Asthma in pregnancy

At a glance

• About 12 to 13% of pregnant women in Australia have asthma, the commonest chronic disease in pregnancy.
• Uncontrolled asthma and exacerbations raise the risk of preterm birth, low birth weight, small for gestational age, pre-eclampsia and perinatal death. Well-controlled asthma carries little excess risk.
• The medicines are safer than the disease. Continue ICS, ICS-formoterol and oral steroids when needed. The usual error is a woman stopping her preventer when she finds out she's pregnant.
• Normal pregnancy causes a compensated respiratory alkalosis. A "normal" PaCO2 in an asthmatic pregnant woman is a danger sign.
• Around birth, avoid carboprost and be cautious with ergometrine and labetalol.

Physiology that changes interpretation

Measure	Change in normal pregnancy	What it means for asthma
Minute ventilation	Up 30 to 50% (progesterone raises tidal volume, rate barely changes)	Breathlessness of pregnancy affects most women, with normal spirometry
PaCO2	Falls to about 28 to 32 mmHg	35 to 40 mmHg in acute asthma means tiring
Arterial pH	About 7.40 to 7.47, with renal compensation (bicarbonate about 18 to 22 mmol/L)	Less buffering reserve when acidosis develops
FEV1 and PEF	Unchanged	Any fall is disease, not pregnancy
Functional residual capacity	Down 10 to 25% as the diaphragm rises	Faster desaturation during apnoea and intubation

Pitfall: attributing breathlessness to pregnancy without spirometry. Physiological dyspnoea doesn't wake women at night, cause wheeze or reduce FEV1.

Course during pregnancy

• The Australian Asthma Handbook puts worsening at about 40%, with at least 20% needing medical care for an exacerbation. Exacerbations peak in the second trimester and are uncommon in labour.
• Asthma severity before pregnancy predicts the course, and it tends to follow the same pattern in later pregnancies.
• Triggers specific to pregnancy: stopping ICS from fear of fetal harm, viral infections (pregnant women are more susceptible to severe influenza), worse reflux and rhinitis of pregnancy.
• Smoking during pregnancy worsens asthma and harms the fetus. NRT is reasonable if she can't quit without it.

Assessment

• Spirometry, PEF and FeNO are safe. Bronchial provocation testing is contraindicated in pregnancy, so diagnosis rests on symptoms and variable airflow limitation shown by bronchodilator response or serial PEF.
• Review control, inhaler technique, adherence and the written action plan at every antenatal visit, or at least each trimester.
• Look for comorbidities that mimic or worsen asthma: GORD, rhinitis, obesity, sleep-disordered breathing, anxiety.
• Differential for new breathlessness in pregnancy: physiological dyspnoea, PE, peripartum cardiomyopathy, mitral stenosis unmasked by the rise in cardiac output, amniotic fluid embolism around delivery, and pulmonary oedema with pre-eclampsia or tocolytics.

Chronic management

Treat to the same goals and steps as outside pregnancy (see Asthma). Plan before conception where possible and refer for review early in the first trimester if control is poor.

Medicine	Pregnancy position
Salbutamol, budesonide, prednisolone	TGA category A; first choices. Budesonide has the most human data
Other ICS (fluticasone, beclometasone) and LABA (formoterol, salmeterol)	Reassuring data; don't switch a woman who is well controlled on them
Budesonide-formoterol as anti-inflammatory reliever or maintenance and reliever	Continue; the Australian Asthma Handbook uses the same regimens in pregnancy
Montelukast	Continue if it clearly helped before pregnancy
Biologics (omalizumab, mepolizumab, benralizumab, dupilumab)	Can be continued through pregnancy, or started after a risk-benefit discussion. Tezepelumab wasn't covered by the Handbook consensus

• Don't step down during pregnancy unless the current dose is inappropriately high (Australian Asthma Handbook). Step up as needed to regain control.
• Severe asthma: review in the first trimester with respiratory and obstetric input, and enter women on biologics in a pregnancy registry. If a biologic is stopped, restart it soon after birth.
• Intranasal corticosteroids are safe for rhinitis and often improve asthma control.
• Vaccines: influenza at any stage, pertussis (dTpa) at 20 to 32 weeks, maternal RSV vaccine (Abrysvo) at 28 to 36 weeks under the National Immunisation Program, and COVID-19 per current recommendations.

Pitfall: writing "step down ICS now she is pregnant". The fetus is harmed by maternal hypoxaemia, not by budesonide.

Acute asthma

Manage as for a non-pregnant adult, with a lower threshold for admission and escalation.

• Oxygen to SpO2 95% or more. Fetal oxygen delivery depends on maternal PaO2.
• Salbutamol by spacer or nebuliser, ipratropium in severe attacks, and systemic corticosteroids (prednisolone 37.5 to 50 mg daily for 5 to 10 days, or IV hydrocortisone).
• IV magnesium sulfate for severe or life-threatening asthma.
• After about 20 weeks, nurse in left lateral tilt to relieve aortocaval compression.
• Blood gas: PaCO2 above about 35 mmHg is a warning sign. Involve ICU and anaesthetics early; the pregnant airway is harder to intubate and desaturates quickly.
• Fetal monitoring (CTG) once the fetus is viable, with obstetric review. Fetal distress usually improves as the mother is oxygenated.
• Chest X-ray if pneumothorax, pneumonia or another diagnosis is possible. The fetal dose is tiny; don't withhold it.
• Discharge with ICS (or a higher dose), a written action plan and review within a week.

Labour, birth and postpartum

Issue	What to do
Asthma medicines in labour	Continue inhalers. Inhaled beta-agonists don't affect labour
Recent or prolonged systemic steroids (or very high-dose ICS)	IV hydrocortisone cover in labour and around caesarean section, per local protocol, for adrenal suppression
Induction	Prostaglandin E2 (dinoprostone) and oxytocin don't worsen asthma
Third stage and PPH	Oxytocin first. Avoid carboprost (PGF2-alpha, bronchoconstrictor). Ergometrine can cause bronchospasm, especially under general anaesthesia. Misoprostol (PGE1) is acceptable
Hypertension or pre-eclampsia	Avoid labetalol; use nifedipine or methyldopa. Hydralazine IV for severe hypertension
Analgesia	Regional anaesthesia preferred over general. Avoid NSAIDs in aspirin-exacerbated respiratory disease

• Breastfeeding: inhaled medicines, prednisolone and montelukast are compatible. Biologics can be continued after discussion; the infant's inactivated vaccines don't need changing.
• Asthma control usually returns to its pre-pregnancy pattern within about 3 months of birth.

Fetal and maternal outcomes

• Maternal risks with poor control: pre-eclampsia, gestational diabetes (partly from oral steroids), caesarean section and severe exacerbations.
• Fetal and neonatal: preterm birth, low birth weight, SGA, neonatal hypoxia and possibly congenital malformations with first-trimester exacerbations.
• Women whose asthma is actively managed have a preterm birth rate close to that of women without asthma.
• Asthma in the mother raises the child's risk of asthma; maternal smoking adds to it.

Pitfalls

• Normal PaCO2 in acute asthma in pregnancy. It isn't reassuring.
• Stepping down or stopping ICS because of pregnancy.
• Withholding oral prednisolone, IV magnesium or a chest X-ray in a pregnant woman who needs them.
• Carboprost for PPH in a woman with asthma, or labetalol for her blood pressure.
• Calling breathlessness physiological without checking spirometry.
• Relying on D-dimer to exclude PE. It rises through normal pregnancy, so image when PE is suspected.
• Ordering a methacholine or mannitol challenge to confirm the diagnosis in pregnancy.