Objective
The objective of the study was to examine fetal/infant mortality by gestational age at term stratified by maternal age.
Study Design
A retrospective cohort study was conducted using 2005 US national birth certificate data. For each week of term gestation, the risk of mortality associated with delivery was compared with composite mortality risk of expectant management. The expectant management measure included stillbirth and infant death. This expectant management risk was calculated to estimate the composite mortality risk with remaining pregnant an additional week by combining the risk of stillbirth during the additional week of pregnancy and infant death risk following delivery at the next week. Maternal age was stratified by 35 years or more compared with women younger than 35 years as well as subgroup analyses of younger than 20, 20-34, 35-39, or 40 years old or older.
Results
The fetal/infant mortality risk of expectant management is greater than the risk of infant death at 39 weeks’ gestation in women 35 years old or older (15.2 vs 10.9 of 10,000, P < .05). In women younger than 35 years old, the risk of expectant management also exceeded that of infant death at 39 weeks (21.3 vs 18.8 of 10,000, P < .05). For women younger than 35 years old, the overall expectant management risk is influenced by higher infant death risk and does not rise significantly until 41 weeks compared with women 35 years old or older in which it increased at 40 weeks.
Conclusion
Risk varies by maternal age, and delivery at 39 weeks minimizes fetal/infant mortality for both groups, although the magnitude of the risk reduction is greater in older women.
Stillbirth and infant death occur in approximately equal proportions in the United States, with the latest data reporting 6.05 stillbirths per 1000 deliveries and 6.68 infant deaths per 1000 live births. It has been shown that maternal age, race and ethnicity, prepregnancy body mass index, maternal comorbidities such as diabetes and hypertension, and tobacco and alcohol use are all independently associated with an increased risk of stillbirth. It is also well known that women aged 35 years or older are at increased risk of stillbirth at term, with reports of nearly double the rate of stillbirth in this cohort. The risk of stillbirth also varies by gestational age (GA), with increased likelihood at later GAs at term and significantly elevated risk during the postterm period at 42 weeks and greater.
Prior work has demonstrated that neonatal and infant outcomes vary by GA at delivery. Increased respiratory complications and neonatal intensive care unit admission have been observed with delivery at 37 weeks, whereas delivery at or beyond 41 weeks has been correlated with macrosomia and meconium. Variation in infant mortality has also been shown based on GA at delivery, with the primary causes of death attributable to sudden infant death syndrome, asphyxia, and sepsis. Infant death has been most strongly associated with preterm delivery, low birthweight, and congenital anomalies. There is variation between the GA at term and risk of infant death, with the highest risk between 37 and 38 weeks, the lowest risk at 39-40 weeks, and a small increase at 41 weeks.
Balancing the risk of stillbirth and infant death is an important component of decision making around the time of delivery. Prior work has been conducted regarding the optimal time of delivery and the mortality risk associated with additional weeks of expectant management. This research has demonstrated that for uncomplicated singleton pregnancies, the risk of infant death is less than that of expectant management at 39, 40, and 41 weeks of gestation. For pregnancies with an increased risk of stillbirth, including those complicated by maternal diabetes, hypertension, and age 35 years or older, the risk of additional weeks of expectant management is particularly relevant to clinical care. It has been shown that for pregnancies affected by gestational diabetes, the infant mortality risk becomes lower than that of continued expectant management at 39 weeks’ gestation.
Given the greater rate of stillbirth in women aged 35 years or older, the GA that minimizes these combined risks may vary by maternal age. We sought to quantify the fetal/infant mortality risk (ie, the combined risk of stillbirth and infant death per each additional week of expectant management) to better evaluate an optimal GA for delivery across varied maternal age ranges.
Materials and Methods
We conducted a retrospective cohort study using 2005 US National birth certificate data to determine the fetal/infant mortality risk during the term period stratified by maternal age. This dataset is comprised of birth cohort–linked live birth, infant death, and fetal death information for the year 2005 from the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention.
For our analysis, the incidence of stillbirth and infant death at each GA was determined for each maternal age subgroup. The primary maternal age stratification included women aged 35 years or older compared with women younger than 35 years. We performed additional subgroup analyses for women aged younger than 20, 20-34, 35-39, and 40 years old and older. Our study population included all singleton pregnancies that were delivered during the term period between 37 0/7 and 42 6/7 weeks.
Pregnancy dating was determined using the best obstetric estimate as opposed to the last menstrual period alone. The NCHS guidelines allow for correction of GA if the estimated age based on the last menstrual period is significantly different from that estimated by ultrasound. The guidelines recommend that early ultrasound be used for this purpose and does not allow for GA correction following birth. Stillbirth was defined as intrauterine fetal death occurring after 20 weeks’ GA and before the time of delivery. Infant death was defined as death occurring within the first year of life.
Exclusion criteria included maternal diabetes (preexisting and gestational), maternal hypertension (chronic and gestational), multiple gestations, and congenital anomalies. This study was deemed exempt by the Institutional Review Board at Oregon Health and Science University because the data source had no identifying patient information.
The risk of stillbirth encountered at each GA was calculated using a pregnancies at-risk life table method, which accounts for all ongoing pregnancies in the denominator and uses the half-week correction described by Smith. This calculation includes the number of stillbirths during a given GA week in the numerator divided by the total number of ongoing pregnancies minus half of the deliveries that occurred during the GA week in question. The half-week correction accounts for the fact that the stillbirths occur evenly distributed throughout the week of gestation ( Figure 1 , Equation 1). The risk of infant death following delivery at each GA was calculated by dividing the number of infant deaths by the total number of live births at the same GA ( Figure 1 , Equation 2). A composite risk score was used to estimate the fetal/infant mortality risk of expectant management at each GA, which included both the risk of stillbirth and infant death. This calculation included the risk of stillbirth during the GA week a patient would remain pregnant plus the risk of infant death incurred following delivery at the following GA week ( Figure 1 , Equation 3).
To investigate the optimal GA for delivery, we compared the risk of delivery, conceptualized as the risk of infant death at each week, with the fetal/infant mortality risk of expectant management for an additional week. This allowed the determination of when the risk of expectant management would exceed the risk of infant death (ie, when the risk of remaining pregnant would be greater than the risk of death due to premature delivery). We analyzed data for each maternal age range described above. All risks were expressed as a rate per 10,000. The stillbirth and infant death data were displayed in table format to facilitate direct comparisons of these outcomes by maternal age ( Tables 1 and 2 ). To evaluate the risk of expectant management vs delivery, the composite risk vs infant death risk was displayed for each maternal age group in table and figure formats ( Tables 3 and 4 and Figures 2 and 3 ).
| GA, wks | Stillbirth per 10,000 ongoing pregnancies (95% CI) | Infant death per 10,000 live births (95% CI) | ||
|---|---|---|---|---|
| Maternal age <35 y | Maternal age ≥35 y | Maternal age <35 y | Maternal age ≥35 y | |
| 37 | 2.2 (1.6–2.8) | 3.3 (1.4–5.1) | 37.1 (34.6–39.6) | 23.9 (18.9–29.0) |
| 38 | 3.0 (2.6–3.5) | 4.0 (2.7–5.3) | 25.4 (24.0–26.7) | 15.9 (13.3–18.4) |
| 39 | 3.9 (3.5–4.3) | 5.0 (3.8–6.2) | 18.8 (17.8–19.7) | 10.9 (9.2–12.7) |
| 40 | 6.8 (6.2–7.4) | 10.0 (8.0–12.0) | 17.4 (16.4–18.4) | 10.3 (8.2–12.3) |
| 41 | 8.5 (7.3–9.8) | 15.4 (10.7–20.2) | 15.6 (13.9–17.3) | 11.9 (7.7–16.0) |
| 42 | 28.2 (20.4–36.0) | 32.5 (10.0–54.9) | 22.6 (15.6–29.5) | 24.4 (4.9–43.9) |
| GA | Stillbirth per 10,000 ongoing pregnancies (95% CI) | Infant death per 10,000 live births (95% CI) | ||||||
|---|---|---|---|---|---|---|---|---|
| Age <20 y | Age 20-34 y | Age 35-39 y | Age ≥40 y | Age <20 y | Age 20-34 y | Age 35-39 y | Age ≥40 y | |
| 37 | 2.7 (0.8–4.5) | 2.1 (1.5–2.7) | 3.0 (1.0–5.0) | 4.5 (0–9.6) | 57.4 (48.7–66.0) | 34.2 (31.6–36.7) | 22.2 (16.7–27.6) | 31.6 (18.1–45.1) |
| 38 | 3.5 (2.0–5.0) | 2.9 (2.5–3.4) | 3.3 (2.0–4.5) | 7.4 (3.3–11.6) | 44.2 (39.0–49.4) | 23.0 (21.7–24.3) | 14.3 (11.7–17.0) | 23.1 (15.8–30.5) |
| 39 | 3.0 (1.9–4.1) | 4.0 (3.6–4.5) | 4.6 (3.3–5.9) | 6.7 (3.3–10.1) | 31.3 (27.8–34.9) | 17.2 (16.2–18.1) | 10.4 (8.5–12.4) | 13.4 (8.6–18.2) |
| 40 | 7.7 (5.9–9.5) | 6.7 (6.0–7.3) | 9.1 (7.0–11.2) | 14.2 (8.5–19.9) | 31.0 (27.5–34.5) | 15.3 (14.4–16.3) | 9.7 (7.5–11.8) | 13.2 (7.7–18.7) |
| 41 | 6.9 (3.9–9.9) | 8.8 (7.4–10.2) | 12.5 (7.8–17.3) | 28.7 (13.1–44.4) | 19.3 (14.3–24.3) | 15.0 (13.2–16.8) | 12.9 (8.2–17.7) | 6.7 (0–14.2) |
| 42 | 46.9 (20.4–73.4) | 25.0 (17.1–32.9) | 10.4 (0–24.7) | 112.6 (23.0–202.1) | 27.5 (7.1–47.8) | 21.7 (14.3–29.1) | 31.1 (6.2–55.9) | 0 — |
| GA | Maternal age <35 years (95% CI) | Maternal age ≥35 years (95% CI) | ||
|---|---|---|---|---|
| Infant death per 10,000 live births | Risk of expectant management for 1 week per 10,000 | Infant death per 10,000 live births | Risk of expectant management for 1 week per 10,000 | |
| 37 | 37.1 (34.6–39.6) | 27.5 (25.4–29.7) | 23.9 (18.9–29.0) | 19.1 (14.6–23.7) |
| 38 | 25.4 (24.0–26.7) | 21.8 (20.6–23.0) | 15.9 (13.3–18.4) | 14.9 (12.5–17.4) |
| 39 | 18.8 (17.8–19.7) | 21.3 (20.3–22.3) | 10.9 (9.2–12.7) | 15.2 (13.1–17.4) |
| 40 | 17.4 (16.4–18.4) | 22.4 (21.3–23.5) | 10.3 (8.2–12.3) | 21.9 (18.9–24.8) |
| 41 | 15.6 (13.9–17.3) | 31.1 (28.7–33.5) | 11.9 (7.7–16.0) | 39.8 (32.2–47.5) |
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