The cells lining the fallopian tube facilitate fertilization. If fertilization does not occur, the egg moves down the fallopian tube to the uterus, where it degenerates, and passes through the uterus with the next menstrual period.
If a sperm penetrates the egg, fertilization results. Tiny hairlike cilia lining the fallopian tube propel the fertilized egg zygote through the tube toward the uterus. The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube to the uterus. The zygote enters the uterus in 3 to 5 days. In the uterus, the cells continue to divide, becoming a hollow ball of cells called a blastocyst. The blastocyst implants in the wall of the uterus about 6 days after fertilization.
If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two twins. Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different. The resulting twins are thus fraternal twins. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide. Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same.
Once a month, an egg is released from an ovary into a fallopian tube. After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg. The fertilized egg zygote divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes.
About 6 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick.
The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta. The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously.
The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother. Some of the cells from the placenta develop into an outer layer of membranes chorion around the developing blastocyst. Other cells develop into an inner layer of membranes amnion , which form the amniotic sac. When the sac is formed by about day 10 to 12 , the blastocyst is considered an embryo. The amniotic sac fills with a clear liquid amniotic fluid and expands to envelop the developing embryo, which floats within it.
The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side. This stage is characterized by the formation of most internal organs and external body structures. By the end of the embryonic period, the basic structures of the brain and central nervous system have been established.
At this point, the basic structure of the peripheral nervous system is also defined. The production of neurons, or brain cells, begins around day 42 after conception and is mostly complete sometime around the middle of pregnancy. As neurons form, they migrate to different areas of the brain. Once they have reached the correct location, they begin to form connections with other neural cells, establishing rudimentary neural networks.
Once cell differentiation is mostly complete, the embryo enters the next stage and becomes known as a fetus. The fetal period of prenatal develop marks more important changes in the brain.
This period of development begins during the ninth week and lasts until birth. This stage is marked by amazing change and growth. The early body systems and structures established in the embryonic stage continue to develop. The neural tube develops into the brain and spinal cord and neurons continue to form. Once these neurons have formed, they begin to migrate to their correct locations.
Synapses, or the connections between neurons, also begin to develop. Between the ninth and twelfth week of gestation at the earliest , reflexes begin to emerge. The fetus begins to make reflexive motions with its arms and legs. During the third month of gestation, the sex organs begin to differentiate. By the end of the month, all parts of the body will be formed. At this point, the fetus weighs around three ounces. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the later stages of pregnancy.
The end of the third month also marks the end of the first trimester of pregnancy. During the second trimester, or months four through six, the heartbeat grows stronger and other body systems become further developed.
Fingernails, hair, eyelashes, and toenails form. So what's going on inside the brain during this important period of prenatal development? The brain and central nervous system also become more responsive during the second trimester.
Around 28 weeks, the brain starts to mature faster, with an activity that greatly resembles that of a sleeping newborn. During the period from seven months until birth, the fetus continues to develop, put on weight, and prepare for life outside the womb.
The lungs begin to expand and contract, preparing the muscles for breathing. While development usually follows this normal pattern, there are times when problems with prenatal development occur.
Disease, malnutrition, and other prenatal influences can have a powerful impact on how the brain develops during this critical period. Brain development does not end at birth. A considerable amount of brain development takes place postnatally, including growing in size and volume while changing in structure.
The brain quadruples in size between birth and preschool. As children learn and have new experiences, some networks in the brain are strengthened while other connections are pruned.
Ever wonder what your personality type means? Sign up to find out more in our Healthy Mind newsletter. National Institutes of Health. What are stem cells, and why are they important? Updated Morning sickness: A mechanism for protecting mother and embryo. Q Rev Biol. In: Anatomy and Physiology. This educational content is not medical or diagnostic advice.
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Once sperm meets egg, a rapid series of events will transform the fertilized cell into a quickly growing, soon-to-be baby. Back to Top. In This Article. What is a zygote? Continue Reading Below. Read This Next.
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