Pregnancy and Development

Question 1

Development of the Embryo

Answer 1

• Immediately after fertilisation, the zygote begins to divide (mitosis) into many smaller cells
• This process is called cleavage, and occurs in as little as 36 hours after fertilisation
• Division keeps occurring, with the cells becoming smaller and smaller because no new cytoplasm is produced
• Once the zygote has started to divide, it is called a Morula
• Eventually a hollow ball of cells called a Blastocyst is produced, and it attaches to the endometrium
• It is engulfed in the endometrium to receive nourishment, this process is called implantation
• The blastocyst consists of a thin layer of cells surrounding a cavity filled with fluid
• At one side of the cavity is a group of cells called the inner cell mass, this eventually become the embryo
• The endometrium needs to be maintained, so the corpus leteum keeps producing oestrogen and progesterone
• The corpus leteum does not degenerate because of a membrane of the zygote, the Chorion, produces a hormone called Human Chorionic Gonadotropin (HCG)
• This membrane eventually becomes the Placenta
• The Chorion eventually becomes the Placenta

Question 2

Function of the Placenta

Answer 2

• The placenta is an organ created during pregnancy to:
• Nutritional and Excretory: Supply nutrients to and remove waste from the foetus
• Endocrine Gland: Produces hormones including oestrogen and progesterone (Endocrine Gland)
• Respiratory: Transport gases (oxygen and carbon dioxide) to the foetus (Respiratory)
• Immune: Transport antibodies to the foetus

Question 3

Placenta

Answer 3

• The placenta is made from both foetal and maternal tissue
• The foetal tissue originates from the blastocyst that is implanted in the endometrium
• Small branching finger-like projections develop from the outer layers of cells, which house numerous blood vessels called Chorionic Villi
• The chorionic villi are surrounded by pools of the mother’s blood which has collected in the spaces within the endometrium
• The villi are bathed in blood, yet the mothers’ and foetal blood does not mix, because there is the membrane of the villi separating them
• Materials such as oxygen, nutrients and wastes are transported across the membrane

Question 4

Umbilical Cord (Placenta)

Answer 4

• Inside the umbilical cord there are:
• Two umbilical arteries (towards mother)
• which carry blood towards capillaries of chorionic villi.
• wastes and carbon dioxide removed from foetus
• And one umbilical vein (away from mother)
• carries blood from placenta to foetus
• delivers oxygen, nutrients and hormones to foetus
• On the maternal side, blood from mother enters the placenta through the UTERINE ARTERIES.
• Blood flows through the spaces where exchange of substances takes place and leaves again through the uterine veins.

Question 5

Embryonic Membrane

Answer 5

5 embryonic layers form:
- Embryonic disc (forms the embryo)
- Amnion (forms amniotic sac and fluid)
- Chorion (becomes the placenta)
- Yolk Sac (becomes umbilical cord)
- Allantois (not used – becomes part of the umbilical cord)

Question 6

Embryonic Period

Answer 6

• Is the first two months of pregnancy
• All of the adult organs are in place by the end of the embryonic period
• After implantation, the blastocyst develops into three layers of cells called the PRIMARY GERM LAYERS, which will differentiate into all the tissues and organs of the body.
• These are the ECTODERM, MESODERM and ENDODERM.

Question 7

Entoderm

Answer 7

• epithelium tissue, of alimentary canal, respiratory system, tonsils, thyroid, parathyroid and thymus, and bladder, urethra and vagina.

Question 8

Mesoderm

Answer 8

• 3 types of muscle, connective tissues (blood, bone, cartilage), lymph tissue, epithelium of body and joint cavities, kidneys and ureters, reproductive tracts, adrenal cortex and dermis of skin.

Question 9

Ectoderm

Answer 9

• epidermis of skin, hair and nails, eyes, receptor cells, epithelium of mouth, nose and sinuses, anal canal, enamel of teeth, anterior lobe of pituitary and adrenal medulla.

Question 10

Embryonic Membranes

Answer 10

• Early in the embryonic period, four EMBRYONIC MEMBRANES form, to protect and nourish the embryo.

Question 11

Amnion

Answer 11

• First membrane to develop, by day 8 it surrounds embryo and encloses a cavity.
• It secretes AMNIOTIC FLUID into this cavity, which protects embryo by acting as a shock absorber, helps to maintain a constant temperature and allows embryo to move freely.
• The amnion expands with growth of the embryo and foetus.
• It ruptures just before birth (breaking of waters).

Question 12

Chorion

Answer 12

• Formed from the outer cells of the blastocyst together with a layer of mesodermal cells.
• It surrounds the embryo and the other three membranes.
• As amnion enlarges it fuses with inner layer of chorion, which eventually becomes the main part of the foetal side of placenta.

Question 13

Allantois

Answer 13

• The ALLANTOIS isn’t as significant in humans as other animals. In humans they become part of the umbilical cord. The allantois is used for waste exchange.

Question 14

Yolk Sac

Answer 14

• The YOLK SAC isn’t as significant in humans as other animals. In humans they become part of the umbilical cord. The yolk sac is used as an early circulation system.

Question 15

Introduction (Birth)

Answer 15

• The period of pregnancy is called gestation and takes 280 days from the beginning of the last menstrual period
• The process by which the foetus is expelled from the mother’s body at the end of gestation is called birth or parturition
• This is preceded by a sequence of events called labour

Question 16

Before Labour (Birth)

Answer 16

• Hormonal changes occur: the level of progesterone falls slightly while the level of oestrogen increase
• These changes cause the softening of the ligaments of the pelvis
• The hormonal changes also increase the irritability and contractibility of the uterus
• Birth begins with the onset of involuntary contractions of the uterine wall, which the mother feels as labour pains
• The cervix has softened and shortened in length and has probably opened up a little
• The foetus is likely to be facing the mother’s left or right hip bone

Question 17

First Stage of Labour (dilation of the cervix)

Answer 17

• This stage lasts, on average, 8-9 hours for the first pregnancy, and 4 hours for subsequent births
• The contractions which began in the last 3 weeks of gestation become stronger and more frequent
• Eventually they become quite strong and occur every 30 minutes
• This marks the beginning of labour
• The first stage of labour is the time from the onset of labour to the complete dilation of the cervix
• Waves of contraction travel from the upper part of the uterus to the cervix
• With each contraction the muscle fibres of the uterus shorten a small amount, pulling the cervix, so it no longer hangs down into the vagina
• As the contractions become more frequent and stronger the head of the foetus is pushed more forcefully against the cervix
• Eventually the cervix is fully dilated (about 10 cm) and the uterus, cervix and vagina form the birth canal

Question 18

Second Stage of Labour (Delivery of the foetus)

Answer 18

• Lasts from 20 minutes to 2 hours and frequently begins with the bursting of the amniotic membrane and a flow of fluids from the vagina
• The foetus moves its head through the cervix, and its head stretches the vagina
  *This stimulates the mother to contract her abdominal muscles.
• These contractions, together with the contractions of the uterus push the foetus through the vagina
• The foetus head is turned to face the mother’s back
• Once the head has emerged it turns to face the mother’s hips, to allow the shoulders and the rest of the body to move more easily through the birth canal.

Question 19

The Stage of Labour(After Birth)

Answer 19

• After the birth of the baby, the placenta, amnion and chorion are still inside the uterus.
• The umbilical cord is clamped, tied in two places and then cut between the ties.
• The arteries and veins in the umbilical cord contract, either before or immediately after they are cut.
• After a few days the stump of cord dries up and falls away.
• The uterus continues to contract, and about 5 minutes after the birth of the baby, the placenta remains of the umbilical cord, the amnion and chorion are expelled.
• Little blood is lost during this stage as the placenta blood vessels constrict and contractions of the uterus; squeeze shut the uterine vessels that supply the placenta
• Blood clots then form to stop all leakage of blood

Question 20

Initiation of Birth

Answer 20

• The factors initiating parturition are not clearly understood, however the balance of certain hormones is involved
• Oxytocin is very effective at stimulating contraction of the uterus, especially late in pregnancy
• Its effect is thought to be due to an increase in the contractions of receptors sensitive to oxytocins

Question 21

Changes before Birth (Baby)

Answer 21

• The factors initiating parturition are not clearly understood, however the balance of certain hormones is involved
• Oxytocin is very effective at stimulating contraction of the uterus, especially late in pregnancy
• Its effect is thought to be due to an increase in the contractions of receptors sensitive to oxytocins

Question 22

Changes after Birth (Baby)

Answer 22

• After the baby is born it can no longer depend on the placenta, so it’s lungs and liver must become fully functional.
• The first breath of life is triggered by the shock of birth.
• If this does not occur, a slap on the baby’s bottom provides enough stimulus.
• Failing this, clamping the umbilical vessels causes CO2 levels to rise which stimulates the respiratory centre in the brain, which causes lungs to function.
• As the lungs expand with the first breath, they no longer give resistance to blood flow, so the blood flow through the ductus arteriosus begins to decrease.
• A few weeks after birth the ductus arteriosus has become fibrous tissue.
• As larger amounts of blood return to the heart from the lungs, the pressure in the left atrium increases, which causes the flap of the foramen ovale to close over against the wall of the right atrium which blocks the passage.
• Again, eventually this becomes permanently closed.
• If the foramen ovale fails to close, the baby is said to be born with a ‘hole in the heart’. The first indication of this is a bluish tinge due to the insufficient oxygen. Surgery is performed to correct this and provide normal circulation.
• As blood no longer flows through the umbilical vessels, the ductus venosus is no longer used, so it gradually constricts until it is permanently closed off.
• All blood then must flow through the liver before it reaches the heart.

Question 23

Changes to the Mother other after Birth

Answer 23

• Reproductive Organs and Uterus return to non-pregnant state
• No baby means the organs and uterus can relax and return to their non-pregnant state
• Uterus continues to contract after delivery
• Still needs to get rid of the placenta after birth
• Discharge of Fluid from the Body
• Mixture of blood and breakdown of tissues
• Blood volume returns to normal
• No baby means that there is less blood needed to transport nutrients to the mother and baby, and get rid of wastes from both mother and child so the volume returns to normal.
• Pulse is slower
• No baby means the heart doesn’t have to work as hard to get blood to the baby therefore lowering pulse.
• Return of Menstruation shows that the reproductive system is back to norma