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A baby girl is born with her entire stocks of eggs, or ova, in place.
There are approximately 2 million ova present at birth, and they
are either released or degenerate over her fertile life.
The ovarian cycle is a woman's reproductive cycle, lasting about
28 days, or one lunar month. At about 14 days before the end of
the cycle, an ovum (or egg) is released from one of the ovaries.
Women have two egg-releasing organs, called ovaries, which release
eggs alternately every month. This egg travels down the fallopian
tubes into the uterus and begins to grow, releasing progesterone
into its environment. It is at this stage that it is fertilisable,
although it is not clear for how long. If the egg is not fertilised,
it is shed with the lining of the uterus as the period begins.
Fertilisation occurs when a sperm fuses with the ovum. This takes
place in the outer part of the fallopian tubes, generally within
one day of ovulation. Only one sperm, out of the millions that are
expelled, will actually penetrate the membrane covering the ovum.
Once this occurs the surface of the zygote (the fertilised egg)
changes, making it impossible to be penetrated by any other sperm.
Within just a few hours of fertilisation the zygote will begin dividing
itself and by the time it reaches the uterus, four days later, it
has become a solid mass of hundreds of cells, called a morula. As
time goes on, these cells continue to divide into a hollow mass
of cells. At this time the cells are called a blastocyst.
Around six days after ovulation the blastocyst will release a hormone
to help it bury itself into the lining of the uterus. The outer
cells will begin joining with the small blood vessels in the uterus
to form the placenta (afterbirth). Meanwhile, the inner cells begin
to develop to form the embryo and the adjoining membranes. Once
the blastocyst is completely imbedded, it begins to develop into
an embryo. This is the state that the baby will stay in until he
or she is born.
Once the embryo is fully imbedded, another set or hormones are released
to trigger a number of changes. These changes occur to make the
woman's body suitable for carrying the embryo safely.
There are two ways that multiple pregnancies can come about. The
first is that more than one egg is produced at one time. These eggs,
if fertilised by different sperm, have their own placenta and develop
side by side within the uterus. These babies will not be identical,
as they have been formed from different eggs.
The second way that a multiple birth can occur is when a single,
egg, fertilised by one sperm, divides and develops into two embryos.
These babies share the same placenta, and will be identical at birth.
Triplets or larger numbers of multiple embryos are formed in one
of these two ways, and will either be identical or not, depending
on the form of conception.
The X and Y chromosomes, just 2 of the 46 that make up a baby determine
its gender. The eggs that are released every month carry only X
chromosomes, while sperm carry both X and Y chromosomes. An egg
fertilised by an X chromosome carrying sperm will become a girl
(XX) baby, while an egg fertilised by a Y chromosome will become
a boy baby (XY).
This is the beginning of a long and complex period of growth, turning
a primitive ball of cells into a fully developed human.
The embedded embryo develops a protective layer around him/her.
This forms the basis of the placenta, the 'organ' that produces
hormones to support the embryo and mother during pregnancy, along
with the rest of the embryo's life support system that is made up
of three separate sections. The first is the amniotic sac, which
is a bag full of fluid in which the embryo will float; the second
is the chorion, which is a protective layer around the amniotic
sac, and the third is the yolk sac, which will support parts of
the baby's development until it's organs are able to function properly.
These layers are vital for a baby's growth, and support him/her
through every stage of development.
The cells that make up the embryo begin to prepare themselves form
specific parts of the body, both internal and external.
Appearance: The being that was a cluster of cells has now become
prawn-shaped, and is about 4 millimetres long, and weighs less than
a gram. It has a tiny, beating heart.
The cells that make up the embryo are moving to their correct places
and rapidly multiplying to form a more recognisable baby-shaped
being. Developments are beginning to take place, hair and skin is
beginning to grow, and the feet and legs are beginning to grow,
with the arms and hands growing at a faster rate. The head is growing
to contain the brain, which is developing at the fastest rate of
all. This means that the head looks out of proportion to the rest
of the body. The internal organs are beginning to form, and the
heart, still tiny, beats very fast, at up to twice the rate of the
mother's. The embryo can now respond to touch.
Appearance: The embryo is now 2.5 centimetres long, and weighs 3
grams. The head is larger than the rest of the body, and looks a
little out of proportion.
The baby is now fully formed in the womb, and is now called a foetus.
The head is still large in proportion to the rest of his/her body,
and it holds a fully formed face with complete eyes that are not
yet open. The details of the body are developing; the finger and
toe nails are in place and the genitals are beginning to grow, meaning
that the gender of the baby is determinable from an ultrasound scan.
Reflexes are also beginning to develop; the sucking, breathing and
swallowing mechanisms are being rehearsed for life outside the womb.
The umbilical cord is now fully mature, and the flow of nutrients
and oxygenated blood from mother to baby, and the flow of waste
products from baby to mother are now fully functioning.
The baby is also able to make quite strong movements, although the
mother will not feel these until a little later in the pregnancy.
Appearance: He/she is growing quickly. It is now 9 centimetres long
and weighs approximately 48 grams.
The foetus is now starting to take on a more human form. His/her
legs are now longer and in proportion with the rest of the body,
and the head is also coming into line. The brain is capable of passing
and receiving messages, but does not yet control the movements made
by the now fully formed limbs.
Some finer movements are also possible; his/her hands are now capable
of grasping, and facial expressions are beginning to occur. The
eyes are beginning to take on more detail, the eyebrows and lashes
are growing, and the insides of the eyes are sensitive to light.
Appearance: The foetus is 13.5 centimetres long, and weighs 180
grams. The head still appears a little larger than the rest of the
body.
There are some more detailed, internal changes taking place at this
stage. The spinal cord has a protective layer around it, which is
in place to prevent damage. The baby also has a primitive immune
system in place that can defend against some infections. His/her
skin is developing fast, and has a waxy secretion all over it to
protect it from the fluid in the amniotic sac. The skin also has
fine hairs all over it, called lanugo.
The other main development at this stage is the addition of more
functioning senses; the baby can now distinguish between bitter
and sweet tastes, and can detect things touching his/her skin. His/her
hearing is also more defined. The sounds of the uterus are becoming
familiar, and he/she will respond to noises from outside the womb,
and will often be especially receptive to rhythm and melody.
Appearance: There is not a great deal of difference in the size
of the baby in this month; he/she is about 18.5 centimetres long
and weighs about half a kilogram.
By this stage, the body is in proportion with itself, the head being
of a regular size. His/her limbs are fully developed with the muscles
and bones ready for the world outside. The sense of hearing is becoming
sensitive to high frequency sounds, and his/her body is able to
react to sounds around it.
The lungs are developing in more and more detail, and breathing
is practiced so the reflex is well rehearsed for the time that air
is passed through them.
By this point in the pregnancy, his/her brain is as developed as
it will be at birth, and a sleep/wake routine is developed for the
time in the womb.
Appearance: Growth has slowed again; the foetus is about 25 centimetres
in length, but his/her weight has doubled to 1 kilogram.
The baby's brain grows larger in this month, and the cells and fibres
become fully functioning. These changes in the brain spark preparations
for birth. Fat grows under the skin to prepare for the colder temperature
to come, his/her eyes open and the fine hair over the body recedes.
The swallowing and sucking reflexes already in place are further
rehearsed in preparation for life outside the womb, as is the breathing.
He/she is also now able to regulate his/her body temperature.
The other main change that takes place in this month is that the
baby learns to orientate him/herself in the womb. This is in preparation
for the time when he/she needs to position him/herself in the right
place for labour to begin.
Appearance: The baby is now 28 centimetres long, and weighs approximately
1.5 kilograms.
Strong movements are now possible form fully coordinated limbs,
but these movements are becoming more constricted, as there is less
space in the womb. The mother will be able to feel these movements
clearly. All organs except the lungs are fully developed, and although
not totally ready, the baby would be able to breathe outside of
the womb if necessary.
His/her eyes are reactive to the light coming from the walls of
the uterus, and he/she is able to blink and focus. The fingernails
are the full length, and there may be hair on his/her head.
Appearance: The baby is now about 32 centimetres long, about the
same size as at birth, and he/she weighs about 2.5 kilograms.
It is in this month that the finer details are prepared for birth.
The skin becomes smoother, and the eyes are now fully functional.
All babies are born with blue eyes, but the colour may change a
few weeks after birth. The reproductive organs are now fully formed,
and virtually in position.
A great deal of the debris from gestation ends up in the intestine.
It is called meconium, and is a darkly coloured substance that is
passed by the baby either during or after birth.
The placenta is still fully active at this time, and the hormones
being produced are still causing reactions in mother and baby. One
hormone causes not only the mother's breasts to produce milk, but
also causes a swelling of the breasts of the baby, whether male
or female. Once this hormone stops affecting a female baby, she
may bleed lightly, in a similar way to a period, a few days after
birth.
Usually at this time the head 'engages' getting into place in the
pelvis to prepare for birth.
At the end of this period the baby is ready for birth. He/she should
have everything that is required for life outside the womb, and
should be able to breathe independently.
Appearance: The length of the baby has not changed a great deal,
he/she will be about 35-37 centimetres long, and will weigh anything
from 3 kilograms upwards.
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