Maternal physiology: what changes, and what that does to your numbers

Cardiovascular

Plasma volume rises by 40 to 50 percent, red cell mass by 20 to 30 percent, so the haemoglobin falls even though iron stores are adequate: physiological anaemia. Cardiac output rises 30 to 50 percent, most of it by 20 weeks, from a rise in stroke volume first and heart rate later (resting rate up 10 to 20 beats). Systemic vascular resistance falls under progesterone and the placental circulation, so blood pressure drops in the second trimester and returns to baseline by term.

The supine position after 20 weeks compresses the inferior vena cava and can drop the cardiac output by a quarter: the reason for left lateral tilt in every emergency, and in every scan and examination that takes more than a moment.

Respiratory

• Tidal volume rises 30 to 40 percent under progesterone; respiratory rate barely changes. Minute ventilation up, so PaCO2 falls to about 28 to 32 mmHg: a compensated respiratory alkalosis is the pregnant baseline.
• A PaCO2 of 40 in a pregnant asthmatic is not normal; it is impending failure.
• Functional residual capacity falls 20 percent as the diaphragm rises, so desaturation on induction of anaesthesia is fast. Oxygen consumption is up. Airway oedema makes intubation harder.
• Dyspnoea of pregnancy is common and benign, but it is a diagnosis after pulmonary embolism, peripartum cardiomyopathy and asthma have been considered.

Renal, gastrointestinal, endocrine

• GFR rises 50 percent: creatinine and urea fall. A creatinine of 80 micromol/L that would be normal outside pregnancy is abnormal in it.
• Mild glycosuria and proteinuria up to 300 mg/day are physiological; the collecting systems dilate (right more than left), which is why hydronephrosis of pregnancy is a finding, not a diagnosis, and why pyelonephritis is commoner.
• Gastric emptying slows and the lower oesophageal sphincter relaxes: reflux, and a full stomach for anaesthesia at any hour.
• Thyroid-binding globulin rises so total T4 rises; free T4 and TSH are what you read, against trimester-specific ranges. hCG has weak TSH activity, so TSH dips in the first trimester.
• Insulin resistance rises through the second and third trimester under placental hormones: the physiology gestational diabetes screening looks for.

Haematology and coagulation

• Fibrinogen and factors VII, VIII, X rise; protein S falls; fibrinolysis is reduced. Pregnancy is a hypercoagulable state and venous thromboembolism risk is four to five times higher, highest in the six weeks postpartum.
• D-dimer rises through pregnancy and cannot be used to exclude thromboembolism with the usual thresholds.
• Platelets fall slightly (gestational thrombocytopenia, usually above 100). A platelet count under 100 needs a cause: preeclampsia and HELLP, immune thrombocytopenia, and the rare serious ones.
• Leucocytosis to 15 is normal, and higher in labour; the white count does not diagnose infection in a labouring patient.

Expected or concerning?

Decide for each finding whether it is a normal pregnancy change or something to act on. The reasoning is the point, not the score.