The correct sequence of prenatal development is

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A baby goes through several stages of development, beginning as a fertilized egg. The egg develops into a blastocyst, an embryo, then a fetus.

At ovulation, the mucus in the cervix becomes more fluid and more elastic, allowing sperm to enter the uterus rapidly. Within 5 minutes, sperm may move from the vagina, through the cervix into the uterus, and to the funnel-shaped end of a fallopian tube—the usual site of fertilization. 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.

From Egg to Embryo

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 10.

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. Most organs begin to form about 3 weeks after fertilization, which equals 5 weeks of pregnancy (because doctors date pregnancy from the first day of the woman's last menstrual period, which is typically 2 weeks before fertilization). At this time, the embryo elongates, first suggesting a human shape. Shortly thereafter, the area that will become the brain and spinal cord (neural tube) begins to develop. The heart and major blood vessels begin to develop earlier—by about day 16. The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day. Blood vessels continue to develop in the embryo and placenta.

Placenta and Embryo at About 8 Weeks

At the end of the 8th week after fertilization (10 weeks of pregnancy), the embryo is considered a fetus. During this stage, the structures that have already formed grow and develop. The following are markers during pregnancy:

• By 12 weeks of pregnancy: The fetus fills the entire uterus.

• By about 14 weeks: The sex can be identified.

• By about 16 to 20 weeks: Typically, the pregnant woman can feel the fetus moving. Women who have been pregnant before typically feel movements about 2 weeks earlier than women who are pregnant for the first time.

• By about 24 weeks: The fetus has a chance of survival outside the uterus.

The lungs continue to mature until near the time of delivery. The brain accumulates new cells throughout pregnancy and the first year of life after birth.

As the placenta develops, it extends tiny hairlike projections (villi) into the wall of the uterus. The projections branch and rebranch in a complicated treelike arrangement. This arrangement greatly increases the area of contact between the wall of the uterus and the placenta, so that more nutrients and waste materials can be exchanged. The placenta is fully formed by 18 to 20 weeks but continues to grow throughout pregnancy. At delivery, it weighs about 1 pound.

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