Placenta, Membranes and Amniotic Fluid – Comprehensive Study Notes

Introduction

The fetus relies on the placenta to survive and grow in utero. It provides nutrients and oxygen, disposes of waste via the maternal circulation, and acts as a protective, endocrine and immunological interface between mother and fetus.
The placenta is the defining feature of all mammals; understanding its development and function underpins interventions that improve fetal outcomes (Gluckman & Hanson 2005).
Key themes: formation, structure, circulation, transfer of substances, amniotic fluid production, and clinical implications of placental and amniotic abnormalities.

Implantation

The placenta derives from embryonic trophoblast cells and a few mesodermal cells from the inner cell mass.
Initial trophoblasts form the cytotrophoblast, which gives rise to the syncytiotrophoblast (trophoblast without cells) via nuclear division without cytoplasmic division.
Syncytiotrophoblast invades the uterine lining to allow embedding by about the 10th day, with a plug of blood clot and cellular debris closing the entry point.
Embryotroph (fluid) forms in lacunae within the syncytiotrophoblast, derived from blood sinusoids and secretions from eroded endometrial glands; this fluid diffuses to the embryonic disc.
Lacunae fuse to form intervillous spaces through which maternal blood flows.
Decidual and myometrial arteries undergo remodeling to become uteroplacental arteries via migratory cytotrophoblasts (MC):
- Endovascular MC invade spiral arterioles, replacing endothelium and degrading tunica media; two waves of invasion occur at 6–10 weeks and 14–16 weeks.
- Interstitial (stromal) MC destroy ends of decidual vessels, promoting blood flow into lacunae.
Maternal arteries become functionally denervated and dilated; local prostacyclin maintains vasodilation of uterine radial arteries.
Mesoderm from the embryonic disc migrates through the primitive streak and joins trophoblast extensions to form the connecting stalk, giving rise to umbilical vessels.
By the end of week 2, trophoblastic cells form primary chorionic villi around the embryo; the chorionic sac consists of a layer of mesoderm near the embryo, cytotrophoblast, and the syncytiotrophoblast adjacent to the endometrium.
The amniotic sac is closest to the uterine wall; the extraembryonic coelom separates the amnion and chorion.

Development of the chorionic villi

Early week 3: a core of loose connective tissue from embryonic mesenchyme invades each primary villus to form secondary chorionic villi.
Some mesenchymal cells differentiate into fetal blood capillaries, forming mature tertiary chorionic villi.
By 15–20 days, arteriocapillary–venous networks connect with embryonic heart vessels; fetal blood begins circulating through villi by end of week 3, enabling exchange between maternal and fetal flows (Figs 12.1–12.2).

Formation of the cytotrophoblastic shell

Cytotrophoblastic cells proliferate and extend through the syncytiotrophoblast to form a cytotrophoblastic shell.
The shell attaches the chorionic sac to the maternal endometrium via specialized stem (anchoring) villi, from which branches grow to maximize exchange.
Placental membrane (up to ~20 weeks) consists of four layers separating maternal and fetal circulations:
- Syncytiotrophoblast
- Cytotrophoblast
- Connective tissue of the mesenchymal core
- Endothelium of fetal capillary
  (Moore et al. 2015)
Maternal and fetal circulations do not mingle unless villous damage occurs.
As pregnancy progresses, the placental membrane thins and fetal capillaries lie very close to the syncytiotrophoblast, increasing exchange efficiency.

Later placental development

After implantation, the decidual reaction spreads, transforming the endometrium into the decidua (shed at term).
Decidua basalis forms the maternal portion of the placenta; decidua capsularis lies over the conceptus; decidua vera/parietalis lines the uterus.
As the conceptus grows, decidua capsularis bulges and fuses with decidua vera; by ~22 weeks, decidua capsularis degenerates and disappears.
The chorionic villi of the decidua basalis branch to form the chorion frondosum (fetal part of the placenta).
By 16 weeks, the placenta reaches full thickness; circumferential growth continues via villous branching; maternal capillaries increase in number and surface area for gas exchange; cellular proliferation stops around 35 weeks, with hypertrophy continuing to term; the fetus can influence placental function via signaling if needs are not met.

The mature placenta: appearance

Structure: flattened discoid organ ≈ 20 cm in diameter; thickness ≈ 2.5 cm at the center; tapers toward the circumference; weight ≈ one-sixth of the baby's weight at term.
Surfaces:
- Maternal surface (attached to decidua) with cotyledons (≈ 20 lobes) separated by sulci; decidua forms septa between lobes.
- Fetal surface covered by amnion; umbilical cord inserts here.
The amnion can be peeled away to reveal the chorionic plate (part continuous with the chorion).

The membranes

Amnion and chorion enlarge until ~28 weeks, then stretch to increase size; rupture during labor is typically due to increased intrauterine pressure and reduced space as contractions occur.
The amnion and chorion are not fused and contain up to ~200 mL of amniotic fluid between them.
The outer chorion adheres to decidua; amnion glides over it aided by mucus; rupture can form amniotic bands, potentially constricting/amputating fetal limbs.
Chorion composition: thick, opaque, friable membrane; thickness at term ~0.02–0.2 mm; chorion laeve cells produce enzymes that can reduce local progesterone and bind it; chorion also produces prostaglandins, oxytocin and platelet-activating factor (PAF) to stimulate myometrial activity.
Amnion structure: inner amnion is tough, smooth, translucent; lines chorion and placenta surface and extends over umbilical cord; at term thickness ≈ 0.02–0.5 mm with five layers; lined by non-ciliated epithelial cells; amnion also produces prostaglandins, particularly PGE2, contributing to labor onset.
Hormonal interactions: rising estradiol–progesterone ratios and prostaglandin activity influence labor; progesterone generally suppresses uterine contractions and modulates membrane potential of myometrial cells.

The umbilical cord

Usually attaches to the center of the placental fetal surface.
Diameter: 1–2 cm; length: 30–90 cm (average ≈ 50 cm).
Contains two arteries and one vein surrounded by Wharton’s jelly (mucoid connective tissue).
Vein is longer than arteries and veins spiral around the vein; loops of vessels may form false knots; rare true knots can cause fetal distress during labor.

The umbilical vesicle (yolk sac) and allantois

By 9 weeks, the umbilical vesicle shrinks to ≈ 5 mm in diameter and detaches from the gut, remaining as a remnant in the umbilical cord.
The allantois degenerates to form the urachus (median umbilical ligament) connecting the umbilicus to the urinary bladder.

The placental circulation

Chorionic villi form a large surface for maternofetal exchange; maternal blood enters intervillous spaces via 80–100 endometrial spiral arteries, bathes the villi, and drains through endometrial veins.
Interference with uteroplacental circulation can cause fetal hypoxia, growth restriction, or fetal death.
Blood flow direction in schematic sections: maternal blood in intervillous spaces; fetal blood remains within chorionic villi capillaries; exchange occurs across the placental membrane.
The fetal–placental circulation is described in Chapter 48 of the text (overview provided here).

Anatomical variations of the placenta

Abnormal placental shapes or attachments require medical evaluation (Table 12.1):
- Succenturiate lobe: a separate placental lobe linked by vessels; risk of infection/hemorrhage if fragments are left after delivery; check for a hole in membranes with vessels leading away from it (Fig. 12.10).
- Battledore placenta: umbilical cord inserts at the placental edge; higher risk of cord detachment late in labor.
- Velamentous cord insertion: cord inserts into membranes outside placental boundary; risk of vessel rupture and fetal hemorrhage (vasa praevia).
- Circumvallate placenta: opaque thickened ridge on fetal surface due to doubling back of membranes; may be associated with growth restriction.
- Bipartite/tripartite placenta: placenta divided into two or three lobes.
- Infarcts and calcifications: infarcts are patches of necrosis from interrupted blood supply; calcifications appear as greyish-white patches; often associated with hypertension; calcifications generally benign.
Major placental pathologies include abruptio placentae and placenta praevia (discussed in Ch. 31).

Functions of the placenta

The placenta facilitates transport of nutrients to the fetus and removal of waste from the fetus.
Immunological role: helps prevent fetal rejection by maternal immune system.
Endocrine role: synthesizes hormones, released into maternal circulation to regulate pregnancy.
Hormones produced include hCG, hPL, placental prolactin, relaxin, progesterone, estrogens; several are steroid or protein hormones.

Endocrine function

Maternal decidua, placenta and fetus coordinate hormonal milieu; decidual production of certain hormones (prolactin, relaxin, prostaglandins) influences pregnancy.
Pregnancy-associated placental protein A (PAPP-A) is produced by decidua and trophoblast; these hormones influence maternal metabolism and immune interactions.
Hormone groups:
- Steroid hormones (e.g., estrogens and progesterone).
- Protein hormones (hCG, hPL, SP, PAPP-A, PAPP-B, PP5).
Box 12.1: Placental protein hormones
- Human chorionic gonadotrophin (hCG): glycoprotein with α and β subunits; β subunit biologically active; rapid rise in early pregnancy; urinary hCG doubles every ~36–48 h; plateaus at ~9 weeks; rises again near term; plasma levels range from $7 ext{–}100 ext{ IU/ml}$ ; used in pregnancy tests via β-subunit antibodies; hCG maintains pregnancy by stimulating ovarian estrogen and progesterone production; hydatidiform mole causes high hCG.
- Human placental lactogen (hPL): 190 amino acids; lacks carbohydrate; levels rise from ≈ 0.3 μg at 10 weeks to ≈ 5.4 μg at 36 weeks, then fall; acts as growth promoter and modulator of glucose metabolism; diabetogenic (antagonizes insulin).
- Schwangerschaftsprotein 1 (SP1): glycoprotein detectable in early pregnancy and late pregnancy; may contribute to immunosuppression.
- Pregnancy-associated proteins A and B (PAPP-A and PAPP-B): PAPP-A rises throughout pregnancy; low levels may indicate poor fetal growth or reduced fetoplacental immune tolerance; PAPP-B increases after 30 weeks and helps assess placental well-being in conditions like preeclampsia and diabetes.
- Placental protein 5 (PP5): small glycoprotein in chorionic villi stroma and syncytiotrophoblast; may inhibit proteolytic activity and placental proteases.
Box 12.1 continues with detailed descriptions of hCG, hPL, SP, PAPP-A, PAPP-B, PP5.
Steroid hormones: estrogens and progesterone
- Estrogens: estrone, estradiol, estriol. Estriol is produced by the fetoplacental unit; fetal liver and adrenals contribute to estriol production; estriol is a marker of fetal well-being; pregnenolone sulfate is converted to estrogens by placental enzymes. In pregnancy, most tissues respond to estrogens; estrogens promote uterine growth, breast development, etc.; estriol rises in normal pregnancy, peak late in gestation; serial estriol assays are not routinely used for fetal well-being.
- Progesterone: produced by the syncytiotrophoblast; supports pregnancy by reducing uterine contractility; participates in endometrial development and decidualization; interacts with relaxin to regulate myometrial cells; progesterone levels rise through pregnancy and are relatively stable; typical values are cited as ~ $275 ext{ nmol/L at }32 ext{ weeks} o 450 ext{ nmol/L at term}$ ; fall in progesterone is not a reliable trigger for labor.

Transfer of substances

The placenta acts as the fetal lung, gut, kidney, and endocrine organ.
Fetal and placental growth increases placental surface area to support fetal needs; surface area of exchange is ≈ $11 ext{ m}^2$ near term.
The placental membrane becomes thinner over pregnancy; vasculosyncytial membranes reduce diffusion distance.
Intracellular vesicles enable transfer of macromolecules (e.g., immunoglobulins).
Transport mechanisms across the placental membrane:
- Simple diffusion of lipid-soluble substances.
- Water pores for water-soluble substances.
- Facilitated diffusion via carrier proteins (e.g., glucose).
- Active transport against gradients (ions like Ca2+ and phosphate; amino acids; some vitamins).
- Endocytosis/pinocytosis of macromolecules.
Transfer rate increases as placental size and blood flow increase; maternal fetal health status (hypertension, alcohol use, etc.) influence transfer efficiency.

Oxygen and carbon dioxide transfer

Fetal respiration relies on diffusion across the placental barrier.
Maternal blood in intervillous spaces is well oxygenated (pO2 ~ $50 ext{ mmHg}$ ).
Fetal blood entering the placenta has much lower oxygen content (pO2 ~ $20 ext{ mmHg}$ ) and rises to ~ $30 ext{ mmHg}$ after oxygenation.
Oxygen diffuses down the partial pressure gradient from mother to fetus.
Three factors augment fetal oxygen delivery:
1) Higher oxygen affinity of fetal hemoglobin (HbF) for O2.
2) Higher fetal hemoglobin concentration (~50% more) than maternal.
3) Bohr effect allows greater O2 loading at lower PCO2, aiding placental transfer.
Carbon dioxide transfer is rapid due to higher lipid solubility; fetal CO2 is excreted as part of fetal metabolism.

Nutrition and transfer of other substances

Fetal needs include amino acids, glucose, calcium/phosphorus for bones/teeth, iron, and trace elements.
Substances pass from mother to fetus via villous walls; the placenta can deplete maternal stores if needed.
Water, electrolytes, and water-soluble vitamins diffuse across membranes.
Glucose transfer: principal substrate for fetal energy; transported by facilitated diffusion via carrier proteins; placenta has some glycogen storage to aid fetal needs.
Amino acids are transported against concentration gradients; placenta stores more amino acids than either circulation.
Lipids: fetus synthesizes fatty acids from carbohydrate; fatty acids can be transferred from mother; cholesterol crosses placenta.
Vitamins: lipid-soluble (A, D, E) transfer down concentration gradient; water-soluble (e.g., vitamin C) transfer may be uphill and not returned to mother.
Trace elements: iron, zinc, copper transferred in small amounts.

Water and electrolyte transfer

Water balance is maintained via hydrostatic and colloid osmotic pressure gradients.
Solutes such as Na+, K+, Ca2+, and PO4^3- freely pass between maternal and fetal compartments.
Excessive hypotonic IV fluids to the mother can disturb fetal water balance, potentially causing hyponatremia in the fetus.

Excretion

The placenta excretes waste products from fetal metabolism into maternal circulation for excretion (urea, uric acid, bilirubin).

Immunological role

The placental membrane acts as a barrier to most bacteria; some infections (syphilis, TB) can cross; most fetal infections are viral (e.g., rubella).
Placental trophoblast shows immune tolerance features, helping prevent maternal rejection of the fetus.
Toward the end of pregnancy, placental transfer of maternal IgG to the fetus (via pinocytosis) provides passive immunity for the first ~3 months after birth.

Amniotic fluid

Production of amniotic fluid (liquor amnii) is an alkaline, clear fluid ~98% water with dissolved substances.
Sources of amniotic fluid:
- Before fetal keratinization, fluid equilibrates freely between fetus and amniotic cavity.
- Amnion cells can secrete fluid (early gestation).
- From week 11 onward, fetal urine becomes a major contributor to amniotic fluid volume; fluid volume increases to ≈ $350 ext{ mL}$ at 20 weeks, 700–1000 mL by 37 weeks, then declines slightly toward term.
- Fetal respiratory and gastrointestinal tracts may secrete fluid.
- Diffusion from maternal interstitial fluid across the amniochorionic membrane (from decidua) also contributes.
Circulation of amniotic fluid: amniotic fluid is in constant circulation; water content changes roughly every $43 ext{ h}$ ; fetal GI tract is a major route of removal (swallowing and absorption into fetal circulation; much diffuses across placenta to maternal circulation; some is excreted by fetus into amniotic sac).
In late pregnancy, the fetal kidneys contribute ~ $700 ext{ mL/day}$ and the lungs ~ $350 ext{ mL/day}$ to amniotic fluid.

Content of amniotic fluid

In early pregnancy the fluid composition resembles fetal tissue fluid; after keratinization (≈17 weeks) the interface with fetal extracellular fluid is reduced and content becomes more like dilute urine.
Mature amniotic fluid includes: electrolytes, proteins and derivatives (urea, creatinine), carbohydrates, lipids, hormones, enzymes, desquamated fetal cells, vernix, lanugo.
As the fetus matures: Na+ and Cl− decrease; urea, uric acid, and creatinine increase; lungs contribute phospholipids (lecithin, phosphatidylcholine) reflecting lung maturity.

Regulation of amniotic fluid quantity

Decidual prolactin and prostaglandin E2 (PGE2) from the amnion regulate amniotic fluid volume.
Amniotic fluid prolactin concentration can reach up to 10x maternal levels, rising in the second trimester and plateauing after 34 weeks.
PGE2 can regulate amniotic fluid by mobilizing it to maternal circulation to balance fetal urine production in the second half of pregnancy.

Functions of amniotic fluid

Critical for normal fetal development:
- Allows symmetric external growth.
- Acts as a barrier to infection.
- Supports normal fetal lung development.
- Prevents adhesion of amnion to fetus.
- Cushions fetus and protects from injury.
- Maintains temperature homeostasis.
- Allows fetal movement and muscular development.
- Helps regulate fetal fluid and electrolyte balance.

Clinical implications: abnormalities of quantity

Abnormal amniotic fluid quantity affects fetal health and development.
Oligohydramnios: amniotic fluid volume is very low, often defined as < $500 ext{ mL}$ at term or reduced below expected values earlier in gestation; associated with fetal growth restriction and renal pathology.
Polyhydramnios: amniotic fluid volume is excessive (often > $2000 ext{ mL}$ or clinically large uterus); fetal swallowing defects, esophageal atresia, anencephaly, and other conditions can contribute; maternal diabetes and Rh incompatibility can contribute; most cases are idiopathic (Box 12.2).

Polyhydramnios

Prevalence: up to ≈ 1.5% of pregnancies.
Types:
- Chronic polyhydramnios: insidious onset around 30 weeks.
- Acute polyhydramnios: early onset around 20 weeks; may be associated with monozygotic twinning or severe fetal anomalies.
Causes (Box 12.2):
- Fetal: multiple pregnancy; CNS anomalies (anencephaly, hydrocephaly, spina bifida); GI anomalies (oesophageal atresia, small bowel atresia, diaphragmatic hernia); cardiac anomalies; hematologic anomalies (α-thalassemia, fetomaternal hemorrhage); skeletal dysplasias; chromosomal/genetic anomalies; intrauterine infections (rubella, syphilis, toxoplasmosis).
- Maternal: diabetes mellitus; Rhesus isoimmunization.
- Placental: chorioangioma; circumvallate placenta.
Box 12.3 (clinical management): management aims to reduce symptoms and risk; maternal comfort measures (upright positioning, antacids), possible amniotic fluid reduction, and monitoring; hydration may modestly increase AFV.
Implications: maternal mortality and fetal mortality rise with increased AFV; obstetric complications include abnormal lie, malpresentation, cord prolapse, preterm labor, rupture of membranes, placental abruption, postpartum hemorrhage.

Oligohydramnios

Definition: amniotic fluid volume less than $500 ext{ mL}$ at term; affects ~4% of pregnancies.
Associations: fetal growth retardation, maternal hypertension or chronic renal disease.
Management depends on gestational age, fetal maturity, and maternal/fetal health; conservative management or delivery may be chosen.
Box 12.4 causes (examples): post-term pregnancy; intrauterine growth restriction (IUGR); PROM; fetal renal anomalies (renal agenesis, urethral obstruction, prune belly syndrome, multicystic/dysplastic kidneys); non-renal anomalies (triploidy, thyroid agenesis, skeletal dysplasia, congenital heart block, twin–twin transfusion); chronic placental abruption.
Box 12.5 management: diagnosis by abdominal exam (uterus small for dates, decreased fetal movement, diminished fetal parts), ultrasound to confirm AFV; amnioinfusion (saline or Ringer's lactate) to restore AFV antepartum or in labor; NICE (2006) cautions that amnioinfusion is invasive and should be done in specialized centers; risks include fetal/neonatal complications.
Prognosis: poor when associated with PROM and fetal anomalies; pulmonary hypoplasia occurs in up to ~60% of fetuses with prolonged oligohydramnios; can be accompanied by amnion nodosum.

Diagnostic uses of amniotic fluid

Biophysical profile (BPP): noninvasive test combining ultrasound for fetal tone, movement, breathing, and AFV to assess fetal well-being; fetal heart rate monitoring can be added.
Amniocentesis: sampling of amniotic fluid for:
- Cellular studies and genetic/chromosomal analysis.
- α-fetoprotein (AFP) for neural tube defects or part of Down syndrome screening.
- Creatinine levels as fetal maturity progresses.
- Bilirubin for hemolytic disease (Rh incompatibility).
- Lecithin/sphingomyelin ratio to assess fetal lung maturity.
- Enzyme studies for inborn errors of metabolism.
Main points (summary):
- The placenta derives from trophoblast and some mesoderm; syncytiotrophoblast invasion and lacunar formation establish maternal blood spaces and exchange.
- Decidua basalis forms the fetal placenta; chorion and amnion give rise to fetal membranes; placental villi form the fetal part of the placenta.
- Placenta produces both steroid and protein hormones; transfer of oxygen, CO2, nutrients, vitamins and minerals supports fetal development.
- Amniotic fluid production and turnover involve the fetus (urine production, lung secretions), maternal diffusion, and placental exchange; AFV is regulated by hormones and can be abnormal in polyhydramnios or oligohydramnios, with significant clinical implications.
- Amniotic fluid analysis and BPP provide important information about fetal well-being and maturity, but procedures carry risks and should be used thoughtfully.

Box highlights (quick reference)

Box 12.1 Placental protein hormones: hCG, hPL, SP1, PAPP-A, PAPP-B, PP5; structural and functional analogies to pituitary hormones; roles in pregnancy maintenance and fetal development.
Box 12.2 Causes of polyhydramnios: fetal (e.g., CNS/GI anomalies), maternal (diabetes, Rh), placental (chorioangioma), and others; most cases idiopathic.
Box 12.3 Polyhydramnios: clinical management considerations; maternal comfort and possible amniotic fluid reduction; monitoring and ultrasound assessment; antenatal planning.
Box 12.4 Causes of oligohydramnios: post-term, IUGR, PROM, fetal renal anomalies, non-renal anomalies, placental insufficiency.
Box 12.5 Oligohydramnios: diagnosis and management strategies, including amnioinfusion as an intervention in select cases.

Summary points (condensed)

The placenta forms early in pregnancy from trophoblast and mesenchyme, with cytotrophoblast giving rise to syncytiotrophoblast that invades the maternal endometrium, establishing fetal–maternal circulation and lacunar spaces.
Chorionic villi development (primary → secondary → tertiary) enables fetal–maternal exchange; the cytotrophoblastic shell anchors the placenta via stem villi.
The placenta acts as a dynamic endocrine organ and a major site of nutrient/gas/waste exchange; it also provides immunological protection to the fetus.
The amniotic fluid environment supports fetal growth, development, temperature regulation, and movement; its quantity is tightly regulated but can be disrupted in polyhydramnios or oligohydramnios with significant fetal/maternal risks.
Diagnostic tools (BPP, amniocentesis) and the interpretation of placental/amnionic abnormalities guide clinical management to optimize fetal outcomes.