Clinical overview
Intravenous fluid is the most frequently prescribed "drug" in obstetric and gynaecological practice, yet it is the one we evaluate least critically. We hang a litre of Ringer's lactate at booking-in, "run it open" for a hypotensive patient on the trolley, and routinely deliver several litres into the postpartum patient with measured blood loss of 600 mL. Each of those reflexes can harm. The pregnant woman is not simply a non-pregnant adult with a bigger abdomen: plasma volume rises by roughly 40–50% from a baseline of ~2.6 L, red cell mass rises proportionately less (producing the physiological dilutional anaemia of pregnancy), colloid oncotic pressure falls, and capillary permeability rises — particularly in pre-eclampsia. These changes mean that the volume a healthy woman tolerates is large, but the margin before pulmonary oedema in the pre-eclamptic woman is dangerously narrow.
Fluid decisions in O&G span a wide clinical range: the empty, vasodilated patient with a ruptured ectopic or major postpartum haemorrhage who needs aggressive resuscitation toward definitive surgical or transfusion control; the woman with hyperemesis gravidarum who needs correction of a contraction alkalosis and ketosis; the pre-eclamptic woman in whom fluid restriction prevents iatrogenic pulmonary oedema; and the perioperative gynaecology patient managed within an enhanced-recovery framework. "Evaluate the use of fluids" is a higher-order skill: it asks you to choose the right fluid, the right volume, the right rate, and the right endpoint for the specific physiology in front of you, and to recognise that more fluid is frequently the wrong answer. This chapter complements fluids-electrolytes-og, shock-management and resuscitation-in-pregnancy.
Core knowledge
Body fluid compartments and what each fluid does
Total body water is about 60% of lean body weight, split into intracellular (~⅔) and extracellular (~⅓) compartments; the extracellular compartment is further divided into interstitial (~¾) and intravascular plasma (~¼). The clinically important consequence: a litre of isotonic crystalloid distributes across the whole extracellular space, so only roughly a quarter to a fifth remains in the circulation after equilibration. Resuscitating an empty circulation with crystalloid therefore requires volumes several times the deficit, and the remainder becomes interstitial oedema — gut, lung, surgical wound.
- Balanced crystalloids (Ringer's lactate / Plasmalyte): physiologically closer to plasma (lower chloride, contain a metabolisable buffer — lactate or acetate). Preferred for most resuscitation and maintenance in the surgical/obstetric patient.
- 0.9% "normal" saline: sodium 154 mmol/L and chloride 154 mmol/L — chloride well above plasma. Large volumes cause hyperchloraemic metabolic acidosis and may worsen renal vasoconstriction. It remains useful where a chloride-rich fluid is wanted (e.g. resuscitation alongside hypochloraemic alkalosis in severe vomiting), but it is not a neutral default.
- 5% dextrose / dextrose-saline: the dextrose is metabolised, leaving free water that distributes across total body water — almost none stays intravascular. It is a maintenance/free-water fluid, not a resuscitation fluid, and large volumes cause hyponatraemia.
- Colloids (albumin, starches, gelatins): synthetic colloids (hydroxyethyl starches) are now avoided for resuscitation because of renal harm and mortality signals in critical care; their routine use is not supported.
Figure M4.1 — Body-water compartments, crystalloid distribution, and why common intravenous fluids behave differently in O&G patients.
Blood and the obstetric circulation
Crystalloid carries no oxygen and no clotting factors. In haemorrhage it is a bridge, not a destination. Excess crystalloid in major haemorrhage causes dilutional coagulopathy, hypothermia and acidosis — the "lethal triad" — so the modern principle is early blood product replacement rather than crystalloid loading. Pregnancy is a procoagulant state with raised fibrinogen (term levels are higher than non-pregnant), so a "normal" fibrinogen in obstetric haemorrhage is actually low and predicts progression. See postpartum-haemorrhage for the haemorrhage drill itself.
Assessment
Estimating the deficit and the trajectory
Evaluation begins with the question: is this patient volume-deplete, euvolaemic, or volume-overloaded — and which way is she heading? Combine:
- History: vomiting/diarrhoea duration and frequency, oral intake, antepartum or postpartum bleeding, ruptured-membranes fluid losses, bowel prep, fasting time, sepsis.
- Examination: heart rate, blood pressure (and postural drop where safe), capillary refill, mucous membranes, skin turgor, urine output, level of consciousness, and — crucially in pregnancy — chest auscultation and oxygen saturation for early pulmonary oedema.
- The obstetric caveat on vital signs: a healthy young pregnant woman compensates for large volume loss with tachycardia and peripheral vasoconstriction while maintaining a near-normal blood pressure; hypotension is a late and ominous sign. Do not be reassured by a "normal" blood pressure in a bleeding obstetric patient. Conversely, in severe pre-eclampsia, hypertension coexists with intravascular volume depletion and a leaky capillary bed, so the woman can be simultaneously "dry" intravascularly and at high risk of pulmonary oedema.
Investigations
- Urine output is the single most useful bedside endpoint of organ perfusion; aim broadly for ≥0.5 mL/kg/h, interpreted alongside the clinical picture rather than as a target to be chased with fluid boluses.
- Bloods: full blood count (serial haemoglobin lags acute bleeding), urea/electrolytes/creatinine, and an arterial or venous blood gas with lactate — see arterial-blood-gas and basic-investigations-analysis. A rising lactate and base deficit signal hypoperfusion; a hyperchloraemic acidosis suggests saline excess.
- Coagulation and fibrinogen in any significant haemorrhage; point-of-care viscoelastic testing where available.
- Electrolytes guide fluid choice: hypochloraemic, hypokalaemic metabolic alkalosis is the signature of protracted vomiting (hyperemesis); hyponatraemia warns against free-water/dextrose excess.
Dynamic assessment
A single set of observations is a snapshot. Fluid evaluation is iterative: give a defined bolus, then reassess the same endpoints (heart rate, blood pressure, perfusion, urine output, lactate, and chest signs) before deciding on the next. A patient who transiently responds and relapses is still bleeding and needs source control, not repeated boluses.
Management
The four questions for every fluid prescription
Before writing up fluid, answer: What for? (resuscitation, replacement of ongoing losses, routine maintenance, or redistribution). Which fluid? How much, and over what time? What is my endpoint and when will I reassess? A maintenance order and a resuscitation bolus are different prescriptions and should never be conflated.
Resuscitation in the hypovolaemic / haemorrhaging O&G patient
This is an emergency drill.
EMERGENCY — major obstetric/gynaecological haemorrhage and hypovolaemic shock
- Call for help and declare a major haemorrhage; activate the massive haemorrhage protocol.
- Two large-bore IV cannulae (14–16 G); take crossmatch, FBC, coagulation, fibrinogen, U&E and a gas with lactate at the same time.
- Position: in the pregnant woman beyond ~20 weeks, left lateral tilt / manual uterine displacement to relieve aortocaval compression.
- Warmed fluids: give an initial balanced crystalloid bolus while blood is mobilised; reassess after each defined bolus rather than running litres in blind.
- Blood early: move to red cells (and a fixed-ratio product approach toward FFP/platelets/cryoprecipitate or fibrinogen as guided) for ongoing loss — crystalloid does not carry oxygen and dilutes clotting factors.
- Tranexamic acid: in postpartum haemorrhage give 1 g IV early — the WOMAN trial showed a death reduction when given within 3 hours of onset.
- Source control is the priority — uterotonics, examination, surgery/embolisation as indicated; fluid is the bridge to definitive control, never the treatment for the cause.
The shift in major-haemorrhage thinking is away from large-volume pre-emptive crystalloid (which dilutes clotting factors and worsens the lethal triad) and toward early balanced blood-product resuscitation with rapid source control. Crystalloid keeps the circulation filled while products are mobilised; it does not replace them. Detailed shock physiology and vasopressor use sit in shock-management and resuscitation-in-pregnancy.
Figure M4.2 — Major obstetric and gynaecological haemorrhage fluid drill: warmed crystalloid as a bridge, early blood products, tranexamic acid, reassessment, and source control.
Pre-eclampsia and HELLP — restrict, do not load
This is the highest-yield "fluid" point in obstetrics and a classic exam discriminator. In severe pre-eclampsia the capillaries are leaky and colloid oncotic pressure is low, so injudicious fluid floods the interstitium of the lung. Iatrogenic pulmonary oedema is a recognised cause of maternal death in pre-eclampsia, and the principle, in line with NICE guidance, is fluid restriction — commonly described as limiting total intravenous and oral fluids to around 80 mL/h (≈1 mL/kg/h) unless there are other ongoing losses such as haemorrhage. Oliguria in stable pre-eclampsia is usually tolerated and managed conservatively rather than treated with repeated fluid challenges, because chasing urine output with boluses is precisely what precipitates pulmonary oedema. When magnesium sulphate is running for seizure prophylaxis or treatment, fluid balance must be charted hour-by-hour. See pre-eclampsia-and-hellp and hypertension-in-pregnancy.
Maintenance and the fasting/labouring patient
For routine maintenance in an adult who cannot drink, standard teaching is approximately 25–30 mL/kg/day of water, with roughly 1 mmol/kg/day each of sodium and potassium and some glucose to limit ketosis, with daily weights and electrolytes for anyone on maintenance fluids for more than a day or two. Over-prescription of maintenance fluid — particularly excess sodium and chloride — is a common iatrogenic harm. In labour, women are generally encouraged to drink; intravenous fluid is not a routine requirement for the well labouring woman and is reserved for specific indications (regional analgesia, augmentation, restricted oral intake).
Hyperemesis gravidarum
Protracted pregnancy vomiting produces dehydration plus a hypochloraemic, hypokalaemic metabolic alkalosis and ketosis. Rehydrate with an isotonic, potassium-containing crystalloid (added KCl titrated to measured potassium), and use the chloride content thoughtfully — saline-based replacement helps correct the chloride deficit. Avoid dextrose-only regimens as the primary resuscitation fluid, both because they do not address the electrolyte deficit and because of the thiamine consideration: give thiamine before or alongside any glucose load in prolonged vomiting to avoid precipitating Wernicke's encephalopathy. Replace potassium before assuming the alkalosis will correct on saline alone. (See RCOG GTG 69.)
Perioperative gynaecology and ERAS
Enhanced-recovery principles have overturned the old habit of prolonged pre-operative starvation followed by liberal intra-operative fluid. The modern approach is avoidance of prolonged fasting (clear fluids until ~2 hours pre-op), carbohydrate loading where appropriate, avoidance of routine bowel preparation, and goal-directed, near-zero-balance intra-operative fluid therapy — enough balanced crystalloid to maintain perfusion without the bowel oedema, ileus and wound complications of overload. Both excessive and insufficient fluid worsen outcomes; the target is euvolaemia, guided by the procedure and the patient. See eras-principles and perioperative-fluids.
Figure M4.3 — Common O&G fluid traps: restrict in pre-eclampsia, replace chloride and potassium in hyperemesis, minimise routine maintenance, and aim for zero balance in ERAS surgery.
South African context
In the South African setting the practical realities shape fluid practice. Ringer's lactate and 0.9% saline are the workhorse crystalloids on the Essential Medicines List and are universally available; Plasmalyte and human albumin are less freely available at district level, which reinforces balanced-salt-solution and blood-product-based resuscitation rather than reliance on colloids. Blood products may be delayed in rural and district facilities, so the crystalloid-as-a-bridge-to-transfer principle matters: resuscitate to maintain perfusion, declare the emergency, and activate the referral pathway early rather than attempting prolonged definitive management where products and theatre are not immediately available. The Saving Mothers reports of the NCCEMD repeatedly identify obstetric haemorrhage and hypertensive disease as leading, often avoidable, causes of maternal death — and within those, both under-resuscitation of the bleeding woman and over-resuscitation (pulmonary oedema) of the pre-eclamptic woman recur as assessor-flagged avoidable factors. The South African National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024) is the operative obstetric reference and should anchor local protocol. The high background HIV prevalence and associated anaemia and sepsis risk further narrow the margin for fluid error in many of our patients.
Red flags / pitfalls
- Treating a bleeding obstetric patient with crystalloid alone. Crystalloid carries no oxygen and dilutes clotting factors; mobilise blood and tranexamic acid early and pursue source control. A "normal" blood pressure in a young pregnant woman who is tachycardic and peripherally shut-down is a late-compensation warning, not reassurance.
- Liberal fluids in pre-eclampsia/HELLP. Chasing oliguria with boluses precipitates pulmonary oedema — a documented cause of maternal death. Restrict (≈80 mL/h) unless replacing measured ongoing losses, and listen to the chest.
- Using 0.9% saline as a neutral default. Large volumes cause hyperchloraemic acidosis; prefer a balanced crystalloid for resuscitation and maintenance.
- Dextrose/free-water fluids as resuscitation. They do not stay intravascular and cause hyponatraemia; they are maintenance or free-water replacement only.
- Glucose before thiamine in prolonged vomiting risks Wernicke's encephalopathy — give thiamine first or alongside.
- Ignoring potassium in hyperemesis or in any patient on maintenance fluids; the hypokalaemic alkalosis will not correct without potassium replacement.
- One-snapshot prescriptions. Failing to set an endpoint and reassess after each bolus is the commonest reason patients are over- or under-resuscitated. Chart fluid balance hour-by-hour in the sick obstetric patient, especially on magnesium.
- Routine maintenance fluid in the well labouring woman — encourage oral intake instead unless there is a specific indication.
- Synthetic starch colloids for resuscitation — avoid; the evidence shows renal harm.
Evidence anchors
- South African National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024) — the operative obstetric reference for South African practice, including haemorrhage and hypertensive-disease management.
- Saving Mothers / NCCEMD reports (cite the latest triennium) — repeatedly identify obstetric haemorrhage and hypertensive disease as leading, frequently avoidable, maternal-death causes, with both under-resuscitation and fluid-overload (pulmonary oedema) flagged as avoidable factors.
- South African EML — Hospital Level (Adults) — for the available crystalloid and blood-product formulary.
- NICE NG133 — Hypertension in Pregnancy (2019) — fluid restriction in severe pre-eclampsia/eclampsia to prevent pulmonary oedema; magnesium sulphate for seizure prophylaxis/treatment.
- RCOG GTG 52 — Prevention and Management of Postpartum Haemorrhage, and the WOMAN trial (Lancet 2017) — tranexamic acid 1 g IV given within 3 hours of PPH onset reduces death from bleeding; early blood-product resuscitation over large-volume crystalloid.
- RCOG GTG 47 — Blood Transfusions in Obstetrics — transfusion strategy in obstetric haemorrhage.
- RCOG GTG 56 — Maternal Collapse in Pregnancy and the Puerperium — left-uterine-displacement and resuscitation in the collapsed pregnant patient.
- RCOG GTG 69 — Hyperemesis Gravidarum — fluid and electrolyte (potassium, chloride) correction and the thiamine caution.
- ERAS Society gynaecologic/oncology guidelines — avoidance of prolonged fasting, carbohydrate loading, and goal-directed near-zero-balance perioperative fluid therapy.
Note: the maintenance volume figure (~25–30 mL/kg/day), the ~1 mmol/kg/day sodium/potassium estimate, the ~80 mL/h (≈1 mL/kg/h) pre-eclampsia restriction expressed as a per-kilogram rate, the ≥0.5 mL/kg/h urine-output target, and the compartment-distribution physiology are standard teaching presented cautiously and are not all individually line-itemed with exact figures in VERIFIED-SOURCES; confirm exact local thresholds against the SA Maternity Guideline and unit protocol.