PSYCH211 Lecture Notes - Lecture 3: Thyroid, Postpartum Period, Prenatal Care

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Chapter 4
From Conception to Birth
- prenatal development begins when a sperm fertilizes an egg
- prenatal development – the changes that transform the fertilized egg into a newborn human
make up
- health professionals measure the start of your pregnancy from the day of your last period –
which is typically 2 weeks before conception
- prenatal gestation takes about 38 weeks and can sometimes add an additional 5 weeks (37 days)
- 38 weeks of prenatal development is divided into three stages: the period of the zygote, the
period of the embryo, and the period of the fetus
Period of the Zygote
- the first period begins with fertilization and lasts about 2 weeks – it ends with the fertilized egg
called the zygote – grows rapidly through cell division
- it implants itself in the wall of the uterus
- it travels down the fallopian tube toward the uterus, and within hours the zygote undergoes its
first cell division – it consists about 100 cells and resembles a hollow, fluid-filled ball
- at the end of the 1st week the zygote is in the uterus
- Implantation – the zygote burrows into the uterine wall and connects to the mothers blood
vessels – takes about a week and triggers hormonal changes that prevent menstruation
- some women fail to become pregnant because the zygote fails to implant itself
- less than 1mm in diameter
- germ disc – a small cluster of cells near the zygote’s centre that develops into the baby
- other cells become structures that support, nourish and protect to develop the child
- the layer of cells closest to the uterus becomes the placenta – a structure for the exchanging
nutrients and wastes between the mother and her baby
- implantation and differentiation of the cells is the end of the zygote
- the zygote is well prepared for the remaining 36 weeks of the journey to birth
Period of the Embryo (3-8 weeks)
- once the zygote completely embeds itself in the uterine wall, it becomes an embryo
- body structures and internal organs develop
- three layers form in the embryo – outer layer or ectoderm, will become hair, outer layer of skin
and the nervous system; the middle layer or mesoderm, will form muscles, bones and the
circulatory system; the inner layer or endoderm, will from the digestive system and the lungs
- 3-week old embryo is about 2 mm long, and weighs less than 30 grams – cell specialization is
underway, but embryo looks more like a salamander – by week 8 it looks different and you can
see the eyes, jaws, arms and legs
- the brain and nervous system are also developing and the heart has been beating for nearly a
- sex organs are the only ones that are not in place
- mother cannot feel it at this point
- embryo reset in a sac called the amnion, which is filled with amniotic fluid to cushion the
embryo and maintain a constant temperature
- it is linked to the mother by the umbilical cord and placenta
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- umbilical cord – contains blood vessels that joins the developing baby through the baby’s
abdomen to the mothers placenta
- in the placenta, the blood vessels from the umbilical cord run close to the mothers blood
vessels from the umbilical cord, but aren’t actually connected to them – instead the blood flows
through villi, finger like projection from the umbilical blood vessels, villi lie in close proximity
to the mothers blood vessels, which allows nutrients, oxygen, vitamins, and waste products to be
exchanged between mother and embryo
- growth follows two principles, they are characterized as “from the top down” and “from the
centre out”
- first, the head develops before the rest of the body – grown from the head to the base of the
spine illustrates the cephalocaudal principle (from the top down)
- second, arms and legs develop before hands and feet – illustrates proximodistal principle –
growth after birth also follows these principles
Period of the Fetus (weeks 9-38)
- about four months in the fetus weighs 100-225 grams
- in the last five months, the fetus gains an additional 3-3.6 kg
- nervous, respiratory and digestive systems are put in place
- at four weeks after conception, a flat set of cells curls to form a tube – one end swells to form
the brain and the rest form the spinal cord
- during the period of the fetus, all regions of the brain grow, particularly the cerebral cortex
the folded surface of the brain that regulates many important human behaviours
- near the end of the embryonic period, the males develop testes and the female develop ovaries
- in the third month a female fetus develops a vagina and labia – the male fetus, the testes secrete
a hormone that causes them to develop a penis and scrotum – if the hormone is not secreted, the
fetus will have female genitalia
- in the 5th and 6th months after conception, eyebrows, eyelashes and scalp hair emerge – the skin
thickens and is covered with a thick, greasy substance called vernix, which protects the fetus
during its long bath in amniotic fluid
- by 22-28 weeks most systems function well enough to support the fetus if it is born at this time,
this is call the age of viability
- babies have a hard time breathing at this stage though because the lungs have not fully matured,
they cannot regulate their body temperature very well because they lack the insulating layer of
fat that appears in the eighth month of gestation
- changes of the fetal period also mean that the fetus actually emits behaviours and responds to
- external sounds can cause the fetal heart rate to increase
- some are more active than others, and these differences predict infant’s behaviour at 6 months
of age
- some newborns apparently can recognize some sounds they heard during prenatal development
- *cat in the hat experiment – newborns recognized the familiar, rhythmic quality of the story
from their prenatal story-time
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Stage Duration (after conception) Principal Changes
Period of the zygote 0-2 weeks Egg is fertilized, zygote
becomes implanted in wall of
Period of the embryo 3-8 weeks Period of rapid growth, most
body structures begin to form
Period of the fetus 9-38 weeks Huge increase in size, most
body systems begin to
* prenatal development prepares the fetus with skills for living as a newborn – they only take
place when a family provides a healthy environment for the baby-to-be
Influences on Prenatal Development
General Risk Factors
- scientists have three general risk factors: nutrition, stress and the mothers age
- need to have a balanced diet with all the major food groups
- women need to increase their calorie intake about 10-20%
- a woman should expect to gain 11-16 kg during pregnancy and a woman who is underweight
will gain 18 kg and a woman who is overweight should expect to gain 7 kg – 1/3 of this is the
baby, placenta and the amniotic fluid
- another 1/3 comes from increases in a woman’s fat stores and the last comes from the increased
volume of blood and increases in the size of her breasts and uterus
- folic acid is important for the baby’s nervous system – when mothers do not obtain enough of
this and the baby can be a risk for spina bifida
- spina bifida – a disorder in which an embryo’s neural tube does not close properly during
development - the neural tube develops into the brain and spinal cord, when it does not close
properly the result is permanent damage to the spinal cord and the nervous system
- when a woman does not provide adequate nourishment, the infant is likely to be premature and
underweight – inadequate nourishment during the last few months can affect the nervous system
because this is the time of rapid brain growth
- chronic stress – a person’s physical and psychological responses to threatening or challenging
- when a nonhuman mother has constant stress it can often effect the offspring to be smaller or
prone to physical and behavioural problems
- it’s harder to tell the effect on a mother because it would be unethical to experiment – must rely
on correlational studies – women who have more anxiety or stress have smaller children, her
body secretes hormones that reduce the flow of oxygen to the fetus while increasing its heart rate
and activity level; also stress can weaken a women’s immune system making her more
susceptible to illness, this can damage the fetal development; third, pregnant women are more
likely to smoke and drink and less likely to rest, exercise and eat properly
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