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 and electrolytes in O&G, 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 pre-eclampsia with severe features, 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.