Anatomy, Physiology and Assessment of the Pregnant Patient

Pregnancy changes most organ systems. Blood and plasma volume go up. Cardiac output goes up. Glomerular filtration goes up, and gastric emptying slows. Each change alters how the heart handles bleeding, how the lungs meet oxygen demand, how drugs are cleared and how fast the stomach empties. Start with the same primary survey you would use on anyone. Then judge vital signs and drug response against pregnancy physiology and the patient's own trend.

• Gestational age: count it from the last menstrual period or the obstetric estimate. The 20-week mark matters twice: the hypertension definitions use it (see Preeclampsia, Eclampsia and HELLP Syndrome), and so does positioning. An enlarging uterus can compress the aorta and vena cava when the patient lies flat.
• Postpartum window: up to 6 weeks after delivery. Hypertensive disease and eclampsia can still happen in this window.
• Hyperemesis gravidarum (HG): severe vomiting in pregnancy that needs hospital care. Each year it affects 0.3–3% of pregnant women. It is the leading cause of hospitalization in early pregnancy (AOGS systematic review, 2024).
• Nausea and vomiting of pregnancy (NVP): the common, milder form. Before treating it as pregnancy-related, ACOG Practice Bulletin No. 189 tells clinicians to separate NVP from nausea and vomiting with other causes.

Normal findings that can mislead

A heart rate that runs high is not automatically shock. Higher cardiac output and blood volume are expected in pregnancy. Judge the rate against the patient's trend and perfusion. Do not use a normal-looking blood pressure to rule out shock or preeclampsia. Repeat it and look at the trend.

• Blood volume: rises about 45%, or about 1,200–1,600 mL above non-pregnant values. Plasma rises 30–40%. That expansion can hide early blood loss. A normal heart rate or blood pressure does not exclude significant hemorrhage.
• Cardiac output: the rise is about 40%, and most of it comes through stroke volume. Resting heart rate runs 10–20 bpm higher than non-pregnant values.
• Blood pressure: dips in the first and second trimesters, then climbs back toward non-pregnant levels in the third.
• Respiration and blood gas: minute ventilation climbs 40–50%. A PaCO₂ of 30 mm Hg or less is normal in pregnancy. That is a compensated respiratory alkalosis, with bicarbonate 18–22 mmol/L. A PaCO₂ of 35–40 mm Hg is the non-pregnant range. It sits above the pregnancy reference, so it is not reassuring.
• Hematology: plasma expands faster than red-cell mass. Hemoglobin and hematocrit fall as a result, which is physiologic anemia. Fibrinogen rises by up to 50%. The patient is hypercoagulable.
• Supine position: late in pregnancy, the uterus can press on the inferior vena cava. Less venous return means less stroke volume and less cardiac output.

Hyperemesis gravidarum presentation

Persistent vomiting with dehydration, weight loss, inability to keep fluids down, or an altered mental status.

Red Flags

• SBP ≥160 or DBP ≥110 mm Hg after 20 weeks, or up to 6 weeks postpartum, confirmed on repeat (see Preeclampsia)
• Any seizure in pregnancy or up to 6 weeks postpartum with no other cause found
• SpO₂ ≤94%, shortness of breath or pulmonary edema
• Confusion, gait instability or eye movement changes in a patient who has been vomiting (possible Wernicke encephalopathy)
• Vaginal bleeding, abdominal or pelvic pain, or contractions in a pregnant patient
• Inability to keep fluids down for a prolonged period, with dehydration

Don't wait on labs. Most of that work happens in the hospital. In the field, five things change what you do: pregnancy status, blood pressure, glucose, oxygenation and the neurologic screen.

• Preeclampsia or eclampsia: new hypertension after 20 weeks, with a seizure or a severe symptom. Do not blame the vomiting until you have checked BP and protein.
• Hypoglycemia or withdrawal seizure: these are seizure causes the eclampsia definition excludes. Check glucose. Ask about substances.
• Wernicke encephalopathy: confusion, ataxia and eye movement change after prolonged vomiting. Treat before the full triad shows up. The thiamine guidance advises exactly that.
• Other causes of vomiting: gastroenteritis, a medication effect or an abdominal emergency. Sort these out first. ACOG Practice Bulletin No. 189 asks clinicians to separate NVP from them.
• Bleeding or abdominal pain in early pregnancy: could be an ectopic pregnancy or miscarriage. Until one is excluded, treat it as an obstetric emergency.

1. Position: after 20 weeks, place the patient on the left side or in the recovery position. If she must lie flat for airway or access, displace the uterus to the left (NAEMSP 2025 guideline).
2. Vomiting and airway: keep the patient upright or on her side and suction the mouth. A slow stomach raises the aspiration risk.
3. Oxygen: give oxygen for SpO₂ ≤94% and assist ventilation as needed.
4. Vital signs: measure BP with repeat readings. Record the trend, not one number.
5. Glucose: check it per protocol when the patient is vomiting, confused or weak.
6. Notify early: give the receiving facility the gestational age or days postpartum, the vomiting history and last oral intake, and the BP trend.

Doses vary by protocol; follow local protocols and medical direction. These medication steps are for advanced-level clinicians (Paramedic/RN/CCT). Where local scope allows, an AEMT may give them.

• IV access and fluids (Paramedic/RN/CCT): Start an IV for dehydration. Give a crystalloid bolus per protocol, then reassess after each bolus for lung crackles, SpO₂ and dyspnea. RCOG 2024 prefers normal saline with added potassium chloride for hyperemesis hydration, and electrolytes need monitoring. Lactated Ringer's is the choice where protocol specifies it. Be cautious with fluids in preeclampsia. Pulmonary edema is a risk there. See Lactated Ringer's
• Antiemetic order: Know the order. ACOG Practice Bulletin No. 189 names pyridoxine alone or with doxylamine as first-line drug therapy. RCOG 2024 lists antihistamines, phenothiazines and doxylamine with pyridoxine as first-line. Ondansetron and metoclopramide are second-line in hospital guidance. EMS choice depends on protocol. Protocols differ on the first-trimester choice, so follow local medical direction.
• Ondansetron (Paramedic/RN/CCT; second-line; off-label for pregnancy-related nausea): 4 mg IV, IM or ODT for nausea and vomiting. Only chemotherapy and postoperative nausea are on the label. Repeat interval and total dose per protocol. The label caps the total daily dose at 8 mg in severe hepatic impairment (Child-Pugh score 10 or greater). Avoid in congenital long QT. Get ECG monitoring when hypokalemia, hypomagnesemia or other QT-prolonging drugs are present. Hyperemesis often causes those imbalances. Watch for serotonin syndrome with other serotonergic drugs (for example SSRIs, SNRIs, MAOIs, tramadol, fentanyl). See Ondansetron
• Thiamine (Paramedic/RN/CCT): Give it with dextrose to any patient with prolonged vomiting who is confused or unsteady. For suspected Wernicke encephalopathy: 500 mg IV every 8 hours for at least 3 days, per the E3 thiamine page. That is the hospital regimen. The EMS dose is per protocol. See Thiamine
• Dextrose (Paramedic/RN/CCT): treat hypoglycemia per protocol. Do not delay dextrose for a patient with documented hypoglycemia. Thiamine goes in at the same time or right after. See Dextrose
• Metoclopramide (second-line; Paramedic/RN/CCT): 10 mg IV or IM for nausea and vomiting when first-line antiemetics fail. In the US this is off-label for pregnancy-related nausea, so dose per protocol. Give IV slowly. The US label allows undiluted 10 mg over 1–2 minutes, since a rapid push causes anxiety and restlessness. Do not give to a patient with epilepsy or pheochromocytoma, with an active or suspected eclamptic seizure (manage the seizure first), or where stimulating GI motility could be dangerous (obstruction, perforation, GI bleeding). Use with caution in hypertension (catecholamine release). With MAOI therapy, the injection label says to use it cautiously, if at all. Watch for akathisia and dystonia. The labeled dystonia treatment is diphenhydramine 50 mg IM. Boxed warning: tardive dyskinesia. Avoid treatment longer than 12 weeks except in rare cases. See Metoclopramide
• Avoid promethazine injection: ISMP lists it as high-alert. It can cause severe tissue injury. Use another antiemetic per protocol. See Promethazine
• Seizure or severe-range BP: follow the Preeclampsia, Eclampsia and HELLP Syndrome pathway, with magnesium first for seizure.
• Hypotension: Left lateral positioning, a fluid bolus per protocol (cautiously in preeclampsia), then a vasopressor per medical direction if the patient remains hypotensive (Paramedic/RN/CCT). Protocol and medical direction set the agent and the target.

• Hyperemesis gravidarum admission: IV fluids, electrolyte replacement, thiamine and antiemetics, all per ED protocol. A 2024 systematic review found few high-quality trials of HG treatments. Protocol and obstetric input guide the regimen.
• Obstetric consultation: bring in the obstetric team early. That covers any pregnancy beyond the local threshold of viability (set by hospital policy), and any bleeding, abdominal pain or contractions.
• Labs: glucose, electrolytes, urine ketones and a pregnancy test. With bleeding, check blood type and Rh status. Read every result against the patient's gestational age.
• Neurologic deficit with vomiting: treat it as Wernicke encephalopathy until another cause turns up. Thiamine as above.
• Preeclampsia with severe features or eclampsia: manage as in the Preeclampsia, Eclampsia and HELLP Syndrome pathway.

• Destination: a hospital with obstetric services, or the most appropriate local or regional facility.
• In transit: keep the patient on her left side after 20 weeks. Monitor SpO₂, mental status and BP trend. Continue fluids and antiemetics per protocol.
• Aspiration: suction as needed and keep the head elevated. A slow stomach is what raises the risk.
• Handoff: gestational age or days postpartum, vomiting history and last oral intake, every drug with its time (including thiamine and dextrose), glucose, BP trend and any bleeding.

• Pediatrics: pregnant adolescents follow the adult regimens above. Some adolescents conceal a pregnancy or do not recognize one, so ask privately. Follow local confidentiality rules for consent and disclosure. Children who are not pregnant but have vomiting or dehydration get weight-based dosing: Weight-Based Dosing and Vital Signs.
• Pregnancy: Ondansetron safety data are limited and cannot support a firm conclusion. The E3 ondansetron page summarizes the findings. No association with overall major malformations was found. A small oral-cleft signal was reported with first-trimester oral use, but not with IV. Fetal harm from thiamine appears remote, so it is used when Wernicke encephalopathy is possible. Avoid promethazine injection. For breastfeeding decisions, use the lactation statement on each drug page.
• Renal Impairment: pregnancy raises glomerular filtration, and drug clearance shifts with it. Adjust renally cleared drugs per the drug page and protocol. Metoclopramide: with creatinine clearance under 40 mL/min, the FDA label starts at about half the usual dose.
• Hepatic Impairment: ondansetron total daily dose is limited to 8 mg in severe hepatic impairment (Child-Pugh score 10 or greater, per the FDA label). The metoclopramide label makes no hepatic dose adjustment. When preeclampsia is possible, track liver enzymes. See Preeclampsia, Eclampsia and HELLP Syndrome for liver involvement.

• Error: reading a normal-looking blood pressure as reassurance in late pregnancy. Compare the trend and look for bleeding, hypoperfusion and neurologic change.
• Error: dextrose alone for a confused, vomiting patient. Thiamine goes in alongside it.
• Error: lying a pregnant patient flat after 20 weeks with the uterus not displaced. Left lateral position or left displacement per NAEMSP 2025.
• Error: reaching for promethazine injection in HG. ISMP calls it high-alert, and the label warns of tissue injury.
• Error: blaming new confusion or an unsteady gait in a vomiting patient on dehydration alone.
• Error: stopping the assessment at pregnancy status. Ask about a recent delivery, bleeding and the postpartum window.
• Error: running trauma or cardiac arrest in pregnancy as routine. Follow the Trauma in Pregnancy and Cardiac Arrest in Pregnancy pathways.