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Traditionally, doctors have measured pregnancy from a number of
convenient points, including the day of last menstruation, ovulation,
fertilization, implantation and chemical detection. In medicine,
pregnancy is often defined as beginning when the developing embryo
becomes implanted into the endometrial lining of a woman's uterus. In
some cases where complications may have arisen, the fertilized egg might
implant itself in the fallopian tubes, the cervix, the ovary or in the
abdomen, causing an ectopic pregnancy. In the case of an ectopic
pregnancy, there is no way for the pregnancy to progress normally. If
left untreated, it can cause harm and possibly death for the mother when
a rupture occurs. Sometimes it will go away on its own, but otherwise a
surgical procedure or medicine is given to remove the tubal pregnancy,
since there is no way of the pregnancy being able to continue safely.
Most pregnant women do not have any specific signs or symptoms of
implantation, although it is not uncommon to experience minimal bleeding
at implantation. Some women will also experience cramping during their
first trimester. This is usually of no concern, unless there is spotting
or bleeding as well. After implantation, the uterine endometrium is
called the decidua. The placenta, which is formed partly from the
decidua and partly from outer layers of the embryo, connects the
developing fetus to the uterine wall to allow nutrient uptake, waste
elimination, and gas exchange via the mother's blood supply. The
umbilical cord is the connecting cord from the embryo or fetus to the
placenta. The developing embryo undergoes tremendous growth and changes
during the process of fetal development.
Morning sickness occurs in about seventy percent of all pregnant women,
and typically improves after the first trimester. Although described as
"morning sickness", women can experience this nausea during afternoon,
evening, and throughout the entire day.
In the first 12 weeks of pregnancy, the nipples and areolas darken due
to a temporary increase in hormones.
The first 12 weeks of pregnancy are considered to make up the first
trimester. The first two weeks from the first trimester are calculated
as the first two weeks of pregnancy even though the pregnancy does not
actually exist. These two weeks are the two weeks before conception and
include the woman's last period.
The third week is the week in which fertilization occurs and the 4th
week is the period when implantation takes place. In the 4th week, the
fecundated egg reaches the uterus and burrows into its wall which
provides it with the nutrients it needs. At this point, the zygote
becomes a blastocyst and the placenta starts to form. Moreover, most of
the pregnancy tests may detect a pregnancy beginning with this week.
The 5th week marks the start of the embryonic period. This is when the
baby's brain, spinal cord, heart and other organs begin to form. At this
point the embryo is made up of three layers, of which the top one
(called the ectoderm) will give rise to the baby's outermost layer of
skin, central and peripheral nervous systems, eyes, inner ear, and many
connective tissues. The heart and the beginning of the circulatory
system as well as the bones, muscles and kidneys are made up from the
mesoderm (the middle layer). The inner layer of the embryo will serve as
the starting point for the development of the baby's lungs, intestine
and bladder. This layer is referred to as the endoderm. A baby at 5
weeks is normally between 1⁄16 and 1⁄8 inch (1.6 and 3.2 mm) in length.
In the 6th week, the baby will be developing basic facial features and
its arms and legs start to grow. At this point, the embryo is usually no
longer than 1⁄6 to 1⁄4 inch (4.2 to 6.3 mm). In the following week, the
brain, face and arms and legs quickly develop. In the 8th week, the baby
starts moving and in the next 3 weeks, the baby's toes, neck and
genitals develop as well. According to the American Pregnancy
Association, by the end of the first trimester, the fetus will be about
3 inches (76 mm) long and will weigh approximately 1 ounce (28 g).
Weeks 13 to 28 of the pregnancy are called the second trimester. Most
women feel more energized in this period, and begin to put on weight as
the symptoms of morning sickness subside and eventually fade away.
In the 20th week, the uterus, the muscular organ that holds the
developing fetus, can expand up to 20 times its normal size during
pregnancy. Although the fetus begins to move and takes a recognizable
human shape during the first trimester, it is not until the second
trimester that movement of the fetus, often referred to as "quickening",
can be felt. This typically happens in the fourth month, more
specifically in the 20th to 21st week, or by the 19th week if the woman
has been pregnant before. However, it is not uncommon for some women not
to feel the fetus move until much later. The placenta fully functions at
this time and the fetus makes insulin and urinates. The reproductive
organs distinguish the fetus as male or female.
Final weight gain takes place, which is the most weight gain throughout
the pregnancy. The fetus will be growing the most rapidly during this
stage, gaining up to 28 g per day. The woman's belly will transform in
shape as the belly drops due to the fetus turning in a downward position
ready for birth. During the second trimester, the woman's belly would
have been very upright, whereas in the third trimester it will drop down
quite low, and the woman will be able to lift her belly up and down. The
fetus begins to move regularly, and is felt by the woman. Fetal movement
can become quite strong and be disruptive to the woman. The woman's
navel will sometimes become convex, "popping" out, due to her expanding
abdomen. This period of her pregnancy can be uncomfortable, causing
symptoms like weak bladder control and backache. Movement of the fetus
becomes stronger and more frequent and via improved brain, eye, and
muscle function the fetus is prepared for ex utero viability. The woman
can feel the fetus "rolling" and it may cause pain or discomfort when it
is near the woman's ribs and spine.
There is head engagement in the third trimester, that is, the fetal head
descends into the pelvic cavity so that only a small part (or none) of
it can be felt abdominally. The perenium and cervix are further
flattened and the head may be felt vaginally. Head engagement is known
colloquially as the baby drop, and in natural medicine as the lightening
because of the release of pressure on the upper abdomen and renewed ease
in breathing. However, it severely reduces bladder capacity, increases
pressure on the pelvic floor and the rectum, and the mother may
experience the perpetual sensation that the fetus will "fall out" at any
It is during this time that a baby born prematurely may survive. The use
of modern medical intensive care technology has greatly increased the
probability of premature babies surviving, and has pushed back the
boundary of viability to much earlier dates than would be possible
without 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 baby survives.
Embryonic and fetal development
Prenatal development is divided into two primary biological stages. The
first is the embryonic stage, which lasts for about two months. At this
point, the fetal stage begins. At the beginning of the fetal stage, the
risk of miscarriage decreases sharply, all major structures including
the head, brain, hands, feet, and other organs are present, and they
continue to grow and develop. When the fetal stage commences, a fetus is
typically about 30 mm (1.2 inches) in length, and the heart can be seen
beating via sonograph; the fetus bends the head, and also makes general
movements and startles that involve the whole body. Some fingerprint
formation occurs from the beginning of the fetal stage.
Electrical brain activity is first detected between the 5th and 6th week
of gestation, though this is still considered primitive neural activity
rather than the beginning of conscious thought, something that develops
much later in fetation. Synapses begin forming at 17 weeks, and at about
week 28 begin to multiply at a rapid pace which continues until 3–4
months after birth. It is not until week 23 that the fetus can survive,
albeit with major medical support, outside of the womb, because it does
not possess a sustainable human brain until that time.
One way to observe prenatal development is via ultrasound images. Modern
3D ultrasound images provide greater detail for prenatal diagnosis than
the older 2D ultrasound technology. While 3D is popular with parents
desiring a prenatal photograph as a keepsake, both 2D and 3D are
discouraged by the FDA for non-medical use, but there are no definitive
studies linking ultrasound to any adverse medical effects.
Some people are confused about the differences between an ultrasound and
a sonogram. An ultrasound is the actual machine that lets you observe
pregnancy. A sonogram is the image of the baby that the ultrasound
produces. 4D Ultrasounds take 3D sonograms. Some people refer to the
procedure as prenatal imaging, 3D imaging, a 3D scan, or 4D scan.
During pregnancy, the woman undergoes many physiological changes, which
are entirely normal, including cardiovascular, hematologic, metabolic,
renal and respiratory changes that become very important in the event of
complications. The body must change its physiological and homeostatic
mechanisms in pregnancy to ensure the fetus is provided for. Increases
in blood sugar, breathing and cardiac output are all required. Levels of
progesterone and oestrogens rise continually throughout pregnancy,
suppressing the hypothalamic axis and subsequently the menstrual cycle.
The woman and the placenta also produce many hormones.