Stillbirth happens when a baby dies in the womb after 20 weeks of pregnancy, and in many cases, the cause involves problems with the placenta, the umbilical cord, or the baby’s development. About 21,000 babies are stillborn in the United States each year, affecting roughly 1 in 175 births. While some stillbirths have a clear medical explanation, between 15% and one-third remain unexplained even after thorough testing.
How Stillbirth Is Classified
A pregnancy loss before 20 weeks is considered a miscarriage. After that point, it’s classified as a stillbirth. The CDC breaks stillbirths into three categories based on timing: early (20 to 27 weeks), late (28 to 36 weeks), and term (37 weeks or later). These distinctions matter because the likely causes shift as pregnancy progresses. Earlier stillbirths are more often linked to genetic problems or infections, while later ones are more frequently tied to placental failure or cord complications.
Placental and Umbilical Cord Problems
The placenta is the baby’s lifeline, delivering oxygen and nutrients from the mother’s blood while clearing waste. When the placenta doesn’t function properly, a condition called placental insufficiency, the baby can slowly be deprived of what it needs to survive. This oxygen deprivation is one of the most common pathways to stillbirth.
Umbilical cord abnormalities account for about 19% of all stillbirths and an even larger share, 28%, of those occurring at or after 32 weeks. These problems include the cord becoming wrapped tightly around the baby (cord entrapment), true knots forming in the cord, twisting or narrowing of the cord, and cord prolapse, where the cord slips ahead of the baby and gets compressed. Any of these can cut off blood flow enough to starve the baby of oxygen. In a large prospective study, nearly half of cord-related stillbirths involved tiny blood clots forming in the cord’s vessels, blocking circulation at a microscopic level.
Birth Defects and Genetic Conditions
About one in five stillborn babies has a major birth defect. These can be structural problems with the heart, brain, kidneys, or other organs, or they can stem from chromosomal abnormalities that make development outside the womb impossible. Some of these conditions are detectable on ultrasound during pregnancy, but others are only discovered after delivery. In the United States, roughly 4,600 stillbirths each year involve a major birth defect.
Maternal Health Conditions
Certain conditions in the mother significantly raise the risk. High blood pressure during pregnancy nearly doubles the odds of stillbirth. Gestational diabetes, particularly in women who were never screened for it, raises the risk nearly fourfold. Preeclampsia, a dangerous combination of high blood pressure and organ damage that develops during pregnancy, can severely restrict blood flow to the placenta.
A history of previous stillbirth is one of the strongest predictors. Women who have experienced a prior stillbirth face nearly ten times the risk in a subsequent pregnancy, making close monitoring essential. Being over 35 also increases risk, with odds roughly 1.8 times higher than for younger mothers.
Infections That Can Cause Fetal Death
Several types of infections can reach the baby or damage the placenta enough to cause stillbirth. Bacterial infections are a major contributor. Syphilis infects the placenta and progressively cuts off blood flow to the baby. Listeria, a foodborne bacterium, can cause both placental damage and direct infection of the fetus. Common bacteria like E. coli can travel upward from the birth canal into the amniotic fluid; if the baby breathes in the contaminated fluid, it develops lung inflammation.
Among viral infections, parvovirus B19 (the virus behind “fifth disease” in children) crosses the placenta and destroys the cells that make fetal red blood cells, causing severe anemia that can be fatal. Rubella damages the placenta and can cause lethal birth defects. Even influenza can contribute to fetal death through high maternal fever and the body’s inflammatory response, without the virus ever reaching the baby directly.
Parasitic infections also play a role, particularly in certain parts of the world. Malaria caused by Plasmodium falciparum infects the placenta and blocks blood flow, restricting oxygen and nutrients. Toxoplasmosis, spread through undercooked meat or cat feces, has been linked to stillbirth in case reports.
Smoking, Obesity, and Other Modifiable Risks
Smoking during pregnancy increases the risk of stillbirth by about 42%. This holds true for both deaths that happen before labor begins and those that occur during labor itself. The encouraging finding is that women who quit smoking by the start of the second trimester show no increased risk at all, with odds essentially returning to baseline.
Maternal obesity is consistently identified as one of the major modifiable risk factors for stillbirth in Western countries. Excess weight contributes to inflammation, poor placental development, and higher rates of conditions like gestational diabetes and preeclampsia that independently raise stillbirth risk.
Why So Many Cases Go Unexplained
Even with a full autopsy and placental examination, roughly 15% of stillbirths in high-income countries have no identifiable cause. When less thorough testing is done, the unexplained rate climbs to between one-third and one-half of all late-term stillbirths. This is one of the most frustrating realities for families. Researchers have proposed that many unexplained cases may involve a combination of subtle vulnerabilities, perhaps a mildly underperforming placenta combined with a baby that was already slightly stressed, where no single factor alone would have been fatal.
Changes in Fetal Movement as a Warning Sign
One pattern that shows up repeatedly in stillbirth research is that mothers often notice their baby moving less in the days or weeks before the death occurs. Studies have found that pregnancies ending in stillbirth were more frequently associated with abnormal fetal movement in the preceding two weeks. This happens because when the placenta isn’t delivering enough oxygen, the baby conserves energy by moving less.
There’s no magic number of kicks or movements that qualifies as “normal,” because every baby has its own pattern. What matters is a change from that pattern. If your baby has stopped moving or is moving noticeably less than usual, that’s considered an urgent warning sign by both the CDC and major obstetric organizations. Seeking medical evaluation promptly when you notice reduced movement gives providers the chance to assess the baby and intervene if needed.
Testing After a Stillbirth
When a stillbirth happens, a thorough evaluation can help identify the cause and guide decisions for future pregnancies. Families are typically offered a fetal autopsy, which is the single most useful diagnostic test available. If a full autopsy feels too difficult, alternatives like a partial autopsy, external examination by a trained pathologist, or MRI imaging can still provide valuable information.
Genetic testing is another key piece. The best samples for chromosomal analysis come from amniotic fluid or a small piece of placental tissue taken from beneath the cord insertion site. Providers also examine the placenta itself under a microscope, looking for signs of infection, blood clots, or structural abnormalities that could explain the death. A detailed maternal history, including medications, infections during pregnancy, and a three-generation family history, helps round out the picture. Women who experience a stillbirth are also typically tested for antiphospholipid syndrome, an immune condition that causes abnormal blood clotting in the placenta.