Stillbirth Risk After 39 Weeks: Why Gestational Age Matters Even in Low-Risk Pregnancies

For many expecting parents, the final weeks of pregnancy are a time of anticipation and preparation. However, for a small but significant number of families in the United States, this period ends in an unimaginable tragedy. Stillbirth—the loss of a baby after 20 weeks of pregnancy—remains one of the most profound challenges in modern obstetrics, leaving parents with a devastating void and clinicians searching for answers that often remain elusive.

While advances in prenatal care and neonatal medicine have drastically reduced infant mortality over the last century, stillbirth rates in the US continue to highlight critical gaps in our healthcare system. From systemic racial disparities to the mysterious nature of “unexplained” losses, the data suggests that the risk of fetal mortality is not always predictable, nor is it limited to those with known medical complications.

As a physician and journalist, I have seen how the clinical data often clashes with the lived experience of patients. The numbers tell us a story of risk and probability, but for the parents, a stillbirth is not a statistic—it is a life-altering event. Understanding why these losses occur, particularly in the final stretch of a full-term pregnancy, is essential for improving outcomes and providing better support for grieving families.

Recent analysis of pregnancy cohorts suggests a concerning trend: the risk of stillbirth does not necessarily plateau at the end of a healthy pregnancy. Instead, there is evidence of a rise in fetal mortality beginning around 39 weeks of gestation. Perhaps most unsettling is the reality that a substantial portion of these losses occur in pregnancies that appeared entirely low-risk, challenging the traditional medical approach to prenatal monitoring.

The Current Landscape of Stillbirth in the United States

To understand the scope of the issue, one must look at the broader epidemiological data. According to the Centers for Disease Control and Prevention (CDC), stillbirth is defined as the loss of a baby at or after 20 weeks of pregnancy. While the overall rate has fluctuated, the US continues to struggle with rates that are often higher than those found in other high-income nations.

The prevalence of these losses is not evenly distributed across the population. One of the most persistent and heartbreaking aspects of maternal health in the US is the racial disparity in outcomes. Data indicates that Black women are significantly more likely to experience a stillbirth than white women, a gap that persists regardless of income or education levels. This suggests that the drivers of stillbirth are not merely biological but are deeply intertwined with systemic inequities, including chronic stress from racism and unequal access to high-quality prenatal care.

Clinically, stillbirths are categorized by the timing of the loss. Early stillbirths (20 to 27 weeks) are often linked to genetic anomalies or severe fetal developmental issues. However, late stillbirths (after 28 weeks) are more frequently associated with placental dysfunction, maternal health conditions, or unexplained causes. It is in this latter category that the most urgent necessitate for research and improved screening exists.

The 39-Week Window: When Risk Increases

Medical convention has long viewed the arrival at 39 weeks—the threshold of “full term”—as a milestone of safety. However, emerging data from national cohorts of pregnancies indicates that the risk of fetal mortality actually begins to climb during this period. This trend is particularly instructive because it occurs even in pregnancies that have progressed without complication.

The biological reasoning behind this rise often centers on the placenta. The placenta is a temporary organ with a finite lifespan; as a pregnancy progresses toward and beyond 40 weeks, the placenta may begin to age, a process known as placental senescence. When the placenta can no longer efficiently deliver oxygen and nutrients to the fetus, the risk of intrauterine fetal demise increases. This “placental insufficiency” can occur silently, without the mother feeling a decrease in fetal movement or showing signs of distress.

The implication of this finding is significant. It suggests that the traditional “wait and see” approach for pregnancies reaching their due date may need to be re-evaluated. If the risk of stillbirth increases substantially after 39 weeks, the conversation between clinicians and patients regarding the timing of delivery—whether through induction or scheduled interventions—becomes a critical component of risk management.

The Mystery of the ‘Unexplained’ Stillbirth

One of the most distressing aspects of perinatal loss is the “unexplained” stillbirth. In a significant number of cases—some studies suggest up to 40% of stillbirths occurring near term—there are no identifiable clinical risk factors. The mother may have had a healthy BMI, normal blood pressure, no history of diabetes, and a pregnancy that appeared textbook-perfect on every ultrasound.

When a stillbirth is labeled as unexplained, it means that after a thorough review of the medical records, a detailed placental examination, and often a fetal autopsy, no clear cause of death could be found. For parents, this lack of closure can be as agonizing as the loss itself. The desire to know “why” is a fundamental part of the grieving process, and when medicine cannot provide an answer, the trauma is compounded.

The fact that so many late-term losses occur without risk factors indicates that our current screening tools may be insufficient. We rely heavily on fetal heart rate monitoring and ultrasound, but these are snapshots in time. They may not capture the intermittent failures of the placenta or sudden thrombotic events (blood clots) that can lead to a rapid fetal demise.

Identifying and Managing Known Risk Factors

While many losses are unexplained, a substantial number of stillbirths are linked to manageable risk factors. Identifying these early allows healthcare providers to implement targeted interventions that can save lives.

• Hypertensive Disorders: Preeclampsia and chronic hypertension can restrict blood flow to the placenta, increasing the risk of fetal growth restriction and stillbirth.
• Diabetes: Both pre-existing type 2 diabetes and gestational diabetes increase the risk of stillbirth, particularly if blood glucose levels are not tightly controlled. The American College of Obstetricians and Gynecologists (ACOG) provides strict guidelines for the management of these conditions to mitigate risk.
• Infections: Certain bacterial or viral infections can cross the placenta or trigger an inflammatory response that leads to fetal loss.
• Placental Issues: Conditions such as placental abruption (where the placenta peels away from the uterine wall) are acute emergencies that can lead to immediate stillbirth.
• Obesity: Higher maternal BMI is associated with an increased risk of various pregnancy complications that can contribute to fetal mortality.

The management of these risks involves a combination of pharmacological treatment, increased surveillance (such as more frequent non-stress tests), and in some cases, the recommendation for an earlier delivery to avoid the risks associated with late-term placental aging.

The Psychological Impact and the Need for Support

The medical community often focuses on the physical causes of stillbirth, but the psychological aftermath is a public health issue in its own right. The grief associated with stillbirth is unique; it is a “disenfranchised grief,” often overlooked by a society that celebrates birth and pregnancy. Parents frequently report feeling isolated or pressured to “move on” quickly.

Comprehensive care must extend beyond the delivery room. This includes access to bereavement counseling, support groups, and a healthcare system that validates the loss of the baby. In many US hospitals, “bereavement protocols” are now being implemented to ensure that parents are given the space to bond with their baby, take photographs, and receive the emotional support necessary to begin the long process of healing.

Key Takeaways for Expectant Parents and Providers

Moving Toward a Future of Prevention

Reducing stillbirth rates in the US requires a multi-pronged approach. First, we must address the systemic health disparities that make Black women and women of color more vulnerable to loss. This is not a medical failure alone, but a societal one that requires policy changes in healthcare access and maternal support.

Second, there is a desperate need for better diagnostic tools. We need biomarkers—perhaps found in the blood or urine—that can signal placental failure before it becomes catastrophic. Until such tools exist, we must rely on a combination of vigilant fetal movement monitoring and a nuanced approach to the timing of delivery for those reaching the end of their third trimester.

Finally, the medical community must be honest about the limitations of current prenatal care. A “normal” ultrasound or a “healthy” check-up does not guarantee a live birth. By acknowledging that stillbirth can happen even in low-risk pregnancies, we can encourage a more proactive dialogue between patients and doctors, ensuring that no one is blindsided by the risks of the final weeks of pregnancy.

The goal is not to instill fear in expectant parents, but to empower them with knowledge. When parents are aware of the signs of fetal distress and providers are mindful of the risks associated with gestational age, we move closer to a world where every pregnancy ends in the joy of a healthy baby.

The next major step in addressing this crisis will be the release of updated national surveillance data from the CDC, which will help researchers determine if recent changes in induction practices and maternal health policies are successfully lowering the rate of late-term losses. We will continue to monitor these developments as they emerge.

Do you have experience with the US healthcare system’s approach to maternal health, or have you found resources that helped you navigate perinatal loss? We invite you to share your thoughts and stories in the comments below to help others in our global community.