O&G · Critical care — maternal physiology
Physiological changes of pregnancy for critical care
Also known as Pregnancy physiology · Maternal adaptations · Pregnancy haemodynamics · Critical care in pregnancy · Maternal cardiovascular adaptation · Pregnancy respiratory physiology
The physiological adaptations of pregnancy that change critical care — blood volume, cardiac output, respiratory gases, GFR and creatinine, coagulation, gastric emptying and drug handling — with the RCOG Green-top 56 modifications for maternal collapse.
Checked against its sources on 8 Oct 2026
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Red flags
- A creatinine above 0.8 mg/dL in pregnancy may indicate renal dysfunction — GFR rises about 50%
- Late-pregnancy PaCO2 is 28-31 mmHg; a non-pregnant 'normal' value may be ventilatory failure
- FRC and TLC fall 10-20%, predisposing to hypoxaemia
- From about 20 weeks, the supine position can cut cardiac output by up to 30-40% — displace the uterus
- No response to CPR within 4 minutes over 20 weeks — perimortem caesarean section
A 32-year-old woman at 34 weeks with pneumonia has a PaCO2 of 38 mmHg on her blood gas, and the registrar records it as normal. In late pregnancy PaCO2 normally falls to 28-31 mmHg, so this value may signal ventilatory failure rather than reassurance.[1]
Why the baseline moves
Pregnancy changes almost every organ system — maternal fat and total body water rise, plasma proteins (especially albumin) fall, blood volume, cardiac output and renal and uteroplacental blood flow rise, and blood pressure falls.[1] Hegewald and Crapo emphasise that knowing these adaptations is necessary to distinguish common physiological dyspnoea from disease.[2]
References11ShowHide
- [1]Costantine MM Physiologic and pharmacokinetic changes in pregnancy. Frontiers in pharmacology, 2014.PMID 24772083
- [2]Hegewald MJ, Crapo RO Respiratory physiology in pregnancy. Clinics in chest medicine, 2011.PMID 21277444
- [3]Conrad KP, Davison JM The renal circulation in normal pregnancy and preeclampsia: is there a place for relaxin? American journal of physiology. Renal physiology, 2014.PMID 24647709
- [4]Mahendru AA, Foo FL, McEniery CM, et al. Change in maternal cardiac output from preconception to mid-pregnancy is associated with birth weight in healthy pregnancies. Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology, 2017.PMID 27859800
- [5]Mahendru AA, Everett TR, McEniery CM, et al. The feasibility of prospectively studying maternal cardiovascular changes from before conception. Hypertension research : official journal of the Japanese Society of Hypertension, 2013.PMID 23575384
- [6]Salles GF, Schlüssel MM, Farias DR, et al. Blood pressure in healthy pregnancy and factors associated with no mid-trimester blood pressure drop: a prospective cohort study. American journal of hypertension, 2015.PMID 25376641
- [7]Davison JM Kidney function in pregnant women. American journal of kidney diseases : the official journal of the National Kidney Foundation, 1987.PMID 3554993
- [8]Lawson J, Howle R, Popivanov P, et al. Gastric emptying in pregnancy and its clinical implications: a narrative review. British journal of anaesthesia, 2025.PMID 39443186
- [9]Desai N, Lawson J, Elwen F, et al. The pregnant and postpartum stomach. European journal of anaesthesiology, 2026.PMID 41258806
- [10]Chu J, Johnston TA, Geoghegan J, et al. Maternal Collapse in Pregnancy and the Puerperium: Green-top Guideline No. 56. BJOG : an international journal of obstetrics and gynaecology, 2020.PMID 31845507
- [11]Friedman AM, Campbell ML, Kline CR, et al. Implementing Obstetric Early Warning Systems. AJP reports, 2018.PMID 29686937