Stillbirth is defined as the death of a fetus occurring after 20 weeks of gestation, differentiating it from a miscarriage, which is a loss before this time point. While the overall rate of stillbirth is low, it remains a significant public health concern, affecting approximately six out of every 1,000 births in the United States. This article provides clear, evidence-based statistics on how the risk profile changes across the third trimester and the factors that influence these weekly probabilities.
Defining Stillbirth and Classification
Healthcare providers classify stillbirths into three categories based on the timing of the loss, which aids in clinical assessment and statistical reporting. The distinction between stillbirth and miscarriage is determined by the gestational age of 20 weeks. Miscarriage refers to a fetal loss before the 20th week of pregnancy, whereas stillbirth refers to a loss occurring from the 20th week onward.
The classifications are: early stillbirth (20 to 27 weeks of gestation), late stillbirth (28 to 36 weeks of gestation), and term stillbirth (at or after 37 weeks of gestation). These categories provide a framework for discussing statistical data, as the causes and clinical management often differ between these gestational periods.
Statistical Risk Profiles by Gestational Week
The risk of stillbirth, while low across the term period, is not static and changes predictably as a pregnancy progresses beyond 37 weeks. The period known as “term” begins at 37 weeks of gestation, and nearly one-third of all stillbirths occur during this time. The prospective risk for stillbirth shows a steady, week-by-week increase with advancing gestation once a pregnancy reaches term.
At 37 weeks of gestation, the prospective risk of stillbirth is estimated to be approximately 0.11 per 1,000 ongoing pregnancies. This rate serves as a baseline for the term period, but the risk begins to accelerate in the following weeks. By the time a pregnancy reaches 40 weeks, the risk has increased, and it continues to rise sharply as the pregnancy becomes post-term.
A significant increase in risk is observed when comparing the 40th week to the 41st week of pregnancy. Continuing a pregnancy from 40 to 41 weeks carries an approximately 64% increased risk of stillbirth compared to delivery at 40 weeks. This increase translates to one additional stillbirth for every 1,449 pregnancies that continue from 40 to 41 weeks.
The highest risk occurs when a pregnancy extends to 42 weeks of gestation and beyond. At 42 weeks, the prospective stillbirth risk rises to approximately 3.18 per 1,000 ongoing pregnancies. This data is the foundation for the routine clinical practice of offering labor induction to women who reach 41 weeks of gestation.
Key Contributing Factors to Increased Risk
While the statistical profile outlines the general population risk by week, numerous individual factors can elevate a person’s baseline risk, regardless of the gestational period. These factors often relate to maternal health, placental function, or previous pregnancy history.
Maternal Health and History
Certain pre-existing health conditions in the mother are known to increase stillbirth risk, including poorly managed diabetes, chronic hypertension, and obesity (BMI of 30 or above). Maternal age is also a factor, with higher stillbirth rates observed in those who are 35 years of age or older. The risk is also increased for individuals pregnant with multiples. A prior history of stillbirth or other complications, like fetal growth restriction, places a person at a higher risk in subsequent pregnancies.
Placental and Cord Complications
Complications involving the placenta and umbilical cord are frequently implicated in stillbirths, as these structures are responsible for delivering oxygen and nutrients to the fetus. Placental abruption, where the placenta separates from the uterine wall prematurely, accounts for a significant percentage of cases. Issues such as umbilical cord constriction or knots can also interrupt this supply line, compromising fetal well-being.
Lifestyle Factors
Smoking cigarettes during pregnancy is a significant and modifiable risk factor associated with stillbirth. Additionally, certain infections that may go unnoticed in the mother can sometimes cross the placenta and cause fetal death. These individual risk factors interact with the week-by-week statistical risk, requiring personalized monitoring and management.
Active Monitoring and Risk Reduction Strategies
Proactive monitoring and specific behavioral adjustments can help mitigate the risk of stillbirth, particularly in pregnancies with identified risk factors or those approaching term. Patient education focuses on the importance of monitoring fetal movement, often referred to as “kick counts.” Pregnant individuals are encouraged to establish an understanding of their baby’s unique pattern of movement and to report any perceived reduction or change immediately to a healthcare provider.
For pregnancies deemed to be at a higher risk, specialized medical surveillance is employed to assess fetal well-being. This can include:
- Non-stress tests (NST), which monitor the fetal heart rate in response to movement.
- Biophysical profiles (BPP), which use ultrasound to evaluate factors like fetal breathing, movement, and amniotic fluid volume.
These tests are designed to detect signs of fetal compromise before a problem becomes severe.
A simple, actionable recommendation involves maternal sleep position in the third trimester. Research suggests that going to sleep on the side, rather than the back, from 28 weeks onward may reduce stillbirth risk. This is thought to prevent the growing uterus from compressing the vena cava, a major vein that returns blood to the heart, which could otherwise reduce blood flow to the placenta. When monitoring suggests that the risk of remaining in the womb outweighs the risk of delivery, timely intervention through planned early delivery or labor induction becomes a method of risk reduction.