Pregnancy Flashcards
Describe the process of implantation
Implantation occurs around day 5 after fertilisation and arises through the interaction of the trophoblast and the epithelial cells. The embedding within the endometrium depends on the invasiveness of the trophoblast cells.
By the time it has implanted there are two distinct cell layers forming from the trophoblast; the syncitiotrophoblast and the cytotrophoblast.
As there is a breach of the epithelium and the blastocyst sits within the stroma, implantation is said to be interstitial.
What are the aims of implantation?
- to establish the basic unit of exchange. This is through 1) primary villi: early. finger like projections of trophoblast
2) secondary villi: invasion of mesenchyme into core
3) tertiary villi: invasion of mesenchyme core by foetal cells - to anchor the placenta
- establish maternal blood flow with the placenta
What are the features of the placenta?
- becomes thinner as foetal requirements increase
* haemomonochorial - only one layer of cells between maternal and foetal blood flows.
Describe the histology of implantation
- decidualisation (preparation of the endometrium for implantation). This provides the balancing force for the invasiveness of the trophoblast.
- remodelling of spiral arteries creates low resistance vascular bed. This maintains high flow to meet foetal demand.
Describe the structure of the placenta and its adaptations for exchange
- formed of a foetal part (chorion frondsum) and a maternal part (decidua basalis/plate)
- between the chorionic plate and decidual plate are intervillous spaces filled with maternal blood.
- decidual septa form between the intervillous space from the decidual plate, although they do not reach the chorionic plate.
- They divide the placenta into compartments known as cotyledons.
What are cotyledons?
They are sections of the placenta, formed from decidual septa which reach down from the decidual plate towards the chorionic plate between the intervillous spaces.
Describe the arrangement of foetal blood vessels within the placenta
- umbilical arteries and veins project into tertiary villi which are bathed in maternal blood
- two umbilical arteries (takes away deox. blood)
- one umbilical vein ( brings in ox. blood)
Describe the factors influencing the passive diffusion of substances across the placenta
- concentration gradient
- barrier to diffusion (loss of syncitiotrophoblast)
- diffusion distance (haemomonochorial)
What are the major substances that are transported across the placenta?
- Simple diffusion; water, electrolytes, urea and uric acid, gasses
- Foetal O2 stores are low therefore sufficient oxygenation is vital
- Facilitated diffusion: glucose
- Active transport: amino acids, iron, vitamins
- Not a true barrier however, ref teratogens
How does the placenta as an endocrine organ support the pregnancy?
- secretes both protein and steroid hormones
- protein; hCG, hCS, hCT and hCC
- hCG is secreted by the syncitiotrophoblast and supports the secretory function of the corpus luteum.
- steroid; progesterone, oestrogen
- steroids maintain the pregnancy. When the corpus luteum gives up the placenta takes over the secretion of progesterone.
Describe the hormonal basis for testing pregnancy
- the fertilised oocyte will form a morula then blastocyst
- part of this is the syncitiotrophoblast
- this secretes human chorionic gonadotrophin (hCG)
- excreted in maternal urine
Describe the functions of the placenta as a provider of passive maternal immunity to the neonate
- immunological competence occurs late in the first trimester, when the foetus makes all of the components of compliment
- foetal immunoglobulins consist entirely of maternal IgG
- transported from mum to baby via Receptor Mediated Pinocytosis
- newborns produce their own IgG but adult levels are not reached until around the age of 3
Briefly outline haemolytic diseases of the foetus
- mother may have previously been sensitised to rhesus antigen (i.e. previous pregnancy)
- IgG can cross the placenta and attach foetal RBC
- rhesus -ve mothers with rhesus +ve foetus are given a rhesus specific IgG. This will bind to the foetal antigen before maternal immune system can intervene
Describe the maternal physiological adaptations to pregnancy
- CVS; increase blood volume, cardiac output, stroke volume and heart rate. Systolic BP is never raised in pregnancy but hypotension is common. This is because progesterone reduces vascular resistance.
- Urinary: GFR increases. Progesterone relaxes ureter smoother muscle increasing stasis and making infection more likely. Pyelonephritis can induce preterm labour.
- Resp: diaphragm displaced, O2 consumption increases, tidal volume increases. Progesterone also induces physiological hyperventilation to blow off extra CO2. This can induce resp alkalosis, which the kidneys balance by producing and reabsorbing more HCO3.
- Carbs and fat: progesterone promotes adipose storage of glucose and increases appetite. It also increases prolactin secretion which increases maternal resistance to insulin
- ketoacidosis unlikely to occur when lipolysis increases as the foetus is able to utilise ketone bodies, removing them from the circulation
- Thyroid: increased TBG and T3/ T4. No increase in free T4 due to increased TBG. hCG directly stimulates T3/4 synthesis, similar to TSH. Can have lowered TSH due to increase neg. feedback.
What are the risks of gestational diabetes?
- Macrosomic fetus
- Still birth
- Increased risk of congenital defects
