14 - Placenta

Question 1

What day does implanation occur and where does it occur?

Answer 1

• Day 6 after it has hatched from zona pellucida
• Usually implants on superior body of uterus but can implant lower down or on previous C-section scars that are not viable or not in the uterus

Question 2

What are some problems with invasion of the conceptus into the endometrial wall?

Answer 2

- Too deep: placenta accreata

- Incomplete invasion: miscarriage or placental insufficiency that can lead to preeclampsia

Question 3

When does the placenta develop and how?

Answer 3

- Week 2 before anything else

• From fetal membranes

- Chorionic sac and amniotic sac separate but then as the amniotic sac enlarges it displaces the chorion and fuses with the chorionic membrane

- Projections around all surfaces of this membrane concentrated into a small disc like space that becomes the placenta

Question 4

What does implanation achieve?

Answer 4

• Basic unit of exchange
• Anchor the placenta
• Establish maternal blood flow

Question 5

What are chorionic villi?

Answer 5

• Villi that sprout from the chorion membrane

- Inner connective tissue core, where fetal vessels can form, and outer layer of syncytiotrophoblasts

• Maternal blood vessels surround villi so exchange can occur but mixing of circulations never

Question 6

How do the chorionic villi change over pregnancy?

Answer 6

1st trimester: barrier between maternal and fetal blood vessels thick. Full layers of cytotrophoblasts and syncytiotrophoblasts

3rd trimester: barrier less, becomes one layer of trophoblast by reducing cytotrophoblasts

Margination of fetal capillaries and loss of trophoblasts

Question 7

How is the degree of invasion of the conceptus controlled?

Answer 7

Decidua = endometrial becomes specialised through decidualisation

If implantation in correct place but decidual reaction is suboptimal can lead to complications like miscarriage, infertility or placental insufficiency leading to pre-eclampsia

Implanation in incorrect place then no decidual reaction so ectopic

Question 8

Label the gross morphology of the placenta.

Answer 8

• Amnion on outside with chorion vessels underneath
• Need to check placenta for cotyledons to check none of them have fallen out and can cause post partum haemorraghe

Question 9

What are the main blood vessels exchanging the placenta and how are they arranged?

Answer 9

• Endometrial arteries bathe the villi
• Paired umbilical arteries carry waste products to villi
• Single umbilical vein takes oxygen and nutrients to fetus

Question 10

What is the endocrine function of the placenta?

Answer 10

• hCG from syncytiotrophoblasts maintain corpus luteum in first trimester. urine and blood tests
• Steroid hormones made to take over corpus luteum and produce enough oestrogen and progesterone to keep body in pregnant state

Question 11

Why may someone have high levels of hCG in their blood?

Answer 11

• Pregnancy specific, from syncytiotrophoblasts
• Molar pregnancy
• Testicular cancer
• Choriocarcinoma

Question 12

How do placenta hormones change the metabolic state of the mother?

Answer 12

• Progesterone: increases appetite to allow increased fat deposition to support fetus and breast feeding

- Human Placental Lactogen (hCS): creates diabetogenetic state by causing insulin resistance to mother so more glucose availability for fetus

Question 13

How does transport across the placenta occur?

Answer 13

- Simple diffusion: water, gases, electrolytes

- Facilitated diffusion: glucose

- Active transport: aa’s, Fe, vitamins

- Receptor mediated endocytosis: IgG’s

Question 14

How does gas exchange occurs across the placenta?

Answer 14

• Simple diffusion
• Flow limited and fetal O₂ stores are low so adequate flow essential
• Can have fetal distress (hypoxia) in labour contractions as they compress uteroplacental circulation

Question 15

How does the baby confer immunity?

Answer 15

• IgG antibodies from mother can pass across placenta via receptor mediated endocytosis
• Protects baby

Question 16

What is rhesus disease of the newborn?

Answer 16

• Haemolytic disorder that can occur due to being able to transfer across placenta. Not common as screened for now
• Causes baby jaundice and anaemia

Question 17

When do teratogens cause the greatest effect in pregnancy?

Answer 17

• Preembryonic: fatal

- Embryonic (3-6 weeks): sensitive as body systems developing

- Fetal (9-36 weeks): +/- sensitive apart from CNS

Question 18

What are some harmful substances to the fetus?

Answer 18

• Thalidomide
• Alcohol (FAS and ARND)
• Therapeutic drugs (antiepileptic, warfarin, ACEi)
• Smoking (low birth weight)
• Drugs (dependency)

Question 19

What are some dangerous infections to have in pregnancy and why?

Answer 19

Mother is in immunocompromised state so infections more serious, need to check mother’s immunisation status. Poor pregnancy outcomes