 Fertilization - How Pregnancy
Occurs
The first thing in
preventing pregnancy, is understanding the
woman’s ovulation cycle and how pregnancy
occurs. A woman’s menstrual cycle and ovulation
time (also called her fertile time) has
everything to do with achieving
pregnancy and avoiding pregnancy.
What is Ovulation?
Ovulation
occurs when a mature egg is released from the
ovary, pushed down the fallopian tube and is
available to be fertilized. The lining of the
uterus has thickened to prepare for a fertilized
egg. If no conception occurs, the uterine lining
as well as blood will be shed. The shedding of
an unfertilized egg and the uterine wall is the
time of menstruation.
Key Facts of
Ovulation:
-
An egg lives
12-24 hours after leaving the ovary
-
Normally only
one egg is released each time of ovulation
-
Ovulation can
be affected by stress, illness or disruption
of normal routines
-
Some women may
experience some light blood spotting during
ovulation
-
Implantation
of a fertilized egg normally takes place
6-12 days after ovulation
-
Each woman is
born with millions of immature eggs that are
awaiting ovulation to begin
-
A menstrual
period can occur even if ovulation has not
occurred
-
Ovulation can
occur even if a menstrual period has not
occurred
-
Some women can
feel a bit of pain or aching, near the
ovaries during ovulation called
mittelscherz which means "middle pain"
in German
-
If an egg is
not fertilized, it disintegrates and is
absorbed into the uterine lining
Tracking
Ovulation:
A woman's
monthly cycle is measured from the first day of
her menstrual period until the first day of her
next period. On average, a woman's cycle
normally is between 28-32 days, but some women
may have much shorter cycles or much longer
ones. Ovulation can be calculated by starting
with the day the last menstrual period (LMP)
starts or by calculating 12-16 days from the
next expected period. Most women ovulate
anywhere between Day 11 - Day 21 of their cycle,
counting from the first day of the LMP. This is
what many refer to as the "fertile time" of a
woman's cycle, because sexual intercourse during
this time increases the chance of pregnancy.
Ovulation can occur at various times during a
cycle, and may occur on a different day each
month.
The Ovulation Cycle divided into two parts:
The first part of the ovulation cycle is called
the follicular phase. This phase starts
the first day of the last menstrual period (LMP)
and continues until ovulation. This first half
of the cycle can differ greatly for each woman
lasting anywhere from 7 days until 40 days. The
second half of the cycle is called the
luteal phase and is from the day of
ovulation until the next period begins. The
luteal phase has a more precise timeline
and usually is only 12-16 days from the day of
ovulation. This ultimately means that the day of
ovulation will determine how long your cycle is.
This also means that outside factors like
stress, illness, and disruption of normal
routine can throw off your ovulation which then
results in changing the time your period will
come. So the old thought that stress can affect
your period is only partly true. Stress can
affect your ovulation which ultimately
determines when your period will come, but
stress around the time of an expected period
will not make it late—it was already determined
when it would come 12-16 days earlier!
Fertility
Awareness is one way to track when ovulation
occurs and it includes studying the changes in
cervical mucus and using a basal thermometer.
Cervical fluid will change to a wet, slippery
substance that resembles "egg whites" just
before ovulation occurs and until ovulation is
over. A basal thermometer helps track a body
temperature rise, which signals that ovulation
has just occurred.
Another way to
track ovulation is through ovulation kits and
fertility monitors. Tracking ovulation can help
a woman get a better idea of when pregnancy can
and can not occur during her monthly cycle. Once
ovulation has occurred, there is nothing you can
do to increase your chances of pregnancy. Your
next step is to begin watching for
early pregnancy symptoms.
View and print
an
Ovulation Calendar, or use our
Ovulation Calculator, to better understand
your menstruation cycle and ovulation.
From the
Menstrual Period to Ovulation (the details you
may not know!)
When your
menstrual cycle begins, your estrogen levels are
low. Your hypothalamus (which is in charge of
maintaining your hormone levels) sends out a
message to your pituitary gland which then sends
out the follicle stimulating hormone (FSH). This
FSH triggers a few of your follicles to develop
into mature eggs. One of these will develop into
the dominant follicle, which will release a
mature egg and the others will disintegrate. As
the follicles mature they send out another
hormone, estrogen. The high levels of estrogen
will tell the hypothalamus and pituitary gland
that that there is a mature egg.
A luteinizing
hormone (LH) is then released; referred to as
your LH surge. The LH surge cause’s the egg to
burst through the ovary wall within 24-36 hours
and begin its journey down the fallopian tube
for fertilization. The follicle from which the
egg was released is called the corpus luteum
and it will release progesterone that helps
thicken and prepare the uterine lining for
implantation. The corpus luteum will produce
progesterone for about 12-16 days (the
luteal phase of your cycle.) If an egg is
fertilized, the corpus luteum will
continue to produce progesterone for a
developing pregnancy until the placenta takes
over. If fertilization does not occur the egg
dissolves after 24 hours.
At this time
your hormone levels will decrease and your
uterine lining will begin to shed about 12-16
days from ovulation. This is menstruation
(menstrual period) and brings us back to day 1
of your cycle. The journey then begins all over
again.
The time of
ovulation is one of the most important things a
woman should understand about her body since it
is the determining factor in getting pregnant
and preventing pregnancy. The process can be
confusing and somewhat overwhelming when trying
to understand.
How
pregnancy (conception) occurs
Most women are able to become
pregnant from puberty, when their menstrual cycles
begin, until menopause, when their cycles stop. A
pregnancy starts with fertilization, when a woman's egg
joins with a man's sperm. Fertilization usually takes
place in a fallopian tube that links an ovary to the
uterus. If the fertilized egg successfully travels down
the fallopian tube and implants in the uterus, an embryo
starts growing.
Ovulation, fertilization, implantation
All the eggs for a
woman's lifetime are stored in her ovaries. Women do not
continually produce eggs. This is different from men,
who continuously make more sperm.
About once a month, an
egg is released from one of a woman's two ovaries. This
is called ovulation. The egg then enters the nearby
fallopian tube that leads to the uterus.
If a woman and a man
have unprotected sexual intercourse, sperm that is
ejaculated from the man's penis may reach the egg in the
fallopian tube. If one of the sperm cells penetrates the
egg, the egg is fertilized and begins developing.
The egg takes several
days to travel down the fallopian tube into the uterus.
Once in the uterus, a fertilized egg usually attaches to
(implants in) the lining of the uterus (endometrium).
However, not all fertilized eggs successfully implant.
If the egg is not fertilized or does not implant, the
woman's body sheds the egg and the endometrium. This
shedding causes the bleeding in a woman's menstrual
period.
When a fertilized egg
does implant (conception), a hormone called human
chorionic gonadotropin (hCG) begins to be produced in
the uterus. This is the hormone that a pregnancy test
measures. It prevents the uterine lining from being
shed, so the woman does not have a period. Other signs
such as breast changes and nausea occur in a woman's
body, also indicating that pregnancy has begun.
Sources:
American
Pregnancy Assn
WebMD
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