Q5 LONG ANSWER 10M ★ High-yield

Describe the development, structure and functions of the placenta. Add a note on amniotic fluid.

Anatomy · 1st Professional — Pre-Clinical

Anchoring villi attach to the decidua; free (floating) villi hang in the intervillous space and do the exchanging
Cytotrophoblastic shell anchors the whole placenta to the maternal tissue

Feature	Detail
Shape and size	Discoid, 15–20 cm across, 3 cm thick, 500–600 g
Maternal surface	Rough, dark red, divided into 15–20 cotyledons by decidual septa
Fetal surface	Smooth, shiny, covered by amnion; the umbilical cord is attached near the centre with vessels radiating from it
Intervillous space	Contains about 150 mL of maternal blood, replaced 3 to 4 times a minute
Cord	Two arteries and one vein in Wharton jelly; about 50 cm long

Layer	Early pregnancy	At term
Syncytiotrophoblast	Complete	Thinned, with syncytial knots
Cytotrophoblast	Complete	Largely absent
Villous mesenchyme	Abundant	Much reduced
Fetal capillary endothelium	Central	Dilated and peripheral
Total thickness	About 25 micrometres	About 2 micrometres

The barrier thins tenfold as pregnancy advances, because the fetus grows faster than the placenta and exchange must keep pace
It also becomes progressively less protective against drugs and infection

Respiratory — oxygen and carbon dioxide by simple diffusion; assisted by the higher affinity of fetal haemoglobin and the double Bohr effect
Nutritive — glucose by facilitated diffusion; amino acids, iron and calcium by active transport against a gradient
Excretory — urea, creatinine, bilirubin
Endocrine — hCG (maintains the corpus luteum for the first 10 weeks), human placental lactogen (diabetogenic, spares glucose for the fetus), oestrogen and progesterone
Immunological — transfers maternal IgG from about 20 weeks, giving passive immunity for the first months of life
Barrier — incomplete; most drugs and many organisms cross

Feature	Detail
Volume	About 1000 mL at 36 weeks, falling to 800 mL at term
Source	Early — maternal and amniotic secretion; later chiefly fetal urine
Removal	Fetal swallowing, and absorption across the membranes
Turnover	Completely replaced about every 3 hours
Functions	Cushions the fetus; allows movement and so limb and lung development; maintains temperature; prevents adhesions; assists cervical dilatation in labour

Oligohydramnios — too little; from renal agenesis, obstructive uropathy or ruptured membranes. Causes POTTER sequence: pulmonary hypoplasia, limb deformity and a characteristic flattened face
Polyhydramnios — too much; from oesophageal or duodenal atresia, anencephaly (impaired swallowing), or maternal diabetes
The relationship is logical — if the fetus cannot pass urine there is too little fluid; if it cannot swallow there is too much

Circulation	Route
Fetal	Two umbilical arteries → chorionic plate → villous capillaries → one umbilical vein
Maternal	Spiral arteries of the decidua → intervillous space (bathing the villi directly) → endometrial veins

The two circulations never mix normally — they are separated by the placental barrier; this is why fetal and maternal blood groups may differ safely, and why a breach at delivery causes Rh sensitisation
About 150 mL of maternal blood in the intervillous space is replaced 3 to 4 times a minute
Spiral arteries are invaded and remodelled by trophoblast into wide low-resistance channels; failure of this remodelling is central to pre-eclampsia and to growth restriction
Uterine contractions reduce intervillous flow, which is why prolonged or excessive contraction causes fetal distress

Hormone	Source	Peak	Function
HCG	Syncytiotrophoblast	8–10 weeks	Maintains the corpus luteum until the placenta takes over; the basis of the pregnancy test
Human placental lactogen	Syncytiotrophoblast	Rises to term	Diabetogenic — induces maternal insulin resistance so that glucose is spared for the fetus; the reason gestational diabetes appears
Progesterone	Syncytiotrophoblast	Rises to term	Maintains the endometrium; relaxes smooth muscle, causing constipation, reflux and ureteric dilatation
Oestrogen	Feto-placental unit — the placenta lacks the enzymes to make it alone	Rises to term	Uterine growth and breast development
Relaxin	Corpus luteum and placenta	—	Softens the pelvic ligaments and the cervix

Oestrogen synthesis needs both fetus and placenta — the fetal suprarenal supplies the androgen precursor and the placenta aromatises it, so falling oestriol was once used as a measure of fetal wellbeing

Rh isoimmunisation — prevented by anti-D immunoglobulin to Rh-negative mothers at 28 weeks and after delivery, and after any sensitising event
Torch infections — Toxoplasma, Rubella, Cytomegalovirus, Herpes, and syphilis and HIV, all cross and cause congenital disease
Placental insufficiency gives intrauterine growth restriction; assessed by ultrasound and umbilical artery Doppler
Retained placenta is a cause of postpartum haemorrhage; the placenta must always be examined for completeness after delivery
Cord prolapse is an obstetric emergency — the presenting part compresses the cord and the fetal circulation fails within minutes

All questions in General Embryology