What Causes A Baby To Die In The Womb Stillbirth?

Stillbirth is the loss of a baby at or after 20 weeks of pregnancy, before or during birth. It happens in roughly 1 in 175 pregnancies in the United States — about 21,000 babies each year, according to the CDC. The causes are varied and often interconnected. In many cases, more than one factor is at play, and in some cases the exact cause is never identified.

Understanding what causes a baby to die in the womb stillbirth begins with understanding that stillbirth is not a single condition. It is an outcome with many possible pathways — problems with the placenta, infections, umbilical cord complications, chromosomal abnormalities, maternal health conditions, and more. Sometimes several of these overlap.

What Causes A Baby To Die In The Womb Stillbirth?

The most common underlying cause involves the placenta — the organ that delivers oxygen and nutrients to the baby. When the placenta doesn’t function properly, the baby can be deprived of oxygen and nutrients. This is called placental insufficiency. It can develop gradually or happen suddenly.

Placental problems are involved in a large share of stillbirths. Research consistently shows that placental abnormalities — including poor blood flow, blood clots within the placenta, and placental abruption (where the placenta separates from the uterine wall before delivery) — are among the leading contributors.

Other major causes include:

• Chromosomal and genetic conditions. Some babies have chromosomal abnormalities or genetic syndromes that are incompatible with life. These may or may not have been detected during pregnancy.
• Infections. Certain infections can cross the placenta and harm the baby. These include cytomegalovirus (CMV), listeria, group B streptococcus, and syphilis, among others.
• Umbilical cord complications. The cord can become compressed, tangled, or wrapped around the baby in a way that cuts off blood flow. True knots in the cord are uncommon but can be fatal.
• Maternal health conditions. Diabetes, high blood pressure, preeclampsia, and obesity all increase the risk of stillbirth.
• Birth defects. Structural birth defects, particularly those affecting the heart, brain, or lungs, can lead to stillbirth.

In a significant number of cases — estimates vary, but some research suggests roughly one in four to one in three — no cause is ever found, even after a thorough evaluation. That uncertainty is one of the hardest parts for families.

How Common Is Stillbirth and Who Is at Higher Risk?

Stillbirth affects about 1 in 175 pregnancies in the U.S. That number has declined over the past few decades but remains higher than in many other high-income countries.

Several factors are associated with increased risk. These are associations, not certainties — most women with these risk factors have healthy pregnancies.

• Advanced maternal age. Risk increases gradually after age 35 and more sharply after 40.
• Obesity. A BMI of 30 or higher is linked to higher stillbirth risk.
• Diabetes. Both pre-existing and gestational diabetes raise risk, especially if blood sugar is poorly controlled.
• High blood pressure and preeclampsia. These conditions affect placental function.
• Multiple pregnancy. Carrying twins or more increases risk.
• Previous stillbirth. Women who have had one stillbirth have a higher risk in subsequent pregnancies.
• Smoking, alcohol, and drug use during pregnancy. All are linked to increased risk.
• Black race in the U.S. Stillbirth rates are more than twice as high among Black women compared to white women. Research points to systemic factors — including access to care, stress from racism, and socioeconomic disparities — rather than biology alone.

Some risk factors can be managed. Others cannot. And many stillbirths occur in pregnancies with no identifiable risk factors at all.

Can Infections During Pregnancy Cause Stillbirth?

Yes. Certain infections can cross the placenta and cause stillbirth. These are not common, but they are well-documented causes.

Infections linked to stillbirth include:

• Cytomegalovirus (CMV). A common virus that often causes mild or no symptoms in adults but can seriously harm a fetus.
• Listeria. A foodborne bacteria. Pregnant women are advised to avoid unpasteurized dairy, raw sprouts, and certain deli meats because of listeria risk.
• Group B streptococcus (GBS). Many women carry this bacteria without symptoms. It can cause infection in the baby.
• Syphilis. Rates have risen in recent years. Syphilis can cross the placenta and cause stillbirth if untreated.
• Parvovirus B19. The virus that causes fifth disease. It can cause severe fetal anemia.
• Toxoplasmosis. A parasite found in cat feces and undercooked meat.

Routine prenatal care includes screening for some of these. Others require awareness and prevention. Not all infections that reach the fetus cause stillbirth — many are treatable or manageable with monitoring.

What Role Does the Placenta Play?

The placenta is the baby’s lifeline. It delivers oxygen, nutrients, and removes waste. When it fails, the consequences can be severe.

Placental insufficiency means the placenta isn’t delivering enough oxygen or nutrients. This can lead to growth restriction, where the baby is smaller than expected. It can also lead to stillbirth if the insufficiency becomes severe.

Placental abruption is a sudden emergency. The placenta separates from the uterine wall before delivery. This cuts off the baby’s oxygen supply. Abruption can cause heavy bleeding in the mother and is a medical emergency.

Other placental issues linked to stillbirth include blood clots in the placental vessels, infections of the placenta, and abnormal placental shape or cord insertion.

One thing that surprises many people: the placenta is not just a passive filter. It is an active organ that regulates blood flow, hormone production, and immune interactions. When it malfunctions, the effects can be subtle at first and then rapid.

What Is Unexplained Stillbirth?

Even after a full evaluation — including autopsy, placental examination, and genetic testing — some stillbirths have no clear cause. This is called unexplained stillbirth.

The proportion of unexplained stillbirths varies widely depending on how thorough the investigation is. Some studies suggest that with complete evaluation, the unexplained rate drops significantly. But it never reaches zero.

This is frustrating for families and clinicians alike. It also means that “no cause found” does not mean “no cause exists.” It may mean the cause was not detectable with current tools.

Research into the causes of stillbirth is ongoing. Scientists are studying genetic factors, placental biology, and immune responses to better understand what happens in these cases.

What Doesn’t Cause Stillbirth?

Many myths circulate about stillbirth. It’s worth correcting a few.

• It is not caused by the mother’s emotions or stress alone. While severe stress may indirectly affect pregnancy outcomes, there is no evidence that normal emotional ups and downs cause stillbirth.
• It is not caused by having sex during pregnancy. Sex is safe in most pregnancies and does not cause stillbirth.
• It is not caused by exercise. Moderate exercise is generally recommended during pregnancy and does not increase stillbirth risk.
• It is not caused by sleeping on your back for a few minutes. Some research suggests a link between going to sleep on the back in late pregnancy and stillbirth risk, but the evidence is not definitive. The recommendation to sleep on your side is precautionary.
• It is not the mother’s fault. This is not a clinical statement, but it is the truth. In the vast majority of cases, nothing the mother did or didn’t do caused the loss.

What Happens After a Stillbirth Is Diagnosed?

When a stillbirth is diagnosed, doctors typically recommend an evaluation to look for a cause. This can include:

• Autopsy. A thorough examination of the baby. This can identify birth defects, infections, or other conditions.
• Placental examination. The placenta is examined for signs of insufficiency, infection, or clots.
• Genetic testing. Chromosomal analysis and sometimes more detailed genetic testing.
• Maternal blood tests. To check for infections, diabetes, blood clotting disorders, and other conditions.

Not all families choose all of these. But each test increases the chance of finding an answer. And finding an answer can help guide care in future pregnancies.

For many families, the emotional weight of stillbirth is overwhelming. Grief support, counseling, and connecting with others who have experienced similar loss can be important parts of healing. The physical recovery also takes time — the body has just been through pregnancy and birth.

Future pregnancies after stillbirth are often monitored more closely. The specific plan depends on what caused the loss, if known. For unexplained stillbirth, monitoring may include more frequent ultrasounds, fetal heart rate checks, and possibly earlier delivery.