Human Development
Chapter 15 (pp. 506-543)
Introduction
• The period from
fertilization to birth is
called gestation.
• In humans, it’s 39
weeks which roughly
breaks down into three
three month periods
called trimesters.
•
•
However, biologists use a two stage
system to describe prenatal
(prebirth) developmental events
during gestation.
•
The embryonic period lasts for
the first 8 weeks. Tissues and
organs form as do structures that
support and nourish the
developing embryo.
•
The fetal period starts at the 9th
week and carries through to
birth. The body grows rapidly
and organs begins to function
and coordinate to form organ
systems.
Before we start this half of the
unit, let’s consult an expert…
Dr. Seuss
Fertilization
•
Fertilization or conception involves the
joining of the haploid sperm and egg into a
single diploid cell that contains 23 pairs of
chromosomes.
•
In humans, this must occur within 24 hours
of ovulation.
•
Under optimal conditions, sperm can
survive in a female’s reproductive system
for up to 3 days. However, of the several
hundred million sperm that enter a
woman’s vagina only a few hundred will
survive to reach the egg in the oviduct (ie.
most are destroyed by the acidic
environment and many head up the wrong
oviduct).
•
Given a normal menstrual cycle, when can
pregnancy occur?
•
When a sperm meets the egg (secondary oocyte), it must
first content with two outer layers called the corona radiata
(the jelly-like outermost layer) and the zona pellucida
(sandwiched between the corona radiata & the cell
membrane).
•
The sperm’s acrosome releases its enzymes and digests a
path through the corona radiata & zona pellucida. Once a
sperm enters the egg, the cell membrane depolarizes
preventing other sperm from entering or even binding with
it.
•
The secondary oocyte quickly undergoes meiosis II, forming
an ootid which quickly matures into an ovum. Within 12
hours of the sperm’s nucleus entering the egg, the two
nuclei fuse into a single celled zygote. It has 23 pairs of
chromosomes (ie. diploid), with one chromosome in each
pair from each parent.
•
This is the moment that the chromosomal sex of the embryo
is determined. One of the 23 pairs of chromosomes
determines this. Females have two of the same kind of sex
chromosomes (XX), while males have two distinct sex
chromosomes (XY). Depending on whether the haploid
sperm provided a X or Y chromosome, the developing
embryo will become either a female or male respectively.
•
Occasionally two eggs are released during ovulation and are fertilized by different sperm
cells at the same time. This will result in dizygotic or fraternal twins. These twins are not
genetically identical and can even be different sexes from one another.
•
If a single fertilized egg splits into two cell masses, you’ll wind up with monozygotic or
identical twins. These twins share the same genetic makeup.
•
Conjoined twins are identical twins
physically joined when the fertilized egg
splits only partially. This effects 1 in
every 200 000 live births. Approximately
half are stillborn and an additional onethird die within 24 hours of birth.
•
The most famous pair of conjoined twins
was Chang and Eng Bunker, brothers
born in Siam (now Thailand) in 1811.
They traveled with P.T. Barnum’s circus
for many years and were labeled as the
Siamese twins.
•
Abigail and Brittany Hensel born in 1990,
have a single body but two heads. Their
parents rejected the option to attempt
surgical separation after hearing from
doctors that is was not likely that both
would survive the operation.
•
Lakshmi Tatma was born in 2005 with four
arms and four legs. This was the result from
a joining at the pelvis with a headless
underdeveloped parasitic twin.
•
The operation to separate the twins lasted 27
hours:
•
Removal of the parasite’s abdominal
organs.
•
Replacing Lakshmi’s necrotic kidney
with the kidney of the parasite.
•
Moving the reproductive system and
urinary bladder.
•
Amputation the parasite’s legs at the
hip joint and cutting the joined
backbones (care had to be taken to
avoid causing paralysis).
•
Dividing the combined pelvic ring of
the twins.
•
Mohamed and Ahmed Ibrahim were born in
Egypt in 2001 with a one-in-two million
condition - they were joined at the top of
the head.
•
The twins were sent to Dallas for a surgery
to separate them. The parents were warned
that the odds of both twins surviving was
only 10%.
•
50 physicians anesthesiologists and nurses
spent 34 hours performing the successful
procedure.
•
The boys’ condition led to developmental
delays and they are entering the first grade
at eight years old. As well, they will require
future surgeries to add bone to their
recovering skull area.
Assistive Reproductive Technologies
•
Technologies that enhance
reproductive potential are
referred to as assisted
reproductive technologies.
This assists to groups of
individuals:
•
Men and women who are
unable to have any
children (ie. sterile).
•
Couples that are
experiencing difficulty
having children over a
period of at least one year
(ie. infertile).
•
Possible causes of male infertility
include:
•
Blocked epididymis or vas
deferens (ie. arising from a
STI)
•
Low sperm count (Risk
factors include overheated
testes, smoking & alcohol
consumption)
•
Abnormal sperm (Risk factors
include overheated testes,
exposure to toxins & STIs)
•
Impotence (Risk factors
include vascular disease,
nervous system injury, stress,
hormonal imbalance,
medication, smoking and
alcohol consumption).
•
Possible causes of female
infertility include:
•
Blocked oviducts (ie.
caused by a STI)
•
Failure to ovulate (Risk
factors include hormonal
imbalance,
malnourishment & stress)
•
Endometriosis (a painful
condition in which the
endometrium grows on
the outside of the uterus)
•
Damaged eggs (Risk
factors include being
exposed to toxins and/or
radiation)
•
Artificial insemination
(AI) or intrauterine
insemination (IUI) has
been used for decades.
•
Sperm are collected
and concentrated
before being placed in
the woman’s uterus.
•
This technique can be
also be used by women
without a male partner.
•
Sperm banks provide a
source of sperm
samples that have been
gathered for this
purpose.
•
In vitro fertilization (IVF) offers a
solution for women with blocked
oviducts.
•
Eggs that are close to ovulation are
surgically harvested from the
ovaries.
•
These eggs are combined with
sperm in a culture dish.
•
After fertilization, 2 or 3 embryos
are placed in the uterus for
implantation.
•
A slight variation of IVF is gamete
intrafallopian transfer (GIFT), in
which the eggs and sperm are
brought together in the oviduct
rather than in vitro. This procedure
has a higher success rate than IVF.
•
Surrogate mothers can be contracted by an infertile couple to carry
a baby for them. Using AI or IVF, one of both gametes may be
contributed by the contracting couple.
•
Women who ovulate rarely or not at all may consider
superovulation. This cause the production of multiple eggs
during a single ovulation as a result of hormone treatment. This
technique is often used in conjunction with other assistive
reproductive technologies.
•
Considerations of any fertility treatment include:
•
Health risks?
•
Financial costs?
•
Emotional strain?
•
Ethical issues?
•
One area of reproductive
technology that is
extremely controversial is
artificial cloning.
•
Clones are organisms that
are exact genetic copies
(ie. identical twins).
•
Cloning has been around
for 2 millennia (ie.
European cultivars of
grapes).
•
In 1996, Dolly the sheep
was successfully cloned
(after 433 failed attempts).
•
This has raised the
possibility of cloning
endangered or even extinct
species.
•
What about the
possibility of cloning
humans in the future?
Contraception
• Technologies or methods that
reduce the reproductive
potential of a couple are
referred to as contraception.
• The surest way to avoid
conceiving a child is simply
not to have sexual intercourse
- abstinence.
• This is 100% effective (at both
preventing pregnancy and
STIs).
•
Surgical sterilization can make men
or women infertile or even sterile:
•
In women, a tubal ligation
involves cutting the oviducts
and tying off the cut ends. This
way the egg never encounters
sperm and can’t reach the
uterus.
•
In men, a vasectomy involves
cutting or tying off the ductus
deferentia. The man is still able
to have an erection and
ejaculate, but his semen doesn’t
contain any sperm. After the
procedure is complete, it can
take 20-30 ejaculations to clear
all the remaining sperm out of
the ductus deferentia.
•
There are many options of hormonal birth control all based on negative feedback loops within a
woman’s body:
•
Birth control pills trick the body into thinking its pregnant. It raises levels of estrogen and
progesterone in body. It exerts a negative feedback on the hypothalamus, which stops
production of GnRH. This stops the pituitary from producing FSH & LH, which means
that no follicular development. The pill is taken for 21 days of the 28 day menstrual cycle.
The last 7 days of the cycle is when menstruation will occur.
•
The morning after pill is a treatment of several pills taken within 2-3 days of intercourse.
They deliver high levels of synthetic estrogen & progesterone, which disrupt the ovarian
cycle. They can prevent ovulation or implantation. The effectiveness of the pills drop the
further from intercourse that they are taken (ie. 95% effective on the first day, 85% on the
second day and only 5% on the third day).
•
An intrauterine contraceptive
device (IUD) can be inserted into
a woman’s uterus for up to 12
years. They release hormones
that thicken the mucous in the
cervix (ie. it blocks/traps sperm
and stops ovulation). As well,
they are made of copper which
raises the white blood cell count
in the uterus (ie. which will kill
sperm).
•
Depo-Provera birth control shots
are taken every 12 weeks and
release a hormone similar to
progesterone.
•
A progestin implant is the size of
a matchstick and is placed under
the skin of the upper arm. It lasts
up to 4 years.
•
Physical barriers as a means of birth control dates back to ancient
Egypt, when honey, acacia leaves and lint were placed in vaginas
to block sperm. Modern physical contraceptives include:
•
Male and female condoms are sheath-shaped barriers that
are not only effective birth control methods but also
greatest reduce the risk of STIs.
•
The use of contraceptive caps have waned in recent years
because of other methods of contraception. However the
two main forms are still a useful option for a minority of
women, particularly those over 35 years of age.
•
Diaphragms are soft, thin, dome-shaped rubber cups
with a flexible rim. Spermicidal jelly is placed inside
the dome as a form of chemical barrier. The
diaphragm is placed high in the vagina to hold the
spermicide against the cervix. It must be left in place
for several hours after sex. After that time, it can be
washed and reused. It should not be left in the vagina
for more than 30 hours.
•
Cervical caps are a circular domes made of thin, soft
silicone. It works in a very similar fashion to a
diaphragm and used in women who find a
diaphragm too large.
•
Some couples refrain from sexual intercourse during the time of the women’s cycle when
she’s most fertile.
•
This is known as natural family planning or rhythm method.
•
Couples must pay careful attention to the subtle signals of the woman’s body, such as body
temperature (ie. an increase indicate ovulation) and the properties of the cervical mucus.
•
This is among the least reliable forms of birth control, with an effectiveness of roughly 70%.
•
If a pregnancy occurs despite the use of contraception then emergency contraception may
be required. Abortion? Ethical issues?
Let’s Quickly Review…
Embryonic Development
•
The first eight weeks (56 days) after ovulation are divided into 23 embryonic stages, also
known as Carnegie stages.
•
This system is used by embryologists to describe the apparent maturity of the embryo.
•
After fertilization, the zygote
continues to slowly works it’s
way to the uterus (this usually
takes 3-5 days).
•
Within 30 hours of fertilization,
the first mitotic division of the
zygote occurs resulting in 2
cells.
•
Three additional cleavages (ie.
the cells become smaller and
smaller with each division)
results in a 16-cell morula by
roughly day 3.
•
The morula undergoes
blastulation (ie. the cells form a
hollow ball) to become a
blastocyst/blastula by day 4-5.
•
This whole time, the size of the
embryo remains constant and all
of the nutrients and organelles
required for this come from the
original ovum.
•
The blastocyst/blastula is comprised of three parts:
•
The fluid-filled interior blastocyst cavity is
called the blastocoel.
•
The outer layer is called the trophoblast and
will eventually form a membrane called the
chorion (which will become part of the
placenta).
•
Located between the blastocoel and trophoblast
is the inner cell mass/embryoblast which will
develop into the embryo itself.
•
Between day 5-7, the blastocyst attaches to the
endometrium, with the inner cell mass positioned
against it. Trophoblast cells secrete enzymes that
digest some of the endometrium tissue, causing the
blastocyst to slowly sink into the uterine wall. This
is called implantation and is complete by day 10 to
14.
•
In some pregnancies, implantation does not occur in
the uterus but in the oviduct. This is called an
ectopic pregnancy. Women who smoke are twice as
likely to have an ectopic pregnancy as nicotine
paralyzes the cilia required to move the ovum
through the oviduct. Ectopic pregnancies can also
occur in the cervix or the abdominal cavity.
•
At the time of implantation, the
trophoblast cells start to secrete large
amounts of the hormone human
chorionic gonadotropin (hCG) for
roughly two months. This is the
hormone that pregnancy tests detect.
•
Having the same effect as LH, hCG
maintains the corpus luteum
throughout the pregnancy. Estrogen
and progesterone secretions
continue, preventing menstruation.
•
However after the first two months
the corpus luteum’s role is
diminished because the placenta
produces enough estrogen and
progesterone on its own to maintain
the endometrium.
•
In the lab, scientists have
cloned stem cells from
human skin and egg
cells.
•
Embryonic stem cells are
primitive, unspecialized
cells found in the inner
cell mass of a blastocyst.
•
These 12 pluripotent
cells can be used to
produce any organ or
other body part that are
genetically identical to
that of the patient (ie. no
risk of rejection when
transplanted).
•
What are the ethical issues
around this technology?
•
•
During implantation in WEEK 2, the blastocyst begins to undergo
gastrulation:
•
A space forms between the inner cell mass and the trophoblast called
the amniotic cavity.
•
The inner cell mass flattens into the embryonic disk, which
differentiates into three distinct layers:
•
The outer ectoderm (ie. closest to the amniotic cavity) will go on to
become structures such as the nervous system, hair, skin, sweat
glands & teeth.
•
The inner endoderm will go on to become structures such as the GI
& respiratory tracts, the liver, the pancreas & endocrine glands.
•
The mesoderm (sandwiched between the ectoderm & endoderm)
will go on to become structures such as connective tissue, the
kidneys, the gonads, muscles, bone & the circulatory system.
These three layers are called the primary germ layers and the embryo is
now called a gastrula.
•
The forming of the gastrula marks the start of morphogenesis. This is when the
embryo begins to develop organs and begins to take on a more human shape.
•
This is done mainly through the process of differentiation. This process enables
cells to develop into a particular shapes and/or to perform specific functions
different from others.
Let’s Quickly Review…
• During WEEK 3, a thickening band of
mesoderm cells develops into a
notochord which forms the basic
framework of the skeleton.
• The nervous system develops from the
ectoderm located just above the
notochord in a process called
neurulation.
• Cells along this surface begin to
thicken and folds develop along
each side of a groove.
• When the folds fuse, they become
the neural tube which eventually
will become the brain and spinal
cord.
• Four extra-embryonic membranes begin to form (and
continue to do so until week 8):
• Chorion: The outer layer of the embryo will develop into
the fetal portion of the placenta.
• Amnion/Amniotic Sac: A membrane surrounding the
embryo which is comprised of amniotic fluid. It protects
the embryo from physical trauma & temperature
fluctuations. It also allows for movement of the embryo.
• Allantois: Forms the foundation of the umbilical cord. It
degenerates during the second month.
• Yolk Sac: Provides food and blood to the cells of the
embryo. It’s limited in humans and degenerates to become
part of the umbilical cord as soon as the placenta forms.
•
At this
point, the
mother
may begin
experience
morning
sickness
and
pregnancy
tests will
work.
•
On day 18,
the heart
starts
beating…
•
•
WEEK FOUR is a time of rapid
growth and differentiation:
•
Blood starts to form and fill
blood vessels.
•
The lungs & kidneys take
shape.
•
Limb buds begin to form.
•
A distinct head is visible
with evidence of eyes, ears
and nose.
•
The embryo is just over half
a centimetre long.
At this time, the mother’s
menstrual period is
approximately two weeks late.
•
•
During WEEK FIVE:
•
The heart begins pumping blood
•
Eyes begin to open (no eyelids or
irises yet).
•
Cells in the brain begin to
differentiate.
•
The embryo is 1.3 centimetre long.
•
A first prenatal appointment to
determine sexual history and to
possibly take urine/blood samples
is recommended.
During WEEK SIX:
•
The limbs lengthen and flex
slightly.
•
Gonads start to produce hormones
that will influence the
development of external genitalia.
•
During WEEKS SEVEN & EIGHT:
•
Internal organs have formed.
•
Nervous system starts to coordinate
body activities.
•
External genitalia are still forming but
are still undifferentiated.
•
A skeleton of cartilage has formed (bone
begins to replace the cartilage in week
nine).
•
Eyes are well developed (covered by
eyelids).
•
Nostrils are developed and filled with
mucus.
•
The embryo is about the size and weight
of paper clip by the end of week 8.
•
The mother may book a first ultrasound
appointment to detect the embryo’s
heartbeat.
• In biology, hermaphrodites are organisms that have
both male and female sex organs or other sexual
characteristics.
• In humans, true hermaphrodites do not exist, but
pseudohermaphrodism does.
• This can occur when female embryos are exposed to
high levels of male sex hormones. They develop
female internal reproductive organs but male
external genitalia.
• Alternately, genetic defects cause children to be born
with female external genital organs, which change at
puberty with the development of a penis and closure
of the false vagina.
Effect of Human Sex Chromosomes on Development of
Gonads and Sexual Differentiation
Total Number of
Sex
Gonad Apparent Sex
Chromosomes Chromosomes Produced of Individual
Condition
Produced
Description
46
XX
Ovary
Female
Normal
46
XY
Testis
Male
Normal
Ovaries defective/absent; sterile; no pubertal
development; no menstrual cycle; rarely attain adult
height of more than five feet
Individuals have no sexual abnormalities and may
children; some are mentally retarded
45
X
Ovary
Female
Turner’s
syndrome
47
XXX
Ovary
Female
Triple X
syndrome
48 & 49
47
48 & 49
47
XXXX & XXXXX
XXY
XXXY & XXXXY
XYY
Ovary
Testis
Testis
Testis
No known consistent pattern of traits; mental
retardation seems to increase markedly as number of
X chromosomes increases
Female
Genitals are male but testis are small & do not produce
sperm; underdeveloped pubic, facial & body hair;
enlarged breasts; tall; possible mental retardation
Male
Klinefelter’s
syndrome
Male
Variations of Same clinical conditions as Klinefelter’s syndrome but
Klinefelter’s individuals are more severely affected; severe mental
syndrome
retardation is common
Male
XYY
syndrome
Individuals are tall (six feet and over); may show
impulsive behaviour; sperm production often reduced
(may be sterile)
•
By the end of week 8, the yolk
sac shrinks, the amniotic sac
enlarges and the umbilical cord
forms.
•
It contains one vein which
brings oxygen-rich blood to the
fetus and two arteries which
transport oxygen-poor blood
from the fetus to the placenta.
•
Besides serves a nutrient/waste
diffusion role between the uterus
and the umbilical cord, the
placenta also serves an endocrine
function.
•
In the second and third
trimesters, the placenta produces
enough estrogen and
progesterone to maintain the
pregnancy (taking over from the
hCG released from the embryo).
•
•
The placenta arises
from two sources:
•
Chorion begins to
extend finger-like
projections into
the uterine lining
(fetal portion of
the placenta)
•
Blood vessels
from the
mother’s
circulatory
system that form
pools (maternal
portion of the
placenta).
The placenta is fully
developed by week
10.
Fetal Development
•
Fetal development starts during week 9 and lasts until birth.
•
Unlike the embryonic period which is a time of morphogenesis, the fetal
period is a time of growth and “refinement” of existing structures.
•
At WEEK 9, the fetus is roughly 6 inches long and begins to move
(although the mother can’t feel it yet)
•
Near the end of the first trimester of the pregnancy, a prenatal test called
chorionic villus sampling (CVS) can be used if chromosomal or genetic
defects are suspected in the developing fetus.
•
The sample of the chorion can be taken through the cervix or the abdominal
wall and this can be used to look for genetic markers of the suspected disorder.
•
By the end of the first trimester (WEEK 12), the cartilage-based skeleton
begins to harden with the development of bone.
•
External reproductive organs are distinguishable as male of female
•
During the second trimester (WEEKS 13 to
24):
•
The fetus is now actively turning
•
The body becomes larger in proportion to
the head.
•
The fetus is now bent forward into the
“fetal position” because it’s beginning to
outgrow the amniotic sac.
•
Soft hair, eyelids and eyelashes have
formed.
•
Bone cells are now fully developed.
•
The fetus has settled into a regular sleep
cycle.
•
By MONTH 6, the fetus is roughly 12
inches long and 1.5 pounds in weight.
• Obstetric ultrasounds use sound waves to create a realtime visual image of a developing embryo/fetus in the
uterus.
• It’s recommended that pregnant women have an
ultrasound between weeks 11 and 13 and weeks 18 and 22 .
• They are used to confirm pregnancy timing and to measure
the fetus so that growth abnormalities can be recognized
quickly later in the pregnancy.
•
During this stage of the pregnancy, amniocentesis or an amniotic fluid test
(AFT) can be used to diagnosis chromosomal abnormalities and fetal
infections.
•
A sample from the amniotic sac and fetal DNA are collected.
•
This procedure is more invasive that CVS and carries a small risk of
miscarriage.
•
This process can be also be used for prenatal sex discernment.
•
During the third trimester (WEEKS 25 to
39):
•
The fetus turns to an upside-down
position in the uterus.
•
Brain cells form rapidly (up to tens of
thousands per minute).
•
The digestive & respiratory systems
are the last to fully develop. It’s not
until the 7th month, that the lungs are
developed enough to sustain life out
of the womb.
•
Testes descend into the scrotum
(males only).
•
Fatty tissue develops giving the fetus
a more plump, “babyish” appearance.
• After nine months of development, it’s time for the fetus
to make its grand entrance into the world…
Parturition
• Parturition (the birthing process) and
all the events associated with it are
commonly referred to as labour. It’s
broken into three stages:
• Dilation stage
• Expulsion stage
• Placental stage
•
The dilation stage begins when the
head of the fetus pushes against the
cervix, causing nerve impulse to be
transmitted to the posterior pituitary
gland, which releases the hormone
oxytocin.
•
Once oxytocin reaches the uterus, it
stimulates uterine contractions.
These contractions push the fetus’
head even further up against the
cervix, created a positive feedback
loop (ie. more oxytocin is released by
the pituitary). These contractions are
10-15 minutes apart and last 40
seconds or longer.
•
The cervix will dilate to 10
centimetres in a period that lasts from
2 to 20 hours.
• Cyclic fatty acids called prostaglandins are released by
the placenta reinforce this positive feedback loop.
• The amniotic sac breaks and amniotic fluid (along with
the cervical plug) are released through the vagina (ie.
“breaking of water”).
• The expulsion stage sees forceful contractions pushing the
baby through the cervix and birth canal.
• This stage usually last from 30 minutes to 2 hours with
regular contractions 1-2 minutes apart.
• Epidural anaesthesia is an injection into the epidural
space in the spine that blocks sensory pain receptors
from the waist down, helping stop the mother who is
giving birth from feeling pain.
• It provides anesthesia for episiotomy (surgical cut in
the muscular area between the vagina and the anus to
enlarge the opening of the birth canal) or forceps
deliveries.
• It uses remains controversial because
• Of the risk of puncturing the dura (the outer layer
surrounding the spinal core) is roughly 1 in 100.
• It can also lead to nerve damage.
•
The placental stage occurs roughly 10 to 15 minutes after the baby is born.
•
The placenta and umbilical cord are expelled from the uterus.
•
The expelled placenta is called the afterbirth. The umbilical cord is
clamped, cut and tied. The cord eventually shrivels and the place where
the cord was attached to the baby becomes the navel.
• Some women are opting to drink their placenta in a fruit
smoothie within hours of giving birth, keeping it cool,
sending it off to be dried and made into capsule or even
ripping off a chunk and placing it by their gums.
• They’re convinced that this gives them an energy boost,
can encourage breast milk production and even prevent
postpartum depression (experienced by 1 in 10 women).
• Other methods for consumption include making it into a
tincture with drops placed in a glass of water as required
and even cooking it and using it as a beef substitute (ie. in
stir fries, burgers or stroganoff).
• Currently there is not enough evidence to support or not
support placentophagy (placental eating) as being
beneficial to a mother after giving birth.
• Some mothers decide to
have a water birth, in which
they spend the final stages of
labour in a birthing pool.
• Proponents believe water
birth results in a more
relaxed, less painful
experience that promote a
midwife-led model of care.
• Critics argue that is can led
to increased mother and/or
baby infections and the
possibility of infant
drowning.
•
A caesarean sections (C-section)
is a delivery that is done by
cutting an opening through the
abdominal.
•
These are done when:
•
The baby is in a rump-first
position (breech birth).
•
The mother has a STI, such as
herpes, which may be
transmitted to the baby.
•
The mother has a small pelvis.
•
The baby is exceptionally
large.
•
The mother has high blood
pressure.
Let’s Quickly Review…
Lactation
• After birth, two
hormones control
the onset of lactation
(the formation and
secretion of breast
milk in the mother).
• Prolactin
• Oxytocin
• Prolactin, which is needed for milk production, is not
secreted during pregnancy because estrogen and
progesterone levels are too high.
• As estrogen and progesterone levels drop after birth, the
anterior pituitary starts secreting prolactin. Milk
production starts within a few days.
• When the baby starts suckling, nerve endings in the
nipple and areola send signals to the hypothalamus. This,
in turn, stimulates the posterior pituitary to release
oxytocin.
• Oxytocin causes contractions within the mammary
lobules, causing lactation. Continued suckling stimulates
more oxytocin release and more lactation (ie. a positive
feedback loop).
Developmental Abnormalities
•
According to Alberta Health Services, couples in the province have a 2-3% chance of
having a child with some type of birth defect.
•
Quite often these defects are caused by teratogens (ie. any agent that causes a structural
abnormality due to exposure during pregnancy). If the mother is exposed to or consumes
these teratogens they can cross the placenta to affect the fetus. Two of the most common
are cigarette smoke and alcohol.
•
Cigarette smoke can increase the risk of a premature birth, low birth weight and
miscarriages.
•
Alcohol is response for fetal alcohol spectrum disorder (FASD). This include the
more common known fetal alcohol syndrome (FAS).
•
Symptoms include abnormally small head & eyes, low nasal bridge, flat midface,
thin upper lip, permanent brain damage, growth problems, heart & kidney defects
and long-term behavioural problems (often resulting in crime, delinquency &
other anti-social behaviour).
•
FASD is irreversible (there is no cure and the effects last a lifetime). In Alberta, the
rate of FASD births is roughly 3 for every 1000 live births.
•
Although mothers shouldn’t drink alcohol at all when pregnant, the greatest risk to the fetus comes
during weeks 7 to 12. Studies have shown that with each daily drink, a woman’s baby was 12%
more likely to have a smaller-than-normal head & 16% more likely to have a low birth weight.
•
Other drugs can have severe effects on
developing fetuses:
•
Mothers who are drug addicts (ie. morphine
or cocaine) give birth to babies addicted to
these drugs as well. They are often
underdeveloped and below normal birth
weight.
•
The use of Accutane for severe acne can lead
to low birth weight, hydrocephaly
(accumulation of CSF in the brain),
microcephaly (the brain does not develop
properly) and cleft palate.
•
First prescribed in the 1950s, thalidomide
was used to reduce morning sickness in
pregnant mothers. At the time, it was not
known that this drug could cross the
placental wall affecting limb development of
the fetus. It remained legally available in
Canada until 1962. In 2015, the 92 remaining
thalidomide survivors each received a $125
000 lump sum and an annual tax free
pension from the Canadian federal
government by way of compensation.
•
The rubella virus, that causes the German
measles, can result in a condition known as
congenital rubella syndrome (CRS) in a
developing fetus if the mother contracts it
before week 13 of the pregnancy. Symptoms
include
•
Death (miscarriages or stillborn babies are
common)
•
If the fetus survives the infection it can be
born with
•
Severe heart disorders
•
Cataracts and/or blindness
•
Deafness
•
Microcephaly
•
Blueberry muffin rash
•
If the mother contracts rubella during the
second trimester the risk of serious symptoms
drops by half. If it’s during the third trimester,
the fetus is generally not affected.
•
What’s another virus that’s been in the news
recently that causes microcephaly?
•
In some cases, birth defects are related to maternal nutrition.
•
For example, a lack of folic acid can lead to a condition known as spina bifida. This results in an
incomplete closing of the backbone and the membranes around the spinal cord.
•
This neural tube defect can cause leg weakness and paralysis, orthopaedic abnormalities (ie.
club foot), bladder/bowel control issues & abnormal eye movement.
•
There’s no known cure for the nerve damage caused by spina bifida. The standard treatment is
surgery after delivery to prevent further damage of the nervous tissue.
•
A much more serious neural tube developmental disorder is anencephaly. This
results in the absence of a major portion of the brain, skull and scalp.
•
It occurs when the rostral (head) end of the neural tube fails to close, usually between
the 23rd and 26th day following conception.
•
Causes of this disorder are disputed. Folic acid has been shown to be important in
neural tube formation, but there’s some indirect evidence of heredity playing a role.