Stillbirth has no single cause. Placental problems, genetic abnormalities, infections, umbilical cord complications, and chronic maternal health conditions all play a role, and in many cases multiple factors overlap. In the United States, about 1 in 175 pregnancies ends in stillbirth, accounting for roughly 21,000 losses each year. Understanding the known causes can help make sense of what happened and, in some cases, inform future pregnancies.
Placental Problems
The placenta is the organ that delivers oxygen and nutrients from your bloodstream to the baby. When it fails, the baby’s lifeline is cut. Placental disease accounts for roughly 24% of stillbirths that undergo a thorough cause-of-death investigation. Within that group, the most common finding is a blood clot in the fetal blood vessels of the placenta (about 35% of placental cases), followed by areas of dead tissue caused by blocked blood flow, known as infarction (about 26%), and overall placental insufficiency, where the organ simply cannot keep up with the baby’s growing demands (about 20%).
Placental abruption, where the placenta separates from the uterine wall before delivery, is another well-recognized cause. When separation is severe, blood flow to the baby stops abruptly. Smoking during pregnancy is one of the strongest links to placental disease leading to stillbirth.
Umbilical Cord Complications
The umbilical cord connects the baby to the placenta, and problems with the cord are involved in about 19% of stillbirths. Nearly half of those cases involve compromised blood flow through the tiny vessels inside the cord. Cord entanglement, where the cord wraps around the baby’s neck or body, accounts for roughly 29% of cord-related stillbirths. Knots, twisting, and narrowing of the cord make up another 27%.
Cord prolapse, where the cord slips ahead of the baby during labor, is dangerous when it occurs but relatively rare, responsible for about 1% of all stillbirths. True knots in the cord happen in less than 2% of pregnancies overall, but when a knot tightens enough to restrict blood flow, the stillbirth rate associated with it ranges from 8 to 11%.
Genetic and Structural Abnormalities
Chromosomal problems in the baby cause an estimated 10 to 20% of stillbirths. These include conditions like trisomy 18 and trisomy 13, where extra copies of chromosomes disrupt normal development in ways that are often incompatible with life. Major structural birth defects, such as severe heart malformations or brain abnormalities, account for about 14% of stillbirths in large studies that use systematic classification tools.
Many of these genetic causes are not inherited from the parents but arise spontaneously during early cell division. After a stillbirth, genetic testing of the baby’s tissue can identify whether a chromosomal or single-gene disorder was responsible. This information matters not just for understanding the loss but for estimating the risk in a future pregnancy.
Maternal Health Conditions
Certain chronic conditions in the mother significantly raise the likelihood of stillbirth. Chronic high blood pressure roughly triples the risk. Diabetes (type 1 or type 2 diagnosed before pregnancy) increases it by about 2.5 times. Abnormal amniotic fluid levels, either too much or too little, raise the risk by about 2.4 times. Preeclampsia and pregnancy-induced hypertension also appear to increase the odds, though the statistical picture is less clear-cut.
These conditions share a common thread: they damage or narrow the blood vessels that supply the placenta, reducing the flow of oxygen to the baby over time. That is why pregnancies complicated by these disorders are monitored more closely in the third trimester, with more frequent ultrasounds and fetal heart rate checks.
Infections
Bacterial and viral infections can cross the placenta and directly harm the baby. Syphilis is one of the most significant infectious causes worldwide. The bacterium can infect the fetus after about 14 weeks of pregnancy, and if infection occurs, roughly 45% of affected babies die in the womb.
Listeria, a food-borne bacterium found in unpasteurized dairy products, deli meats, and contaminated produce, reaches the baby through the mother’s bloodstream and can cause fetal death. Among viruses, parvovirus B19 (the virus behind “fifth disease” in children) is considered the most common viral cause. It attacks the baby’s developing red blood cells, causing severe anemia and fluid buildup. In some European countries, parvovirus is linked to as many as 10% of stillbirths, though the proportion in the U.S. is lower, under 1%.
Other infections, including cytomegalovirus and malaria in endemic regions, are also recognized contributors. Many infectious causes are preventable through screening, vaccination, or food safety precautions.
Obesity and Maternal Age
Being overweight or obese before pregnancy is one of the most significant modifiable risk factors for stillbirth in high-income countries. More stillbirths are attributed to excess weight than to smoking, low education, or advanced maternal age. The numbers are striking: compared to lean women, those who are overweight before pregnancy have about a 40% higher risk of stillbirth. For obese women (BMI 30 to 35), the risk roughly doubles. For severely obese women (BMI 35 or above), the risk is about 2 to 2.3 times higher.
The cumulative stillbirth rate illustrates the gap clearly. Among lean women, the rate is about 7.7 per 1,000 births. Among severely obese women, it climbs to 17.3 per 1,000. The connection likely runs through the same vascular mechanisms that link diabetes and hypertension to stillbirth: excess weight promotes inflammation, insulin resistance, and blood vessel damage, all of which compromise placental function.
Advanced maternal age (over 35) is also an established risk factor, though it carries a smaller increase in risk than obesity does.
When No Cause Is Found
Even with a thorough investigation, a substantial proportion of stillbirths remain unexplained. The standard evaluation includes an external examination of the baby, a detailed analysis of the placenta and umbilical cord, genetic testing from blood or tissue samples, and imaging. A full autopsy, when parents consent, provides the most complete information, though a minimally invasive alternative using imaging and targeted tissue sampling is increasingly available.
When a cause is identified, it helps parents understand what happened and gives their medical team concrete information for managing future pregnancies. When no cause is found, the uncertainty is painful, but it also means the specific, high-recurrence causes like chromosomal disorders or undiagnosed maternal disease have been ruled out.
Fetal Movement and Early Warning Signs
A noticeable decrease in your baby’s movements is one of the few warning signs that something may be wrong. After 28 weeks, you should become familiar with your baby’s individual pattern of movement. If you feel that movements have slowed or stopped, do not wait until the next day. Contact your maternity provider immediately.
If you are unsure whether movement has truly decreased, lie on your left side and focus on what you feel for two hours. If you do not count at least 10 distinct movements in that time, call your provider right away. At the hospital, the initial assessment typically involves listening to the baby’s heartbeat, running a heart rate tracing, and performing an ultrasound to check fluid levels and growth. These steps can identify a baby in distress early enough for intervention.
Formal kick-counting charts, where you log movements against a fixed threshold every day, have not been shown to reduce stillbirth in studies. What matters more is knowing your baby’s normal pattern and acting quickly when something feels different.