fetal growth varies among pregnancies, and some babies are born smaller or larger than others are. Factors that can affect fetal growth include maternal weight, nutrition, stress, or toxin exposure. Other factors include placental density, umbilical cord blood flow, and fetal hormone production.
Bibliography
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