1st Trimester
Pregnancy is broken down into weeks, and is divided
into three parts called trimesters.
Full term pregnancy is considered 40
weeks and an infant delivered before the 37th week completed is considered
premature. Pregnancy is counted from the first day of a your last period. This
means that at conception, you are already considered two weeks
pregnant.
The first, second, and third
months are the first trimester of pregnancy.
2 weeks after the first day of the last normal
menstrual period - The sperm and egg join in the fallopian tube and
fertilization has occurred. A single sperm penetrates the mother's egg cell, and
the resulting cell is called a zygote. The zygote contains all of the genetic information
(DNA) necessary to become a child. The baby begins to form from this single
fertilized egg, the cell starts to divide itself into 2 cells, then 4, 8 and so
on rapidly. Forty-six chromosomes combine, which pre-determine all of a person's
physical characteristics. The fertilized egg is still rapidly dividing as it
floats down from the fallopian tube and towards the uterus. The embryo first
moves into the uterus at about 80 hours after ovulation. The implantation
process begins about 3 days later.
3 weeks - Now the developing embryo is in the
uterus, it searches for a nice place to implant. When one is found, the zygote burrows beneath the
surface of the uterus. At this stage the fertilized egg is now called a
blastocyst, it is a fluid filled cluster of 50 to 60 cells, still multiplying
madly. Implantation of the blastocyst occurs at about day 5 to day 8 of embryo
development. IVF embryos are usually transferred into the uterus around
this stage. The developing embryo is only one-hundredth of an inch long, but is
rapidly developing. It can reproduce itself through twinning at any time up to
about 14 days after conception; this is how identical twins are formed.
4 weeks - At week four, you might begin to
suspect you are pregnant since the embryo produces hormones which stop the mother's menstrual
cycle. The cluster of cells has attached to the uterus wall and has divided into
two parts. The half attached to the uterine wall will become the placenta, the
vessel filled support system that will nourish the developing life, and the
other half will become the baby. The amniotic fluid that cushions the fetus begins to
form. The cells of the embryo multiply and begin to take on specific functions.
The backbone, spinal column, and nervous system are forming. The heart, kidneys,
liver, and intestines will soon be taking shape. Nerve growth begins when a
sheet of cells on the back of the embryo folds in the middle to form a tube,
which will become the future spinal cord. At one end the tube enlarges to form
the brain's major sections.
5 weeks - On the 26th day after
fertilization, the embryo's tiny heart begins to beat. At this time the embryo
is still size of a raisin. There is rapid growth, and the baby's main external
features begin to take form. Low on the sides of the head are two folds of
tissue that will become the ears. Although not completely developed, all the
major body organs and systems are formed. The neural tube enlarges into three
parts, that will soon to develop to become a very complex brain. Also the
placenta begins functioning, known as the chorionic villi and the umbilical
cord, through which the baby will receive nourishment and oxygen. The spine and
spinal cord is growing faster than the rest of the body at this stage and will
give the appearance of a tail. This will disappear as the baby grows.
6 weeks - The embryo itself is about a 1/4 inch (6mm) long.
Major organs, including the kidneys and liver, have begun to develop, and the
neural tube, which connects the brain and spinal cord closes. The brain develops
into five areas and some cranial nerves are visible. The arm and leg buds are
visible. The physical sensations of pregnancy start -- nausea, fatigue, sore
breasts and frequent urination.
7 weeks -The embryo's hands and feet are shaped like paddles,
but the fingers are beginning to take shape. The embryonic tail has now almost
disappeared. The pituitary gland is also forming and the fetus is beginning to
grow muscle fibers. The heart has divided into the right and left chambers and
is beating about 150 beats a minute which is about twice the rate of an adult.
The baby's facial features are visible, including a mouth and tongue. The eyes
have a retina and lens. The major muscle system is developed, and the baby
starts to practice moving. The baby has its own blood type and the blood cells
are produced by the liver.
8 weeks - The unborn baby is now called a fetus. The fetus is
protected by the amniotic sac and filled with fluid. Inside the child swims and
moves gracefully. The fetus is now about 1/2 inch (12mm) long. The arms and legs
have lengthened. During this time of development, the baby's head appears much
larger than the body because the brain is growing very rapidly. Brain waves can
now be measured. The teeth and the palate are beginning to form and the larynx
is developing. Through its parchment-thin skin, the baby's veins are clearly
visible. By the eighth week the ears begin to take shape.
9 weeks - Though the fetus is constantly moving, you won't be
able to actually feel fetal movement for several more weeks. All of the organs,
muscles, and nerves are in place and beginning to function. The eyelids have
fused shut and won't open again until week 27. As the hands and feet develop fingers and toes, they
lose their paddle-like look, and the touch pads on the fingers form.
10 weeks - The fetus weighs about 1/3 of an
ounce. The heart is almost completely developed and very much resembles that of
a newborn baby. An opening the atrium of the heart and the presence of a bypass
valve divert much of the blood away from the lungs, as the child's blood is
oxygenated through the placenta. The wrists and ankles have formed and the
fingers and toes are clearly visible. Genitals have begun to from, but it is too early
to tell the sex of the fetus. By this week of the pregnancy the placenta has
developed enough to support most of the critical job of producing hormones.
11 weeks - The vital organs - the liver, kidneys, intestines,
brain and lungs are almost fully formed and beginning to function, while the the
head is almost half the length of the entire body. The fetus is only about 2
inches (50mm) long and weighs less than a half ounce but it is busy moving and
kicking.
12 weeks - Vocal chords are complete, and the
baby can and does sometimes cry silently. The brain is fully formed, and the
baby can also feel pain. The fetus may even suck his thumb. The eyelids now
cover the eyes, and will remain shut until the seventh month to protect the
delicate optical nerve fibers. The hair is on the head and the fingers and toes
have developed soft nails. The kidneys are developed and begin to secrete
urine.
Week 12 and the third month is the end of the first
trimester.