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Family
Doctor Books |
Preview of Understanding Pregnancy
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Fertilisation and development of the fetus and
placenta |
The menstrual cycle
The average length of the menstrual cycle is 28 days
and the first day of the menstrual period is referred to as day 1 of
the cycle. Release of an egg from the ovary (ovulation) normally takes
place around day 14 and the interval between ovulation and the onset
of menstruation is usually fixed at 14 days. During the menstrual cycle
there are various hormones at work and these are necessary to cause
the ovary to release an egg (oocyte) and to prepare the lining of the
uterus (endometrium) for a developing embryo. Several hormones are
involved in this process as follows.
Follicle-stimulating hormone
Follicle-stimulating hormone (FSH) is produced from
the pituitary gland, which lies at the base of the brain. Each month
it stimulates the growth and ripening of between 5 and 12 oocytes within
their follicles (fluid-filled swellings in the ovaries). Peak secretion
of FSH coincides with an oocyte reaching maturity. FSH also aids the
release of the oocyte into the fallopian tube. The amount secreted then
falls back to the baseline level. Each ovary has around 20,000 oocytes
at puberty but fewer than 12 are stimulated at the beginning of each
menstrual cycle. Of these only one usually develops into a mature follicle
which releases an oocyte at ovulation in the middle of the menstrual
cycle.
Each
month from puberty to the menopause a women’s body goes
through the menstrual cycle. The cycle is controlled by the interaction
of four hormones, affecting the ovaries and release of mature
eggs and the wall of the uterus (endometrium). |
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Luteinising hormone
Luteinising hormone (LH), like FSH, is produced by
the pituitary gland. There is a surge in LH blood levels just before
ovulation and this results in the final maturation and release of the
oocyte from the follicle. LH then acts on the cells within the follicle
to convert it to the corpus luteum, the cavity left when the follicle
has ruptured. The corpus luteum secretes the hormones oestrogen and progesterone,
preparing the lining of the uterus to receive the newly fertilised ovum.
Oestrogen
This hormone is produced by the cells in the follicle
and corpus luteum and brings about the changes that occur in the endometrium.
Its level rises and falls in the first half of the cycle and reaches
a second, smaller peak again in the second half. During the first half,
it causes the endometrium to thicken and, in the second half, along with
progesterone, it maintains the endometrium in its spongy, secretory phase
in which glands in the endometrium enlarge and start to secrete nutrients
for the possible embryo.
Progesterone
This hormone is produced in the second half of the
menstrual cycle by the corpus luteum and brings about the development
of the secretory endometrium, so that it is ready to receive and nourish
a fertilised oocyte. The blood level of this hormone can be measured
on day 21 of the cycle to confirm that ovulation has occurred.
If fertilisation
fails to occur, the corpus luteum degenerates. Consequently, oestrogen and
progesterone production decreases and the endometrium is no longer supported.
Menstruation then occurs with the shedding of the endometrium.
Conception and the first journey of life
An oocyte is normally fertilised by a single, one-celled
sperm 12 to 24 hours after it is released. Oocytes and sperm each have
23 chromosomes, and when a sperm penetrates an oocyte its chromosomes
can move freely. The chromosomes from the sperm and the oocyte then pair
up to make cells of 46 chromosomes. Conception has occurred.
After about
36 hours the fertilised oocyte divides into two cells and by three days it
has multiplied to 12 to 16 cells and is known as a morula (mulberry). The morula
is wafted down through the fallopian tube and on day 7 enters the cavity of
the uterus. Its development continues and some fluid appears inside it, making
it look like a cyst. It is now called a blastocyst and it implants into the
endometrium (the lining of the wall of the uterus) on day 9.
Of about
100 million sperm that are ejaculated, only around 500 to 1,000
remain by the time they reach the egg at the outer end of the
fallopian tube. |
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Under the influence of progesterone and oestrogen secreted
from the corpus luteum, the glands and blood vessels in the endometrium
have developed so that it has become spongy and ready to receive the
blastocyst, which by then is termed the ‘embryo’. By day
14, the embryo is completely embedded and the implantation site in the
endometrium heals over. When the implantation site is sealed, there is
sometimes a little bleeding and this show of blood may be mistaken for
a period. This sometimes explains why some pregnancies are later found
to be a month further on than thought.
Occasionally this early journey
for the morula may be delayed. Sometimes there is no obvious reason for
the delay but it may be caused by a blockage or adhesions in the tiny
bore of the fallopian tube. This can be a result of a past infection
which may or may not have been obvious to the patient. When this delay
occurs, the morula fails to reach the uterine cavity. However, it continues
to develop and implants in an abnormal (ectopic) site usually within
the wall of the fallopian tube. This becomes an ectopic pregnancy (see
figure) and it cannot survive because the site of implantation is too
weak to support the developing pregnancy. Rupture of the ectopic pregnancy
may occur, sometimes with severe internal bleeding (see ‘Pregnancy loss’,
page 122). Very rarely the pregnancy may implant among the pelvic organs
and continue as an abdominal pregnancy, but this hardly ever reaches
a size and maturity to allow surgical delivery.
The fertilised
egg divides rapidly, first to form a morula and then a blastocyst.
After about six to seven days, the blastocyst reaches the uterine
cavity where it implants in the endometrium. |
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When the blastocyst attaches to the endometrium on
day 9, it has an inner mass of cells which develops into the embryo and
an outer mass of cells that are called trophoblasts (see figure). The
trophoblast cells begin to develop their own blood supply and this outer
cell mass becomes known as the chorion which later develops into the
placenta (or afterbirth). The cells produce a hormone called human chorionic
gonadotrophin (hCG) and this is the hormone that is detected in a pregnancy
test. It is first produced in very small amounts and can be detected
by sensitive tests at the time of your first missed period.
The embryo and formation of the organs
By day 14 after the release of the oocyte, the trophoblast
is completely embedded and the inner cells begin to line up as two layers
of different cells (see figure on page 20). Pregnancy is now suspected
because of the missed period. Doctors and midwives refer to the number
of weeks after the first day of your last period as the gestational age
of your pregnancy (and this dating of the pregnancy will be used from
this point in the book).
An ectopic
pregnancy occurs where the fertilised ovum implants outside of
the uterine cavity. |
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The cell differentiation continues and, as early as
five weeks (from the last period), the inner mass of cells begins to
change into tissues and organs. Tissues include skin and bones and the
basic cells responsible for these are laid down. Organs include the brain,
heart, eyes and ears, and these are formed by the end of nine weeks after
the last menstrual period.
Using pregnancy ultrasound scanning, the blastocyst
is seen as a small circle measuring two to four millimetres within the
uterus at 4.5 weeks after your last period. The first pulsations of the
heart beat can be seen as early as 36 days after the last period when
the embryo measures only two to four millimetres. The yolk sac is seen
close to the embryo and measures about 10 mm.
Illustration
of a human embryo nine days after fertilisation, known as a blastocyst. |
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The gestation period from week 5 to the end of week
9 is called the embryonic period or period of organogenesis. This is
a vital time for development of the organs – most abnormalities
or defects that are found after birth have happened during this time.
We know this from case studies of patients who have had rubella (German
measles). The most damaging effects occur when a mother has the illness
between week 5 and week 7 of the gestation period – her baby may
be born blind, deaf and with heart defects.
Illustration
of a human embryo aged two weeks seen in the uterus. The embryo
measures about 0.2 millimetre and has no nervous system or organs. |
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Rubella is caused by a virus and because it can cause
birth defects it is called a teratogen. Certain drugs have teratogenic
properties (the best known are thalidomide and the retinoids) and should
be avoided during the period of organogenesis. Most medications should
be avoided at this time except supplements such as folic acid or others
that may be vital for your health. Although most birth defects occur
during the period of organogenesis, the cause is not usually known. The
formation of the embryo is a series of complex biological events and
occasionally things go wrong. Some abnormalities may be lethal – the
embryo dies and miscarriage results.
Illustration
of a human embryo aged three weeks seen in the uterus. The embryo
measures about three millimetres in length. |
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Illustration
of a human embryo aged four weeks seen in the uterus.
The embryo measures about four to five millimetres in length. |
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The fetus
By the beginning of 10 weeks’ gestation, the
outline of a baby becomes recognisable and it is now called a fetus.
Organ formation is almost complete and the fetus now starts to mature
and prepare for delivery at around 40 weeks from the last period. The
skin initially appears red and transparent but progressively thickens
in the last few weeks when it is covered in a layer of white grease called
vernix. This prevents the mature skin becoming crinkly and waterlogged
while bathed in amniotic fluid in the uterus.
The sex is recognisable
on external appearance by 14 weeks, although the actual sex was determined
at conception. If you have an ultrasound scan at this time it is rather
early to be certain of the sex but usually at the time of the 20-week scan
it is possible, depending on how your fetus is lying.
At 10 weeks the fetus
weighs only 10 grams and measures three centimetres (cm) from the top
of the head to the base of the spine (crown–rump length). Growth is considerable over the next 30 weeks
and the average weight at delivery is 3,500 grams (7.5 pounds) when the
crown–rump length is 35 cm (see box on page 24). The body proportions
also change over this period. At 10 weeks, the head size is about half
the crown–rump length, but by 40 weeks it is only about a quarter
because of the increase in size of the trunk.
The fetus becomes viable
at 24 weeks. This means that it is possible for it to survive if delivered
after this time. However, delivery at 24 weeks is very hazardous for
a newborn because the organs and tissues are still very immature. The
chance of survival is not good and, if survival does occur, the chance
of handicap is high. As each week passes beyond the 24-week gestational
age of viability, the chances of intact survival increase greatly. With
modern neonatal care, most babies will survive after 30 weeks.
| Fetal
weight and length changes |
Weeks
of pregnancy |
Fetal
weight (grams) |
Crown–rump
length (cm) |
10
weeks |
10
g |
3
cm |
22
weeks |
500
g |
20
cm |
28
weeks |
1,000
g |
25
cm |
40
weeks |
3,500
g |
35
cm |
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Relative
changes in fetal size and proportions between week 8 and week
22 of pregnancy. |
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The placenta
When the trophoblast invades the endometrium it develops tiny stems and
branches. Within these, blood vessels form and develop into a network resulting
in the formation of the chorion. At around 10 weeks the chorionic development
is concentrated in the area nearest the fetus (see figure). It becomes
the true placenta and its circulation links up with the fetus through the
connecting stalk, which becomes the umbilical cord. The chorion has the
same genetic make-up as the fetus and it can be sampled at this stage in
pregnancy for the prenatal diagnosis of specific genetic or chromosome
problems.
The chorion
is the precursor of the placenta. This illustration shows the
approximate stage of development after 10 weeks. Chorionic villi
grow into the uterine wall and establish an intimate connection.
The placental location can be anywhere on the uterine wall, not
just as depicted. |
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The tiny stems and their branches are called villi
and these chorionic villi erode the tiny blood vessels in the endometrium.
They become bathed by the mother’s blood and from this oxygen and
nutrients pass across the thin barrier of cells from the mother to the
blood in the placenta, and then onto the fetus through the umbilical
cord. The uteroplacental circulation has become established (see figure)
and the developing fetus is dependent on its mother for nutrition. The
placenta also produces hormones, which aid the growth of the fetus and
maintain the pregnancy.
| The
placenta |
| The fetus is reliant on the mother for oxygen
and nutrients. The placenta allows the exchange of oxygen and
nourishment from the mother to the fetus.
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KEY POINTS
- The average length of the menstrual cycle is 28
days and ovulation occurs 14 days before the onset of menstruation
- FSH and LH from the pituitary gland cause ovulation
- Oestrogen and progesterone from the corpus luteum
prepare the secretory endometrium for the fertilised egg
- Detection of human chorionic gonadotrophin in
the mother’s urine or blood confirms that conception has occurred
- The embryonic period, when the organs form, lasts
from five to nine weeks after the first day of the last period
- The fetal period lasts from 10 weeks until delivery
at about 40 weeks
- The fetus is viable from 24 weeks after your last
period
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