What Causes Stillbirth Common And Unexplained Factors?

Stillbirth is the loss of a baby at or after 20 weeks of pregnancy in the United States. The causes fall into two broad groups: known complications like placental problems, infections, and umbilical cord accidents, and a large unexplained category where standard testing finds no clear reason. In the U.S., roughly 1 in 175 pregnancies ends in stillbirth, according to the Centers for Disease Control and Prevention.

What surprises many people is how often the answer is “we don’t know.” Even after a thorough evaluation, a cause is never identified in a substantial share of cases. That gap is not a failure of the parents. It reflects real limits in what current testing can detect.

What Causes Stillbirth: The Most Common and Unexplained Factors?

Placental problems are the single largest category of identifiable causes. The placenta is the organ that delivers oxygen and nutrients to the fetus, and when it fails, the consequences can be severe. Conditions like placental abruption (the placenta separating from the uterine wall before birth) and placental insufficiency (inadequate blood flow across the placenta) account for a meaningful portion of stillbirths.

Beyond the placenta, the most frequently identified contributors include:

• Placental complications — abruption, insufficiency, and problems with how the placenta attaches and develops
• Genetic or chromosomal conditions in the baby, some of which are incompatible with survival
• Infections that cross the placenta or affect the fetus
• Umbilical cord accidents — problems with blood flow through the cord
• Maternal health conditions such as poorly controlled diabetes, high blood pressure, and certain clotting disorders
• Birth defects affecting major organs

Then there is the unexplained group. When a full workup — including autopsy, placental examination, and genetic testing — is done, a cause is still not found in a large proportion of cases. The exact share depends heavily on how thorough the evaluation was. When parents decline autopsy or placental testing, the unexplained rate rises simply because fewer answers were sought.

Why Do So Many Stillbirths Have No Clear Cause?

No clear cause is found because current tests cannot detect everything that goes wrong. A “normal” autopsy and a “normal” genetic test do not mean nothing happened. They mean nothing detectable by those tools happened.

Several factors widen this gap. Subtle placental problems can be hard to identify even on careful examination. Some genetic conditions are beyond the reach of standard testing. And timing matters — changes that occurred days before the loss may leave little trace by the time testing is done.

There is also a practical reality. Autopsy and placental examination are the two tests most likely to yield an answer, and both require consent and, in some cases, cost and logistics that families are not prepared for in the middle of grief. When these are skipped, the chance of finding a cause drops.

This is where an honest clarification helps: “unexplained” is a statement about the limits of our testing, not evidence that the loss was random or that nothing could ever have been known.

How Do Placental Problems Lead to Stillbirth?

The placenta is the fetus’s lifeline, and when it fails, the fetus can be deprived of oxygen and nutrients. Placental insufficiency means the placenta is not transferring enough across its surface. Over time this can restrict fetal growth and, in severe cases, lead to fetal death.

Placental abruption is different. Here the placenta pulls away from the uterine wall, cutting off the blood supply suddenly. Abruption can cause severe maternal bleeding and fetal distress, and it is a medical emergency. Some abruption cases are linked to high blood pressure, smoking, and prior abruption, but many occur without any obvious risk factor.

Problems with how deeply the placenta implants into the uterus also matter. When implantation is abnormal, the placenta may not function normally later in pregnancy. These conditions are among the more common findings on placental examination after a loss.

What Role Do Infections and Cord Accidents Play?

Infections can reach the fetus through the placenta or ascend from the birth canal. Some infections known to be associated with stillbirth include certain bacterial infections and, less commonly in well-resourced settings, infections like listeria and cytomegalovirus. Many of these infections cause mild or no symptoms in the mother, which is part of why they go unrecognized until a loss is investigated.

Umbilical cord accidents are another identifiable category. The cord can wrap around the baby’s neck or body, become compressed, or develop problems with blood flow through its vessels. Cord accidents are real and are found in a share of stillbirths. It is also true that many babies are born safely with the cord around the neck, so the presence of a cord issue does not by itself prove it caused the loss. Investigators look for signs that blood flow was actually compromised.

Which Maternal Health Conditions Raise the Risk?

Several maternal conditions are associated with higher stillbirth risk, and most of them are manageable with good prenatal care. Poorly controlled diabetes is one of the strongest. High blood pressure, including preeclampsia, is another. Certain clotting disorders and autoimmune conditions also appear on the list.

Risk also rises with factors like being 35 or older at delivery, carrying multiples, and having had a prior stillbirth. Smoking and obesity are associated with increased risk as well. These are associations, not predictions — most pregnancies with these factors end with a healthy baby.

One point worth stating plainly: the majority of stillbirths occur in pregnancies without any of these risk factors. Risk factors help guide monitoring, but they do not explain most individual cases.

What Tests Help Find a Cause After a Loss?

Several tests can improve the chance of finding an answer. Placental examination is widely considered one of the highest-yield steps and is recommended in most cases. Autopsy of the baby can identify genetic, structural, and infectious causes. Genetic testing — including chromosomal analysis and, in some cases, more detailed testing — can reveal conditions standard screening missed.

Blood tests on the mother can screen for infections, clotting disorders, and other conditions. Reviewing the pregnancy record, ultrasound findings, and any monitoring data helps put the picture together.

Not every family wants every test, and that is a legitimate choice. It is also fair to ask what each test is likely to find and how the answer might change future care. Some clinicians recommend these tests routinely; the strength of evidence varies by test, with placental examination and autopsy generally having the strongest support for identifying causes.

Can Stillbirth Be Prevented?

Many stillbirths cannot be prevented with current knowledge, and it is dishonest to suggest otherwise. What is well established is that managing maternal health conditions, avoiding smoking and alcohol, and attending regular prenatal care reduce risk at the population level.

For pregnancies at higher risk, clinicians sometimes recommend closer monitoring, such as more frequent growth ultrasounds or fetal monitoring near term. Whether and when to recommend early delivery in a higher-risk pregnancy is a genuine clinical judgment call, balancing the risks of prematurity against the risk of continuing the pregnancy. The evidence on optimal timing is not uniform across situations.

One thing that does not have strong evidence behind it: home fetal movement monitors and apps marketed for stillbirth prevention. No large trials have shown these devices reduce stillbirth, and they can create false reassurance or unnecessary alarm. Counting kicks and reporting a change in your baby’s normal movement pattern to your clinician is the approach with broader clinical support.