From the Guidelines
The risk of fetal demise increases significantly with advancing gestational age, particularly after 39 weeks, and induction of labor is generally recommended between 41-42 weeks of gestation if spontaneous labor has not occurred. This recommendation is based on the most recent and highest quality study available, which suggests that the risk of stillbirth rises significantly after 39 weeks, approximately doubling from 1 in 1,000 at 40 weeks to 2 in 1,000 at 42 weeks 1.
Key Factors Influencing Fetal Demise Risk
- Advanced maternal age (especially over 35)
- Obesity
- Hypertension
- Diabetes
- History of prior stillbirth
- Intrauterine fetal growth restriction (IUGR)
- Specific structural anomalies
- Genetic syndromes
- Fetal arrhythmias
- Blood group incompatibilities
- Fetal anemia
- Congenital infections
- Multiple gestations
Recommendations for Fetal Monitoring and Delivery
- Regular fetal monitoring with twice-weekly non-stress tests and amniotic fluid assessment is recommended for pregnancies that continue beyond 41 weeks 1
- Delivery at 37 weeks of gestation is recommended in pregnancies with fetal growth restriction (FGR) and an umbilical artery Doppler waveform with decreased diastolic flow (S/D, RI, or PI greater than the 95th percentile) but without absent or reversed end-diastolic velocity (AEDV/REDV) 1
- Delivery at 33-34 weeks of gestation is recommended for pregnancies with FGR and AEDV 1
- Delivery at 30-32 weeks of gestation is recommended for pregnancies with FGR and reversed end-diastolic velocity (REDV) 1
Rationale for Recommendations
The increased risk of fetal demise with advancing gestational age occurs because the placenta, which provides oxygen and nutrients to the fetus, begins to function less efficiently as pregnancy advances beyond term, potentially leading to decreased fetal oxygenation 1. Additionally, amniotic fluid volume typically decreases in post-term pregnancies, which can lead to umbilical cord compression and further compromise fetal well-being 1.
From the Research
Fetal Demise Risk and Advancing Gestational Age
- The risk of fetal death increases with advancing gestational age, especially after 37 weeks of gestation 2, 3.
- Studies have shown that the risk of stillbirth at term increases with gestational age, from 2.1 per 10,000 ongoing pregnancies at 37 weeks to 10.8 per 10,000 ongoing pregnancies at 42 weeks 3.
- Advanced maternal age is also an independent risk factor for stillbirth, and the risk of stillbirth increases at term of pregnancy in all maternal age groups, especially in older mothers 4.
- The hazard risk of stillbirth doubles from 39 to 40 weeks of gestation, from 0.60 per 1000 ongoing pregnancies to 1.16 per 1000 ongoing pregnancies, in women aged 40 years or older 4.
Gestational Age and Fetal Demise Risk
- The risk of fetal demise is substantially higher for high-risk pregnancies than for low-risk pregnancies 2.
- Delivery at 39 weeks of gestation in both high and low-risk pregnancies may result in an increased number of perinatal deaths 2.
- The number of fetal deaths that can be avoided by delivery exceeds the neonatal death rate between 37 and 38 weeks of gestation in low-risk pregnancies and at 36 weeks of gestation in high-risk pregnancies 2.
Assessment of Fetal Well-being
- Antenatal fetal assessment methods, such as the nonstress test, oxytocin challenge test, and biophysical profile, can be used to evaluate fetal well-being 5, 6.
- The combination of the nonstress test and oxytocin challenge test has been shown to have a higher positive predictive value for fetal acidosis than individual tests 6.