Maternal hyperoxygenation in fetal CHD: why doctors give oxygen before delivery

If your fetal cardiologist asks you to wear an oxygen mask for ten or fifteen minutes near the end of pregnancy while they repeat your fetal echocardiogram, the request can feel startling. Parents naturally wonder whether the baby is running low on oxygen or whether the mask is meant to fix the heart defect before birth. Neither is true. Across three major 2026 studies in Prenatal Diagnosis, Ultrasound in Obstetrics & Gynecology, and Pediatric Cardiology, researchers clarify what this test actually does: it acts as a safe physiological rehearsal for the delivery room.

What maternal hyperoxygenation actually is (and why it is a test, not a therapy)

Before birth, a baby's lungs are filled with fluid and do not yet exchange gas. Because the fetal lungs are not breathing air, the blood vessels inside them remain tightly constricted, keeping pulmonary vascular resistance high and sending only a small fraction of cardiac output through the lungs. Most blood bypasses the lungs through two fetal shunts: the foramen ovale (an opening between the upper heart chambers) and the ductus arteriosus (a vessel connecting the pulmonary artery to the aorta).

That quiet, low-flow fetal lung circulation creates a blind spot for ultrasound. Certain dangerous bottlenecks, such as a tight foramen ovale that cannot let enough blood cross after birth, or narrowed pulmonary veins that back up under full blood flow, may look borderline or even normal on a standard room-air scan simply because very little blood is trying to pass through the lungs yet.

When an expectant parent breathes high-concentration medical oxygen through a face mask for roughly 10 to 15 minutes in the third trimester (typically between 34 and 38 weeks of gestation), extra oxygen crosses the placenta into the fetal bloodstream. Fetal pulmonary vessels respond by relaxing and dilating, temporarily increasing blood flow into the lungs. In 2026, a systematic review and single-arm meta-analysis published in Prenatal Diagnosis (registered in PROSPERO as CRD420251274436) pooled nine observational studies evaluating maternal hyperoxygenation (MH) in pregnancies with congenital heart disease. Across those studies, MH consistently increased the fetal pulmonary artery velocity-time integral (VTI), confirming augmented pulmonary blood flow, and reduced the middle cerebral artery pulsatility index Z-score (MCA-Z), reflecting normal cerebral circulatory adaptation.

Crucially, the authors of the 2026 Prenatal Diagnosis meta-analysis concluded that MH is best supported as a physiological assessment tool rather than a routine therapeutic intervention. While chronic daily oxygen therapy to try to grow small left-heart structures remains investigational with heterogeneous protocols, brief acute maternal hyperoxygenation (AMH) during a third-trimester echo gives the care team a clear preview of how the baby's heart will behave the moment the umbilical cord is clamped and the lungs take over.

Predicting urgent balloon atrial septostomy in d-TGA: the 2026 UOG cohort

Nowhere is that delivery-room preview more decisive than in dextro-transposition of the great arteries (d-TGA) with an intact ventricular septum or only a tiny ventricular septal defect. In d-TGA, the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle, creating two parallel circuits. After birth, the newborn can only survive the first hours if oxygen-rich blood from the lungs crosses the atrial septum through the foramen ovale to reach the body.

If the foramen ovale is too small or covered by a rigid flap (a restrictive atrial septum), the newborn develops severe hypoxemia immediately after delivery and requires an emergency catheter procedure called a balloon atrial septostomy (BAS) within the first hours of life. Historically, predicting which d-TGA babies would need an emergency BAS in the first three hours versus those who could wait safely has been notoriously difficult on room-air ultrasound alone.

A prospective 2026 single-center cohort study published in Ultrasound in Obstetrics & Gynecology evaluated 34 pregnancies with fetal d-TGA (29 with intact ventricular septum and 5 with a small VSD of 4 mm or less) at 34 to 38 weeks' gestation using concurrent fetal echocardiography and cardiac MRI (CMR) first in room air and then during acute maternal hyperoxygenation. Overall, 23 of the 34 newborns (68%) underwent BAS after birth, and 12 of the 34 (35%) required urgent BAS within 3 hours of delivery due to persistent preductal oxygen saturation below 70%.

What made the UOG study clinically striking was how clearly the oxygen challenge separated urgent cases from stable ones. On room-air ultrasound, abnormal atrial septal flap morphology (such as a tethered, redundant, or bowed flap) already signaled risk (P = 0.004). Once the mother breathed oxygen during AMH, two dynamic signs emerged as the strongest predictors of needing urgent BAS within 3 hours:

• Constricted or continuously reversed diastolic flow across the ductus arteriosus during AMH (P = 0.0006): When pulmonary resistance drops during oxygen breathing, a healthy fetal ductus adjusts smoothly; continuous diastolic reversal or constriction warns that the fetal shunts are under strain and postnatal mixing will be severely limited.
• Effective foramen ovale size of 3.4 mm or less during AMH (P = 0.0024): As extra blood returns from the dilated fetal lungs to the left atrium, an adequate foramen ovale stretches open. If the effective opening stays 3.4 mm or smaller under that flow challenge, the newborn's atrial shunt is unlikely to keep up after birth.
• Ascending aorta to main pulmonary artery oxygen saturation ratio (AAo/MPA SaO2) above 1 on CMR during AMH: Among the 26 fetuses who also underwent fetal cardiac MRI, an inverted saturation ratio during AMH carried an odds ratio of 28.6 (95% CI 1.1 to 620.3; P = 0.0084) for urgent postnatal BAS.

Unmasking hidden pulmonary venous obstruction in TAPVR

The same principle, using extra oxygen to boost fetal lung blood flow so hidden bottlenecks become visible, also applies to rare lesions involving the pulmonary veins. In total anomalous pulmonary venous return (TAPVR), the four veins that carry oxygenated blood back from the lungs connect abnormally instead of draining directly into the left atrium. Before birth, babies with TAPVR are usually stable, but immediately after birth, survival depends on whether that abnormal venous pathway is wide open or physically pinched (obstructed).

Obstructed TAPVR is one of the few true neonatal surgical emergencies in pediatric cardiology, yet it is famously easy to miss on a routine fetal echocardiogram because baseline fetal lung blood flow is so low that a narrowed vertical vein may show normal, low-velocity Doppler waveforms.

A 2026 clinical report in Pediatric Cardiology examined late-gestation maternal hyperoxygenation (MHO) testing in three fetuses with prenatally diagnosed TAPVR. All three fetuses showed preserved pulmonary vasoreactivity when the mother breathed oxygen, increasing flow through the pulmonary veins:

• In the first fetus, vertical vein Doppler waveforms remained normal both at baseline and during MHO; after birth, the baby remained stable without emergent intervention.
• In the second fetus, vertical vein velocities were already elevated at baseline and worsened further during MHO; the baby required emergent surgical repair shortly after birth.
• In the third fetus, vertical vein Doppler findings looked completely normal at baseline on room air, but became distinctly abnormal as soon as MHO increased pulmonary blood flow. That MHO stress response accurately predicted postnatal clinical deterioration and the need for early catheter-based intervention.

For families, this explains why a cardiologist may still ask for a late-gestation oxygen test even when an earlier second-trimester scan looked reassuring. The test is designed to catch the silent bottleneck before labor begins, never after.

Translating oxygen-test results into delivery room Level of Care and PGE1 readiness

Once your fetal cardiology team reviews your third-trimester echo and any maternal hyperoxygenation testing, the findings feed directly into your delivery room Level of Care (LOC) plan. Not every baby with a heart diagnosis needs an emergency catheter lab waiting down the hall, and not every baby needs prostaglandin E1 (PGE1) started at minute one. Understanding which tier your baby falls into helps you know who will be in the delivery room and what the first hour will actually look like.

It is also worth remembering that how teams communicate these plans matters as much as the measurements themselves. A 2026 qualitative study of initial fetal cardiology consultations in the Journal of Pain and Symptom Management found that while clinicians consistently approached parents with empathy, discussions around management options and long-term quality of life varied widely, and families were most engaged when doctors explained concrete day-to-day expectations rather than abstract medical risk. Asking your team to translate any oxygen-test result into a written delivery-room tier removes guesswork during the final weeks of pregnancy.