The ____ Occurs When the Baby Can First Survive Outside the Womb.

March 11, 2022 Post a Comment

A baby goes through several stages of evolution, beginning as a fertilized egg. The egg develops into a blastocyst, an embryo, and so 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 motion from the vagina, through the cervix into the uterus, and to the funnel-shaped stop of a fallopian tube—the usual site of fertilization. The cells lining the fallopian tube facilitate fertilization.

If fertilization does non 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 downward the fallopian tube to the uterus. The zygote enters the uterus in 3 to 5 days.

In the uterus, the cells keep to separate, 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 i egg is released and fertilized, the pregnancy involves more one fetus, normally two (twins). Considering the genetic material in each egg and in each sperm is slightly unlike, each fertilized egg is different. The resulting twins are thus fraternal twins. Identical twins result when 1 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

In one case a month, an egg is released from an ovary into a fallopian tube. Afterwards sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where ane sperm fertilizes the egg. The fertilized egg (zygote) divides repeatedly as it moves downward the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. And 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 afterward fertilization, the blastocyst attaches to the lining of the uterus, usually near the elevation. This process, called implantation, is completed past mean solar day nine or x.

The wall of the blastocyst is one cell thick except in i surface area, where it is three to four cells thick. The inner cells in the thickened expanse 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 material 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 side by side phase in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side. This phase is characterized by the formation of most internal organs and external body structures. Most organs begin to form well-nigh 3 weeks afterward fertilization, which equals v weeks of pregnancy (because doctors date pregnancy from the first day of the adult female's last menstrual period, which is typically 2 weeks before fertilization). At this fourth dimension, the embryo elongates, first suggesting a man shape. Shortly thereafter, the area that will go the encephalon and spinal string (neural tube) begins to develop. The heart and major blood vessels begin to develop before—by nigh twenty-four hours sixteen. The middle begins to pump fluid through blood vessels by day 20, and the first red blood cells announced the next day. Blood vessels proceed to develop in the embryo and placenta.

Placenta and Embryo at About 8 Weeks

At eight weeks of pregnancy, the placenta and fetus have been developing for six weeks. The placenta forms tiny hairlike projections (villi) that extend into the wall of the uterus. Blood vessels from the embryo, which pass through the umbilical string to the placenta, develop in the villi.

A sparse membrane separates the embryo's blood in the villi from the mother's blood that flows through the space surrounding the villi (intervillous space). This system does the following:

Allows materials to exist exchanged betwixt the claret of the mother and that of the embryo
Prevents the mother'southward immune system from attacking the embryo because the mother's antibodies are too large to laissez passer through the membrane (antibodies are proteins produced by the allowed organization to help defend the body against foreign substances)

The embryo floats in fluid (amniotic fluid), which is contained in a sac (amniotic sac).

The amniotic fluid does the post-obit:

Provides a space in which the embryo tin can grow freely
Helps protect the embryo from injury

The amniotic sac is strong and resilient.

At the end of the eighth week subsequently fertilization (10 weeks of pregnancy), the embryo is considered a fetus. During this phase, 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 almost 14 weeks: The sex can exist identified.
Past about xvi to twenty weeks: Typically, the meaning adult female can feel the fetus moving. Women who have been pregnant before typically feel movements virtually 2 weeks earlier than women who are pregnant for the first time.
By nearly 24 weeks: The fetus has a gamble of survival exterior the uterus.

The lungs go along to mature until near the time of delivery. The brain accumulates new cells throughout pregnancy and the first yr of life afterward 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 organization greatly increases the surface area of contact between the wall of the uterus and the placenta, then that more nutrients and waste matter materials can be exchanged. The placenta is fully formed past 18 to xx weeks but continues to abound throughout pregnancy. At delivery, it weighs about 1 pound.