A baby can die in the womb from problems with the placenta, genetic abnormalities, infections, umbilical cord complications, or maternal health conditions like high blood pressure and diabetes. In the United States, about 21,000 babies are stillborn each year, affecting roughly 1 in 175 births. Despite advances in prenatal care, more than 60% of these losses remain unexplained even after thorough medical evaluation.
A pregnancy loss before 20 weeks is classified as a miscarriage. After 20 weeks, the death of a baby before birth is called a stillbirth. The causes overlap in some cases, but stillbirth involves a more developed baby and often has distinct underlying factors.
Placental Problems
The placenta is the organ that delivers oxygen and nutrients from your blood to the baby. When it fails, the baby slowly or suddenly loses its lifeline. Placental abnormalities account for roughly 24% of stillbirths, making them one of the most common identifiable causes.
Placental insufficiency develops when blood vessels connecting the uterus to the placenta don’t form properly early in pregnancy. This creates a low-flow, high-resistance blood supply that progressively starves the baby of oxygen. The baby’s body responds by redirecting blood toward the brain and heart at the expense of other organs like the kidneys and gut. Over time, chronic oxygen deprivation leads to slowed growth, reduced movement, and in severe cases, death.
Placental abruption is a more sudden event where the placenta partially or completely separates from the uterine wall before delivery. This cuts off blood flow abruptly and can cause fetal death within minutes to hours depending on the severity. Abruption is one of the obstetric emergencies most closely linked to stillbirth.
Umbilical Cord Complications
The umbilical cord is the baby’s only connection to the placenta, and problems with it contribute to about 10 to 19% of stillbirths. In a large study analyzing 496 stillbirths, 94 involved some type of cord abnormality.
The most common cord-related issue isn’t a dramatic event like the cord wrapping around the baby’s neck. Nearly half of cord-related stillbirths involved compromised blood flow through the tiny vessels within the cord itself. Cord knots, twisting, and narrowing accounted for about 27% of cord-related deaths, and these occur in less than 2% of all pregnancies. Cord entanglement around the neck or body made up 29%. Cord prolapse, where the cord slips ahead of the baby and gets compressed, is rare (about 1% of all stillbirths) but carries a high mortality rate when it happens.
Genetic and Chromosomal Abnormalities
Roughly 10 to 20% of stillbirths are linked to chromosomal problems in the baby. These are errors in the baby’s DNA that occur at or near conception, not something caused by anything the parents did during pregnancy. Some of these abnormalities cause structural defects in the heart, brain, or other organs that are incompatible with survival. Fetal structural abnormalities overall account for about 14% of stillbirths.
A 2019 study published in the New England Journal of Medicine used advanced genetic sequencing on 246 stillbirth cases and found molecular diagnoses in about 6% that involved specific disease-causing genes, some of which had never been previously connected to stillbirth. This suggests that genetic causes may be more common than older estimates captured, because standard chromosome testing misses smaller mutations.
Infections
Infections cause roughly 13% of stillbirths. At least 40 different organisms, including bacteria, viruses, and parasites, have been linked to fetal death. These infections can reach the baby in two main ways: traveling through the bloodstream and crossing the placenta, or ascending from the vagina through the cervix.
Syphilis is the single most significant infectious cause of stillbirth worldwide and is almost entirely preventable with routine prenatal screening and treatment. Listeria, a bacteria found in contaminated deli meats, soft cheeses, and other ready-to-eat foods, can cross the placenta and directly infect the baby. Group B streptococcus and E. coli are among the bacteria that can ascend from the birth canal.
On the viral side, parvovirus B19 (the virus that causes “fifth disease” in children) is one of the best-documented viral causes of stillbirth. It infects red blood cell precursors in the baby, potentially causing severe anemia and heart failure. Cytomegalovirus, herpes simplex, and some enteroviruses can also cross the placenta and cause fetal death, though these are less commonly investigated in stillbirth evaluations.
Maternal Health Conditions
High blood pressure during pregnancy, including preeclampsia, nearly doubles the risk of stillbirth. Preeclampsia damages blood vessels throughout the body, including those supplying the placenta, which restricts the baby’s growth and oxygen supply. Hypertensive disorders are identified as a contributing factor in about 9% of stillbirths.
Diabetes also raises the risk, particularly when it goes undiagnosed or unmanaged. Women with gestational diabetes who never received screening had nearly four times the odds of stillbirth compared to women without diabetes. Even among women diagnosed and receiving care, the risk remained modestly elevated. The mechanism involves abnormal blood sugar levels affecting placental function and fetal development, particularly in the third trimester.
Risk Factors That Increase the Odds
Several factors raise the overall likelihood of stillbirth without being direct causes on their own. Smoking during pregnancy increases the odds by 47% overall. Women who smoke 10 or more cigarettes per day face a 52% higher risk compared to nonsmokers, while lighter smoking (under 10 per day) shows a smaller, less statistically certain increase.
Maternal age plays a significant role. Women 35 and older have roughly three times the risk of stillbirth compared to younger women, and the risk climbs further after 40. A previous stillbirth is the strongest individual risk factor: women who have experienced one stillbirth have nearly 10 times the odds of it happening in a subsequent pregnancy. Lower education levels and lack of prenatal care are also consistently associated with higher rates, likely because they correlate with delayed diagnosis of treatable conditions like diabetes and high blood pressure.
Why So Many Cases Go Unexplained
Even with the best available testing, more than 60% of stillbirths have no identifiable cause. This is one of the most frustrating realities for families and researchers alike. The standard evaluation after a stillbirth includes a fetal autopsy, microscopic examination of the placenta and umbilical cord, and genetic testing. X-rays may be taken to look for skeletal abnormalities. If families are not comfortable with a full autopsy, imaging with MRI or a partial examination can still provide some answers.
The most useful tissue for genetic testing is amniotic fluid, collected before delivery when possible. Placental tissue from just below the cord insertion site and cartilage samples from the baby also work well. Fetal skin, somewhat counterintuitively, is a poor source for chromosome analysis.
The high rate of unexplained cases reflects real limitations in what current medicine can detect. Some causes, like brief cord compression or a transient infection that resolves before testing, leave no trace. Others may involve genetic variants or placental malfunctions that science hasn’t yet learned to identify. The National Institutes of Health has launched research consortiums specifically aimed at closing this gap, but for now, many families leave the hospital without the answer they were looking for.
Reduced Fetal Movement as a Warning Sign
One of the few warning signs that something is wrong is a noticeable decrease in how much your baby moves. When a baby is under stress from oxygen deprivation or placental failure, one of the first adaptive responses is to become less active, conserving energy and redirecting blood flow to vital organs. This means a baby who suddenly moves less than usual may be in distress.
There is no single “normal” number of kicks that applies to every pregnancy, because each baby has its own pattern of activity. What matters is a change from your baby’s usual pattern. If you notice significantly fewer movements than what has been typical, or if you can’t feel movement at all during a time when your baby is usually active, contact your healthcare provider promptly. Kick counting in the third trimester, where you track how long it takes to feel 10 movements, is one simple tool that can help you notice changes early.