Haemostasis, Coagulation, Thrombogenesis and Fibrinolysis
Haemostasis is the controlled formation of a clot at a site of vascular injury. Thrombosis is clot formation in the wrong place, at the wrong time, or at the wrong scale. The difference is not the chemistry of fibrin: it is the localisation, trigger, surface, flow state and regulatory balance around the clot.
In O&G, this system is never abstract. It decides whether a woman survives placental separation, whether an abruption becomes DIC, whether a woman with pre-eclampsia develops microangiopathy, whether postpartum VTE occurs, whether tranexamic acid helps, and why a "normal" fibrinogen can be dangerous during major obstetric bleeding. The same biology also explains heavy menstrual bleeding, von Willebrand disease, antiphospholipid syndrome, inherited thrombophilia, HELLP, amniotic fluid embolism, sepsis-associated coagulopathy and transfusion strategy.
Before any molecule, hold one idea: competent haemostasis rests on four pillars — a healthy vessel wall, enough working platelets, intact coagulation factors and a functioning fibrinolytic system. Every bleeding or clotting problem in obstetrics is a failure (or a deliberate shift) in one or more of these four. When you meet a deranged result, name the pillar first; the molecules follow.
The central idea is then simple:
Endothelium keeps blood fluid. Injury exposes procoagulant surfaces. Platelets build the first plug. Tissue factor and activated platelets generate thrombin. Thrombin converts fibrinogen to fibrin. Fibrinolysis later removes the clot.
Pregnancy moves the entire system toward haemostatic readiness: high fibrinogen, higher factor VII/VIII/X and von Willebrand factor, lower protein S, venous stasis, reduced fibrinolytic capacity and a huge placental wound at delivery. This protects against haemorrhage, but it also creates the highest lifetime VTE-risk window for many women — pregnancy raises venous thromboembolism risk roughly four- to five-fold compared with the non-pregnant state, and the puerperium is higher still.
The System in One Page
| Layer | Core job | Main molecules/cells | O&G meaning |
|---|---|---|---|
| Vascular tone and endothelium | Keep blood flowing; expose injury when damaged | Nitric oxide, prostacyclin, thrombomodulin, tissue factor, vWF | Pre-eclampsia, sepsis, abruption, thrombosis |
| Primary haemostasis | Make a platelet plug | Platelets, vWF, collagen, ADP, thromboxane A2, GPIb, GPIIb/IIIa | Thrombocytopenia, HELLP, ITP, platelet dysfunction |
| Secondary haemostasis | Make fibrin through thrombin generation | Tissue factor, factors VII, X, V, II, VIII, IX, fibrinogen, XIII | PPH coagulopathy, haemophilia carrier states, DIC |
| Natural anticoagulation | Restrict clotting to the injury site | Antithrombin, protein C, protein S, TFPI, endothelial heparan sulphate | Thrombophilia, LMWH action, pregnancy protein S fall |
| Fibrinolysis | Remove fibrin once repair is underway | Plasminogen, plasmin, tPA, PAI-1, PAI-2, alpha-2 antiplasmin | TXA, PPH, abruption, DIC, postoperative bleeding |
| Inflammation-coagulation link | Trap pathogens and repair tissue, but may overshoot | Complement, cytokines, NETs, monocytes, endothelium | Sepsis, COVID-type endotheliopathy, pre-eclampsia, DIC |
Think of haemostasis as a staged emergency response:
- Seal the hole with vasoconstriction and platelets.
- Reinforce the seal with fibrin.
- Limit the repair with anticoagulant brakes.
- Clear the scaffold with fibrinolysis once the vessel wall has healed.
The obstetric examiner often gives you a deranged laboratory value. Your job is to ask: which layer has failed?
Endothelium: the Blood-Contact Organ
The vascular endothelium is not passive lining. It is a dynamic haemostatic organ.
Normal Antithrombotic Endothelium
Resting endothelium prevents clotting by several mechanisms:
| Endothelial action | Mechanism | Result |
|---|---|---|
| Repels platelets | Nitric oxide and prostacyclin reduce platelet activation and vasoconstriction | Blood remains fluid |
| Presents anticoagulant surface | Heparan sulphate enhances antithrombin; thrombomodulin binds thrombin | Thrombin is redirected away from clotting |
| Activates protein C | Thrombin-thrombomodulin activates protein C, with protein S as cofactor | Factors Va and VIIIa are degraded |
| Limits tissue factor | Tissue factor pathway inhibitor restrains TF-FVIIa-FXa | Initiation is controlled |
| Supports fibrinolysis | tPA release promotes plasmin generation | Fibrin is removable |
This is why healthy blood does not clot inside intact vessels despite containing all the required proteins.
There is also a continuous low-grade maintenance role. Tiny capillary defects appear all the time, and platelets are constantly sealing these microdefects with small fibrin plugs that fibrinolysis then quietly clears. This is why severe thrombocytopenia produces spontaneous capillary bleeding (petechiae) even without obvious trauma — the routine repair of microvascular wear-and-tear has failed.
The prostacyclin–thromboxane balance is worth understanding as a threshold, not a switch. Endothelial prostacyclin inhibits platelet aggregation at much lower concentrations than are needed to prevent platelet adhesion. So minor injury allows platelets to adhere but not necessarily to aggregate, and small platelet thrombi are simply washed away; only more significant endothelial damage tips the balance toward thromboxane-driven aggregation and a real clot. The size of the injury, not merely its presence, determines whether a thrombus forms — a principle that recurs in pre-eclampsia, where the prostacyclin-to-thromboxane ratio shifts toward thromboxane.
Activated or Injured Endothelium
Endothelium becomes procoagulant when exposed to trauma, hypoxia, cytokines, complement activation, sepsis, antiphospholipid antibodies, pre-eclampsia biology or placental injury.
| Trigger | Endothelial response | O&G consequence |
|---|---|---|
| Placental separation | Exposes decidual and placental tissue factor | Rapid thrombin generation at the placental bed |
| Pre-eclampsia | Endothelial activation, platelet consumption, vasoconstriction | Hypertension, proteinuria, HELLP, microangiopathy |
| Sepsis | Cytokine-driven tissue factor expression and leak | DIC, capillary leak, organ dysfunction |
| Surgery/trauma | Tissue factor exposure and local inflammation | Postoperative VTE and bleeding risk |
| Abruption | Decidual tissue factor enters maternal circulation | Hypofibrinogenaemia and DIC pattern |
| Amniotic fluid embolism | Acute inflammatory and coagulation activation | Cardiopulmonary collapse plus coagulopathy |
The high-yield concept is endothelial phenotype switching. The same endothelium that normally prevents clotting can become an amplifier of thrombin generation when inflamed or injured.
Primary Haemostasis: Platelets Build the First Plug
Primary haemostasis is the platelet-vessel wall response. It is fast, local and surface-dependent.
Step 1: Adhesion
Vascular injury exposes subendothelial collagen and bound von Willebrand factor (vWF). Platelets tether to vWF through the platelet GPIb-IX-V receptor complex, especially under high shear. Platelet collagen receptors then stabilise adhesion.
| Molecule | Role | Defect pattern |
|---|---|---|
| vWF | Bridges collagen to platelet GPIb; carries factor VIII | Mucocutaneous bleeding, heavy menses, PPH risk |
| GPIb | Platelet receptor for vWF | Bernard-Soulier-type large platelet bleeding pattern |
| Collagen receptors | Stabilise platelet-vessel attachment | Platelet adhesion failure |
This is why vWF is so important in gynaecology. A woman with low vWF may have heavy menstrual bleeding from menarche, surgical bleeding, postpartum bleeding and normal-looking routine PT/aPTT if the abnormality is mild.
Step 2: Activation
Adherent platelets change shape, expose negatively charged phospholipid, and release granule contents.
| Platelet signal | Effect |
|---|---|
| ADP | Recruits and activates nearby platelets |
| Thromboxane A2 | Amplifies platelet activation and vasoconstriction |
| Serotonin | Supports vasoconstriction |
| Calcium | Supports granule release and coagulation enzyme complexes |
| Phosphatidylserine exposure | Provides the catalytic surface for thrombin generation |
Aspirin matters because it blocks platelet cyclo-oxygenase and reduces thromboxane A2 generation. Low-dose aspirin in pre-eclampsia prevention is not "an anticoagulant"; it modifies platelet-endothelial thromboxane/prostacyclin balance.
Step 3: Aggregation
Activated platelets express active GPIIb/IIIa, which binds fibrinogen. Fibrinogen bridges platelets to one another, producing a platelet aggregate.
| Bleeding clue | Likely layer |
|---|---|
| Petechiae, bruising, epistaxis, gum bleeding, menorrhagia | Platelet/vWF/vascular problem |
| Deep muscle haematoma, haemarthrosis, delayed re-bleeding | Coagulation-factor problem |
| Diffuse oozing from wounds and cannula sites in a sick bleeding patient | DIC, dilution, fibrinogen/factor failure |
Obstetrics can obscure these patterns because the uterus can bleed massively from a mechanical source. Still, mucosal bleeding and puncture-site oozing should immediately make you ask whether platelets, fibrinogen or coagulation factors are failing.
Secondary Haemostasis: the Thrombin-Fibrin System
Secondary haemostasis stabilises the platelet plug with fibrin.
The Cascade Is a Laboratory Map
The classic pathway model remains useful for interpreting tests:
| Pathway | Major factors | Screening test |
|---|---|---|
| Extrinsic | Tissue factor, factor VII | PT/INR |
| Intrinsic | Factors XII, XI, IX, VIII | aPTT |
| Common | Factors X, V, II, fibrinogen, XIII | PT and aPTT may both be affected |
This map explains why:
- Warfarin and vitamin K deficiency often raise PT/INR early because factor VII has a short half-life.
- Unfractionated heparin prolongs aPTT because it potentiates antithrombin against thrombin and factor Xa.
- Severe liver disease can prolong both PT and aPTT because many factors are reduced.
- Low fibrinogen may prolong clotting time and, more importantly, produces weak clot.