Pulmonary hypertension in newborns happens when the blood vessels in the lungs fail to relax after birth, keeping blood pressure in those vessels dangerously high. This condition, called persistent pulmonary hypertension of the newborn (PPHN), affects roughly 1 to 2 out of every 1,000 live births and is most common in full-term and post-term babies. The causes range from lung diseases and birth complications to structural problems and even certain medications taken during pregnancy.
How Circulation Normally Changes at Birth
Understanding what goes wrong starts with understanding what’s supposed to happen. Before birth, a baby’s lungs are filled with fluid and don’t participate in breathing. The blood vessels in the lungs are tightly constricted on purpose, because the placenta handles oxygen delivery instead. Chemicals called vasoconstrictors keep those vessels narrow, while vasodilators that would open them up remain at low levels.
When a baby takes its first breath, everything shifts rapidly. Air fills the lungs, oxygen levels rise, and the body releases a surge of chemicals that relax the pulmonary blood vessels. Blood pressure in the lungs drops dramatically, and blood flows freely through them to pick up oxygen. PPHN occurs when this switch doesn’t happen. The blood vessels stay constricted, blood bypasses the lungs, and the baby doesn’t get enough oxygen.
Lung Diseases That Trigger PPHN
The most common cause of PPHN is an underlying lung problem that prevents normal oxygen exchange and triggers a dangerous feedback loop: low oxygen causes the lung vessels to constrict, which reduces blood flow to the lungs, which drives oxygen even lower.
Meconium Aspiration
Meconium is the thick, tar-like stool a baby produces before birth. If a baby inhales it during a difficult delivery, the meconium can block airways. Complete blockages cause portions of the lung to collapse. Partial blockages create a “ball-valve” effect where air gets in during inhalation but can’t escape during exhalation, trapping air and damaging tissue. In severe cases, the resulting oxygen deprivation and blood acidity worsen pulmonary hypertension in a self-reinforcing cycle.
Pneumonia and Sepsis
Infections acquired before, during, or shortly after birth can also trigger PPHN. Group B Streptococcus, one of the most common bacteria involved, produces specific compounds that directly constrict the lung’s blood vessels. Gram-negative bacterial infections work through a different but equally harmful path: toxins released by these bacteria cause the body to flood the bloodstream with inflammatory chemicals that tighten the pulmonary vessels. The inflammation from pneumonia also damages the lung tissue itself, compounding the problem.
Respiratory Distress Syndrome
Premature babies sometimes lack surfactant, the substance that keeps the tiny air sacs in the lungs from collapsing. Without it, the lungs can’t expand properly, oxygen levels fall, and the pulmonary vessels constrict in response.
Structural and Developmental Problems
Some babies develop PPHN not because of a triggering event at birth, but because their lungs or lung blood vessels didn’t develop fully in the womb.
Congenital diaphragmatic hernia (CDH) is a well-known example. In CDH, a hole in the diaphragm allows abdominal organs to push up into the chest cavity during fetal development, crowding the lungs. For years, doctors assumed this crowding alone explained why the lungs were underdeveloped. Research has since shown that at least some of the lung underdevelopment in CDH appears to be an intrinsic problem with how the lung tissue forms, not just a result of physical compression. Either way, the baby is born with smaller lungs that have fewer blood vessels, making the remaining vessels unable to handle normal blood flow. Pulmonary hypertension and lung underdevelopment are the primary factors that determine whether a baby with CDH survives the newborn period.
Low amniotic fluid during pregnancy (oligohydramnios) can cause similar lung underdevelopment, because the fetus needs to “breathe” amniotic fluid in and out of the lungs to stimulate their growth.
Idiopathic PPHN: No Obvious Lung Disease
In some newborns, the lungs look clear on a chest X-ray and there’s no infection, no meconium, and no structural problem. Yet the baby has severe oxygen deprivation. This is called idiopathic PPHN, and it results from abnormal remodeling of the blood vessels in the lungs before birth. The muscular walls of the pulmonary arteries become excessively thick during fetal development, making them resistant to the normal signals that should relax them after delivery. Why this remodeling happens isn’t always clear, though chronic oxygen deprivation in the womb and certain maternal exposures are suspected contributors. Doctors sometimes refer to chest X-rays in these cases as showing “black lungs” because the lung fields appear dark and clear, with no visible disease to explain the baby’s condition.
Birth Complications and Oxygen Deprivation
Perinatal asphyxia, meaning the baby is deprived of oxygen during or just before delivery, is a direct and sometimes reversible cause of PPHN. When oxygen drops suddenly, the pulmonary vessels clamp down hard. This response is mediated by a specific signaling pathway that makes the muscle cells in vessel walls extra sensitive to calcium, causing them to contract forcefully. The good news is that asphyxia-related PPHN is often reversible once adequate oxygen and ventilation are restored, because the underlying blood vessels are structurally normal. The constriction is a reaction, not a permanent change.
Other conditions that thicken the blood itself can also contribute. Polycythemia, where the baby has too many red blood cells, makes the blood more viscous and harder to push through the small pulmonary vessels, raising pressure.
Maternal Medications as a Risk Factor
Certain medications taken during the second half of pregnancy are linked to PPHN. The strongest evidence involves SSRI antidepressants. A study published in the New England Journal of Medicine found that babies exposed to SSRIs after the 20th week of pregnancy had roughly six times the odds of developing PPHN compared to unexposed babies. Importantly, SSRI use limited to the first half of pregnancy showed no increased risk, and non-SSRI antidepressants were not associated with PPHN at any point during pregnancy. This timing pattern suggests that SSRIs may interfere with the normal development or relaxation capacity of the pulmonary blood vessels during a critical window in the second and third trimesters.
NSAIDs, a class of common pain relievers, have also been investigated as potential contributors. These medications can cause premature narrowing of a key blood vessel (the ductus arteriosus) in the fetus, which may alter blood flow patterns and contribute to pulmonary vascular changes before birth.
Long-Term Outlook for Babies With PPHN
PPHN is treatable, but it’s a serious condition. Treatment typically involves supplemental oxygen, mechanical ventilation, and in moderate to severe cases, inhaled nitric oxide, which is a gas that directly relaxes the pulmonary blood vessels. Severe cases may require a heart-lung bypass machine (ECMO) to keep the baby alive while the lungs recover.
Recovery depends heavily on the underlying cause. Babies whose PPHN stems from a reversible trigger like asphyxia or mild meconium aspiration generally do better than those with underdeveloped lungs or severe vascular remodeling. A Johns Hopkins study tracking babies with moderately severe PPHN found that at one year of age, 46% had some form of neurodevelopmental or hearing impairment. Specifically, 30% had cognitive delays, 19% had hearing loss, and 13% had major neurological abnormalities. About 22% required rehospitalization during their first year. These numbers reflect babies at the more severe end of the spectrum, and milder cases often have better outcomes, but they underscore why early identification and aggressive treatment matter.