Physiological changes of pregnancy for critical care

A 32-year-old woman, 34 weeks pregnant, is admitted to your labour ward with pneumonia. The registrar charts her respiratory rate at 24 and her SpO2 at 94% and writes "stable". Both are wrong. A respiratory rate of 24 in late pregnancy is normal only at rest, not in pneumonia; an SpO2 of 94% in a woman whose PaO2 should be 101–105 mmHg is the slope toward respiratory failure. The physiology of pregnancy is the lens through which every critical care number is read; without it, normal looks sick and sick looks normal. This topic is the lens.[1][11]

Overview and definition

The physiological adaptations of pregnancy are a coordinated, hormonally-driven programme that begins in the first trimester and affects every organ system. They are not pathology — they are how a woman carries a fetus and survives the blood loss of delivery. But in critical illness they become the trap: the baseline shifts, the reserve narrows, and the woman who looks well on a non-pregnant chart is already decompensating on a pregnant one.[1][2]

Three principles run through the whole topic:[1][11]

• The pregnant baseline is not the non-pregnant baseline. Heart rate is faster, blood pressure lower, PaCO2 lower, creatinine lower, fibrinogen higher, white cell count higher. Reference ranges must be re-set.
• The adaptations are compensations with a ceiling. Cardiac output, minute ventilation and GFR are already maxed out — the system has little additional reserve to give when illness strikes.
• Position and timing matter. Supine hypotension of late pregnancy (aortocaval compression) is iatrogenic until proven otherwise; left lateral position is the first manoeuvre of every resuscitation above 20 weeks.[11]

References12Show ledgerHide ledger

1. [1]Costantine MM Physiologic and pharmacokinetic changes in pregnancy Front Pharmacol, 2014.PMID 24772083
2. [2]Sanghavi M, Rutherford JD Cardiovascular physiology of pregnancy Circulation, 2014.PMID 25223771
3. [3]Hegewald MJ, Crapo RO Respiratory physiology in pregnancy Clin Chest Med, 2011.PMID 21277444
4. [4]Conrad KP, Davison JM The renal circulation in normal pregnancy and preeclampsia: is there a place for relaxin? Am J Physiol Renal Physiol, 2014.PMID 24647709
5. [5]Mahendru AA, Foo FL, McEniery CM, Everett TR, Wilkinson IB, Lees CC Change in maternal cardiac output from preconception to mid-pregnancy is associated with birth weight in healthy pregnancies Ultrasound Obstet Gynecol, 2017.PMID 27859800
6. [6]Mahendru AA, Everett TR, McEniery CM, Wilkinson IB, Lees CC The feasibility of prospectively studying maternal cardiovascular changes from before conception Hypertens Res, 2013.PMID 23575384
7. [7]Salles GF, Schlüssel MM, Farias DR, Franco-Sena AB Blood pressure in healthy pregnancy and factors associated with no mid-trimester blood pressure drop: a prospective cohort study Am J Hypertens, 2015.PMID 25376641
8. [8]Davison JM Kidney function in pregnant women Am J Kidney Dis, 1987.PMID 3554993
9. [9]Lawson J, Howle R, Popivanov P, Sidhu J, Gordon C, Leong M, Onwochei D, Desai N Gastric emptying in pregnancy and its clinical implications: a narrative review Br J Anaesth, 2025.PMID 39443186
10. [10]Desai N, Lawson J, Elwen F, Howle R The pregnant and postpartum stomach Eur J Anaesthesiol, 2026.PMID 41258806
11. [11]Chu J, Johnston TA, Geoghegan J Maternal Collapse in Pregnancy and the Puerperium: Green-top Guideline No. 56 BJOG, 2020.PMID 31845507
12. [12]Friedman AM, Campbell ML, Kline CR, Wiesner S, D'Alton M, Shields LE Implementing Obstetric Early Warning Systems AJP Rep, 2018.PMID 29686937