Stillbirth results from a wide range of causes, but placental problems and pregnancy complications account for the majority of cases. In the United States, about 1 in 175 pregnancies ends in stillbirth, totaling roughly 21,000 each year. A stillbirth is defined as the loss of a baby at or after 22 weeks of pregnancy, though the WHO uses a 28-week threshold for international comparisons. Despite advances in prenatal care, more than 60% of stillbirth cases remain unexplained even after thorough investigation.
Placental Problems Are the Leading Cause
Half of all stillbirths involve disorders or conditions affecting the placenta, the organ that delivers oxygen and nutrients from your bloodstream to your baby. The placenta can fail in several ways. In placental abruption, it separates from the uterine wall before delivery, cutting off blood supply. In placental insufficiency, the organ gradually loses its ability to support the baby’s growth, often without obvious symptoms. A large NIH study found that abnormalities of the placenta itself accounted for about 24% of stillbirths, while broader pregnancy and birth complications, including preterm labor, premature rupture of membranes, and abruption, contributed to another 29%.
Because placental problems often develop silently, they can be difficult to detect before a crisis. Slowed fetal growth is sometimes the first sign that the placenta isn’t working well, which is one reason prenatal visits track your baby’s size over time.
Umbilical Cord Complications
The umbilical cord is your baby’s lifeline, and problems with it can restrict oxygen flow rapidly. A cord prolapse occurs when the cord slips ahead of the baby during labor and gets compressed. True knots in the cord, which form when the baby moves through a loop of cord early in pregnancy, are usually harmless unless they pull tight and block blood flow. A condition called vasa previa, where fetal blood vessels cross over the cervix unprotected, carries an especially high risk: torn vessels cause death in at least half of affected babies if the condition isn’t diagnosed before labor.
Cord accidents are unpredictable and account for a notable share of otherwise unexplained stillbirths. There is no reliable way to prevent them, but vasa previa can sometimes be detected on ultrasound, allowing doctors to plan a cesarean delivery before labor begins.
Maternal Health Conditions
Several chronic and pregnancy-related health conditions raise stillbirth risk. Preeclampsia, a dangerous rise in blood pressure during pregnancy, can damage the placenta and restrict blood flow to the baby. Diabetes, both pre-existing and gestational, is another significant factor. Research published in Diabetologia found that in women with type 1 diabetes, higher average blood sugar levels throughout pregnancy were linked to stillbirth. For type 2 diabetes, the pattern was different: blood sugar levels before pregnancy were a more important predictor than levels during pregnancy itself. High body mass index compounded the risk in both groups.
Obesity on its own is an independent risk factor for stillbirth, partly because it increases rates of preeclampsia, congenital abnormalities, and abnormal fetal growth. Advanced maternal age, generally defined as 35 and older, also raises the baseline risk. These factors don’t guarantee a bad outcome, but they do mean closer monitoring is warranted.
Infections That Reach the Baby
Certain bacterial and viral infections can cross the placenta and harm or kill a developing baby. Listeria, a bacterium found in unpasteurized dairy, deli meats, and some ready-to-eat foods, is particularly dangerous during pregnancy. The infection often feels like a mild flu in the mother, and some women have no symptoms at all. Yet nearly 25% of pregnancy-associated listeria cases result in fetal loss or newborn death. The bacteria typically reach the baby through the placenta rather than during delivery.
Other infections linked to stillbirth include syphilis, cytomegalovirus (CMV), and parvovirus B19. Malaria is a major contributor in tropical regions. Many of these infections are treatable or preventable, which is why routine prenatal screening and food safety precautions matter.
Genetic and Chromosomal Abnormalities
Some stillbirths are caused by genetic conditions in the baby that are incompatible with survival. Chromosomal abnormalities, where the baby has too many or too few chromosomes, can disrupt organ development so severely that the pregnancy cannot continue. These issues are more common in earlier losses but still play a role later in pregnancy. After a stillbirth, genetic testing of the baby and placenta can sometimes identify a chromosomal cause, which helps parents and doctors understand recurrence risk for future pregnancies.
Restricted Fetal Growth
Babies who are significantly smaller than expected for their gestational age face higher stillbirth risk. Restricted growth often signals that the baby isn’t getting adequate nutrition or oxygen, usually because of placental problems, high blood pressure, or smoking. Growth restriction is one of the most important modifiable risk factors for stillbirth in the general population, because it can be detected through routine ultrasound measurements and managed with closer surveillance or early delivery when necessary.
Sleep Position in Late Pregnancy
How you fall asleep during the third trimester may influence stillbirth risk. A large meta-analysis combining data from multiple studies found that going to sleep on your back after 28 weeks was associated with 2.6 times the odds of late stillbirth compared to falling asleep on your left side. Sleeping on your right side carried no increased risk compared to the left.
The biological explanation is straightforward. Lying flat on your back compresses two major blood vessels, the vena cava and aorta, reducing blood flow by as much as 85% in the vena cava and 30% in the aorta. This decreases the amount of blood reaching the uterus and placenta, lowering oxygen delivery to the baby. Researchers estimated that if every pregnant person past 28 weeks fell asleep on their side, late stillbirths could be reduced by about 5.8%. That’s a small but meaningful number for something that requires no medical intervention.
Smoking, Substance Use, and Other Lifestyle Factors
Smoking during pregnancy is one of the most well-established modifiable risk factors for stillbirth. It damages blood vessels in the placenta, restricts fetal growth, and increases the chance of placental abruption. Recreational drug use, particularly cocaine and methamphetamine, carries similar vascular risks. Alcohol use in pregnancy also raises stillbirth risk, though the relationship is harder to quantify because of underreporting.
Why So Many Cases Remain Unexplained
Even with autopsy, placental examination, and genetic testing, more than 60% of stillbirths have no identifiable cause. This is one of the most frustrating realities for families and researchers alike. The NIH has launched research consortiums specifically to close this knowledge gap, focusing on biological mechanisms that current diagnostic tools may miss. For parents who experience an unexplained stillbirth, the lack of answers does not mean nothing went wrong. It means the tools available today cannot yet detect what happened, and the absence of a diagnosis does not indicate that anything the parents did or didn’t do was responsible.