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Pregnancy
Pregnancy is the process by which a mammalian female carries
a live offspring from conception until it develops to the point
where the offspring is capable of living outside the womb. It
starts from the fertilization of an embryo (conception) to form
a zygote, and ends in birth, miscarriage or abortion.
In humans, pregnancy takes approximately 40 weeks between the
time of the last menstrual cycle and delivery. It is divided into
three trimesters of three months duration each. The first trimester
carries the highest risk of miscarriage, the unintentional abortion
of a fetus. It is often a result of defects in the fetus, its
parent, or damage caused after conception.
Fertilization
The first step of pregnancy usually begins with sexual intercourse
where male gametes or sperm are deposited into the vagina. The
semen produced by the male contains not only sperm cells but also
sugars, proteins and other substances to help keep the sperm viable.
Human sperm generally survive for about 48 hours in the female
body. Sperm have a long flagellum, which they use to swim; they
are the only human cells with this property. These cells are haploid,
having divided by meiosis from germ cells in the testes and possessing
only one half of the chromosomes of ordinary body cells. Typically,
between 100 million and 300 million sperm are released in one
ejaculation.
Ova, or oocytes, are the haploid female egg cells, and their
role is to fuse with one sperm cell to form a fertilized zygote
which will then grow in the uterus to form a developing fetus.
These cells are produced by meiosis in the ovaries and stay in
a state of suspended animation until activated by hormonal changes
in the woman's menstrual cycle. Typically, only one ovum is released
during each menstrual cycle.
At ovulation, the fimbriae at the end of the fallopian tube move
over the ovary to catch the released ovum. If fertilization takes
place, the sperm usually meet the ovum in the fallopian tube,
requiring the sperm cells to swim from the upper vagina through
the cervix and across the length of the uterus before reaching
the fallopian tube a considerable distance compared to the size
of the sperm cell. Once there, the successful sperm swim towards
the ovum and each attempts to fertilize it. Each sperm cell contains
a small pouch of enzymes that it uses to break through the outer
layer of the ovum in order to fuse with it. This can take up to
twenty minutes. Once the ovum fuses with a single sperm cell,
its cell membrane changes, preventing fusion with other sperm.
The fusion of the ovum's and sperm's nuclei to form a diploid
cell completes the first stage of pregnancy.
Alternative methods of fertilization, including artificial insemination
and in vitro fertilization, are sometimes used in cases of infertility
and by single women and lesbian couples.
Development
Pre-implantation
At this point, there exists a single totipotent cell with the
potential to create an entire human being. Mitotic cell division
is the next process to occur, where each cell doubles to produce
another diploid cell. The zygote divides to produce two smaller
cells, called blastomeres, roughly every 20 hours. These cells
get progressively smaller until about 4 divisions have taken place
resulting in 16 individual cells. This cluster of 16 cells, called
a morula, leaves the fallopian tube and makes its way to the uterus.
Ectopic pregnancy
Occasionally, the developing zygote will implant somewhere other
than the uterus; this is called ectopic pregnancy. The most common
location for ectopic pregnancies is in a fallopian tube, but they
can arise in any tissue that the fertilized zygote can reach (essentially
anywhere within the abdomen). Since only the uterus is suited
to the development of the embryo, ectopic pregnancy usually results
in complications when the developing embryo grows too big for
the blood supply of the ectopic location to sustain it any more.
When this happens, the ectopic pregnancy is at risk of rupture,
potentially resulting in serious blood loss. An ectopic pregnancy
can thus be a life-threatening condition.
Post-implantation
A blastocele is a small cavity on the center of the cells, and
the developing cells will grow around this. There will be a flat
layer of cells on the exterior of this cavity, and the zona pellucida
will remain the same size as before. Cells are growing increasingly
smaller to fit in. This new structure with a cavity in the center
and the developing cells around it is known as a blastocyst. The
presence of the blastocyst means that two types of cells are forming,
inner cell mass growing on the interior of the blastocele and
cells growing on the exterior of it. In 24 to 48 hours, the blastocyst's
barrier, the zona pellucida breaches. The cells on the exterior
of the blastocyst begin excreting an enzyme which erodes epithelial
uterine lining and creates a site for implantation. The blastocyst
also secretes a hormone called human chorionic gonadotropin which
in turn, stimulates the corpus luteum in the mother's ovary to
continue producing progesterone, which acts to maintain the lining
of the uterus so that the embryo will continue to be nourished.
The glands in the lining of the uterus will swell in response
to the blastocyst, and capillaries will be stimulated to grow
in that region. This allows the blastocyst to receive vital nutrients
from the mother. Pregnancy tests detect the presence of human
chorionic gonadotropin.
Placental Circulation System
The cells surrounding the blastocyst now destroy cells in the
uterine lining, forming small pools of blood which in turn stimulates
the production of capillaries. This is the first stage in the
growth of the placenta. The inner cell mass of the blastocyst
divides rapidly, forming two layers. The top layer will become
the embryo and cells from there will be used in the amniotic cavity.
At the same time, the bottom layer will form a small sac. (If
the cells begin developing in an abnormal position, an ectopic
pregnancy may also occur at this point.) Several days later, chorionic
villi in the forming placenta anchor the implantation site to
the uterus. A system of blood and blood vessels now develops at
the point of the newly forming placenta, growing near the implantation
site. The small sac inside the blastocyst begins producing red
blood cells. For the next 24 hours, connective tissue will develop
between the developing placenta and the growing fetus. This will
later develop into the umbilical cord.
Cellular Differentiation
Following this, a narrow line of cells appears on the surface
on the embryo. Its growth shows the fetus will undergo gastrulation,
in which the three layers of the fetus, the ectoderm, mesoderm
and endoderm, will develop. The narrow line of cells begin to
form the endoderm and mesoderm. The ectoderm begins to grow rapidly
as a result of chemicals being produced by the mesoderm. These
three layers will give rise to all the various types of tissue
in the body. The endoderm will later form the lining of the tongue,
digestive tract, lungs, bladder and several glands. The mesoderm
will form muscle, bone and lymph tissue, as well as the interior
of the lungs, heart, reproductive and excretory systems. It will
also give rise to the spleen, and will be used in the production
of blood cells. The ectoderm will form the skin, nails, hair,
cornea, lining of the internal and external ear, nose, sinuses,
mouth, anus, teeth, pituitary gland, mammary glands, eyes and
all parts of the nervous system.
Approximately 18 days after fertilization, the embryo has divided
to form much of the tissue it will need. It is shaped like a pear,
where the head region is larger than the tail. The embryo's nervous
system is one of the first organs to grow. It begins growing in
a concave area known as the neural groove. The blood system continues
to grow networks which allow the blood to flow around the embryo.
Blood cells are already being produced and are flowing through
these developing networks. Secondary blood vessels also begin
to develop around the placenta, to supply it with more nutrients.
Blood cells will begin to form on the sac in the center of the
embryo, as well as cells which will begin to differentiate into
blood vessels. Endocardial cells begin to form the musculature
which will become the heart.
At about 24 days past fertilization, there is a primitive S-shaped
tubule heart which begins beating. The flow of fluids throughout
the embryo will begin at this stage.
Morning sickness
Morning sickness (also called "nausea and vomiting of pregnancy,"
or "NVP") affects between 50 and 85 percent of all pregnant
women. It is not confined to the morning: nausea can occur at
any time of the day. The cause is not known: some of the theories
are:-
- An increase in the hormone progesterone relaxes the muscles
in the uterus, which prevents early pregnancy labor, but may
also relax the stomach and intestines, leading to excess stomach
acids.
- An increase in hCG (human chorionic gonadotropin).
- An increase in sensitivity to odors.
- Eating vegetables. One theory is that the small amounts of
toxins vegetables produce to deter insect infestation are normally
harmless to humans but extremely dangerous to embryos; therefore,
becoming nauseous during pregnancy was an evolutionary measure
to protect the embryo. Other studies however have linked consumption
of fruits and vegetables to higher birth weights (higher birth
weights tend to mean healthier babies).
For 50% of all sufferers, it ends by the 16th week of pregnancy.
Studies have shown that women who suffer from NVP are less likely
to have miscarriages.
Trimester
The human gestation period of approximately 40 weeks between
the time of the last menstrual cycle and delivery is traditionally
divided into three periods of three months, or trimesters.
- In the first trimester the embryo undergoes most of its early
structural development. Most miscarriages occur during this
period.
- In the second trimester the embryo, now known as a fetus,
is recognisable as human in form, but is not developed enough
to be viable if born.
- In the third trimester the fetus reaches viability, and may
survive if born prematurely.
The use of modern medical intensive care technology has greatly
increased the probability of premature babies living, and has
pushed back the boundary of viability to much earlier dates than
would be possible without intensive medical assistance. In spite
of these developments, premature birth remains a major threat
to the fetus, and may result in ill-health in later life, even
if the fetus survives the birth and subsequent intensive care.
The actual boundaries of when an embryo is regarded as a fetus
or a fetus becomes regarded as potentially viable depend on the
definitions of these terms, and do not necessarily fit neatly
on the classic trimester boundaries. Note also that these boundaries
are the matter of both medical and political controversy.
Some timings that have been used are:
embryo to fetus: eight weeks
Nevertheless, the concept of pregnancy as divided into three
trimesters continues to be used in practical healthcare.
Teenage pregnancy / Teen pregnancy
Teenage pregnancy, the phenomenon of teenage girls getting pregnant,
is a contemporary social issue in some nations, especially in
the United States. The perceived problem with teenage pregnancies
is that teenagers are not ready, emotionally and financially,
to raise children, even if they are physiologically able to. Teenage
girls are generally enrolled in high school or middle school and
usually still depend on their parents. In many cases, the father
of the baby is of similar age and equally unprepared to raise
a child. In such instances, abortion is often considered, or the
child is raised by a single mother.
Scientists have long argued that sex education (about contraception
and safe sexual behavior) would effectively reduce the number
of teenage pregnancies, and countries that do use progressive
sex education at a young age, such as the Netherlands, have a
much lower rate of teenage pregnancy than the United States. Conservatives
in the US, on the other hand, believe that sexual abstinence is
the only safe way to reduce teenage pregnancies, even as studies
that have examined such programs have shown no effectiveness at
preventing pregnancies or even revealed a statistical increase
in some programs. The religious right in particular feels that
giving teenagers sexual information would lead to an increase
in sexual behavior and that this is undesirable, and that juveniles
have therefore to be shielded from such information and instead
be informed strictly about the possible negative effects of sexual
behavior, such as sexually transmitted diseases. The George W.
Bush administration has extensively funded abstinence programs
in deference to this point of view.
Source:
information here is licensed under the GNU
Free Documentation License. It uses material from the Wikipedia
article Pregnancy.
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