Stillbirth, the loss of a pregnancy at 20 weeks or later, affects about 1 in 175 births in the United States, resulting in roughly 21,000 cases each year. The causes range from problems with the placenta and umbilical cord to maternal health conditions, infections, and genetic abnormalities. In 15 to 60 percent of cases, no definitive cause is found even after thorough investigation.
Placental Problems
The placenta is the organ that delivers oxygen and nutrients from your bloodstream to the baby. When it fails, the baby can be deprived of what it needs to survive. Placental problems are the single most common identifiable cause of stillbirth, and they take several forms.
Placental abruption occurs when the placenta separates from the uterine wall before delivery. This cuts off blood flow to the baby and can happen at any point during pregnancy. Placental insufficiency is a broader term for when the placenta simply doesn’t work well enough, often because of poor blood vessel development early in pregnancy. The placenta may not have embedded deeply enough into the uterine wall, or its tiny blood vessels may have narrowed over time, reducing the flow of nutrients. This can lead to restricted fetal growth, which itself is a warning sign. Placental infarcts, areas of dead tissue caused by blocked blood flow, can also contribute.
Umbilical Cord Complications
The umbilical cord is the baby’s lifeline, and problems with it account for about 19 percent of stillbirths. These complications include the cord becoming wrapped tightly around the baby’s neck or body (cord entrapment), true knots or twisting in the cord, narrowing at a specific point (stricture), and cord prolapse, where the cord slips through the cervix ahead of the baby and gets compressed.
Knots, twisting, and strictures occur in roughly 0.3 to 2 percent of all pregnancies but carry a stillbirth rate of 8 to 11 percent when present. Cord prolapse is less common but particularly dangerous, usually happening after the membranes rupture prematurely. In many cord-related stillbirths, the damage shows up as blocked blood flow in the cord’s tiny vessels, something that can only be confirmed through examination of the cord and placenta after delivery.
Maternal Health Conditions
Several chronic and pregnancy-related health conditions raise the risk of stillbirth, primarily by damaging the blood vessels that supply the placenta.
Preeclampsia, a disorder marked by high blood pressure and organ damage during pregnancy, disrupts normal blood vessel function and increases the risk of dangerous clotting. The same mechanism that raises blood pressure in the mother also starves the placenta of adequate blood flow. Preeclampsia can progress rapidly, which is why frequent monitoring of blood pressure and protein in the urine is standard in prenatal care.
Diabetes, both pre-existing and gestational, is another significant risk factor. When blood sugar levels remain poorly controlled, the risk of stillbirth rises. Research dating back to the 1960s first identified this link, and more recent work confirms that improved blood sugar management reduces the chance of fetal death. For women with gestational diabetes, the timing of delivery often depends on how well blood sugar has been controlled throughout the pregnancy.
Other maternal conditions that increase risk include blood clotting disorders, kidney disease, and thyroid problems.
Infections
Infections that reach the uterus, placenta, or amniotic fluid can cause stillbirth, sometimes without obvious symptoms in the mother. The list of known culprits spans bacteria, viruses, and parasites.
Among bacteria, listeria (from contaminated food) and syphilis are two of the most well-documented causes. Group B streptococcus and E. coli can also lead to fetal death if they infect the uterine environment. Viral causes include parvovirus B19 (sometimes called “fifth disease”), cytomegalovirus (CMV), and rubella. In regions where malaria and Chagas disease are common, these parasitic infections are recognized contributors as well.
Some of these infections are preventable through screening, vaccination, or food safety precautions. Syphilis screening is routine in prenatal care, and avoiding unpasteurized dairy, deli meats, and undercooked foods reduces the risk of listeria exposure.
Genetic and Chromosomal Abnormalities
Chromosomal problems like extra or missing chromosomes account for 6 to 13 percent of stillbirths. This is notably lower than in earlier pregnancy losses, where chromosomal abnormalities are found in about 50 percent of cases. The difference reflects the fact that the most severe genetic problems tend to cause miscarriage earlier in pregnancy, while those compatible with longer survival may not cause fetal death until later.
Structural birth defects affecting the heart, brain, or other organs can also contribute, sometimes in combination with chromosomal issues.
Demographic and Lifestyle Risk Factors
Maternal age plays a measurable role. Women 35 and older face a stillbirth risk roughly 1.3 to 1.9 times higher than women under 35, and the risk climbs further after age 40.
Race is one of the starkest risk factors in the United States. In 2021, the stillbirth rate among non-Hispanic Black women was 9.89 per 1,000 births, compared to 4.85 among non-Hispanic White women and 3.94 among Asian women. Native Hawaiian and Pacific Islander women experienced a similarly elevated rate of 9.87 per 1,000. These disparities reflect systemic differences in access to care, chronic health conditions, and environmental stressors rather than inherent biological differences.
Substance use during pregnancy increases risk through direct effects on the placenta. Tobacco, cocaine, and methamphetamine cause blood vessels in the placenta to constrict, reducing oxygen delivery and increasing the chance of abruption by roughly threefold. Opioid use has been linked to delayed development of the placenta’s nutrient-exchange structures, a finding seen in nearly 39 percent of placentas from opioid-exposed pregnancies compared to about 16 percent in unexposed ones.
Why Many Cases Remain Unexplained
Even with a full workup, including autopsy, placental examination, genetic testing, and a review of maternal health records, between 16 and 36 percent of stillbirths have no identifiable cause. Some studies using broader definitions put that figure as high as 60 percent. The wide range depends on how thorough the evaluation is and which classification system is used.
A complete evaluation after a stillbirth typically includes a detailed examination of the baby for structural abnormalities, chromosomal analysis, and microscopic examination of the placenta and umbilical cord. Not all families choose to pursue a full evaluation, and in many parts of the world these tests are not routinely available, which means the true proportion of “unexplained” cases may be lower than reported if better diagnostic tools were universally applied.
For families who experience an unexplained stillbirth, the lack of a clear answer is one of the most difficult aspects. It also complicates planning for future pregnancies, since without a known cause, it is harder to estimate whether the risk is elevated again or whether specific interventions could help.