Objective
The purpose of this study was to evaluate the risk of respiratory morbidity in neonates delivered at “early term” (37-38 weeks) compared with those delivered at 39 weeks.
Study Design
We conducted a retrospective cohort study of singleton deliveries from 37 0/7 to 39 6/7 weeks’ gestation. Our primary outcome was composite respiratory morbidity.
Results
Of 2273 deliveries at 37-39 weeks, 51% (n = 1169) delivered in the early term period. Infants delivered at 37-38 weeks had a 2-fold increased risk of respiratory distress syndrome, oxygen use, continuous positive airway pressure use, and composite respiratory morbidity (risk ratio [RR], 2.9; 95% confidence interval [CI], 1.0–7.9; oxygen usage RR, 2.0; 95% CI, 1.4–2.9; continuous positive airway pressure RR, 1.9; 95% CI, 1.1–3.2; composite respiratory morbidity RR, 2.0; 95% CI, 1.4–2.8).
Conclusion
The 2-fold increased risk of composite respiratory morbidity of infants in the early term period supports the urgency for limiting nonindicated deliveries to ≥39 weeks’ gestation.
A term pregnancy is defined as one that has reached 37 weeks’ gestation. This implies a level of fetal maturity beyond which morbidity is negligible. Recent literature, however, has shown that significant morbidity persists at term, more specifically the “early term” period defined as 37-38 weeks’ gestation. Tita et al performed a secondary analysis of data from the Maternal-Fetal Medicine Units Network evaluating neonatal outcomes following early term elective cesarean deliveries. Among elective deliveries in the absence of maternal or obstetrical indications, they evaluated a composite outcome of neonatal morbidities. The authors demonstrated a decrease trend in the incidence of adverse respiratory outcomes (37 weeks 8.2%, 38 weeks 5.5%, 39 weeks 3.4%; P < .001) and ventilator support (37 weeks 1.9%, 38 weeks 0.9%, 39 weeks 0.4%; P < .001) as gestational increased >37 weeks. They found that the attributable risk of morbidity for delivery at 37 weeks was 48%, while this same risk at 38 weeks was 27%. In addition, the morbidity at 38 4/7 to 38 6/7 weeks remained increased (risk ratio [RR], 1.2; 95% confidence interval [CI], 1.0–1.4).
Respiratory morbidity may be higher as well in infants delivered in the early term gestation window. The Consortium for Safe Labor reviewed respiratory morbidity in neonates born in the late preterm period (34-36 weeks’ gestation) from a multicenter retrospective cohort study of >200,000 births in the United States. As expected, respiratory morbidity was higher in late preterm infants compared with those born at term. Interestingly, they also reported that some respiratory morbidity remained high at the early term gestations compared with 39 weeks. Specifically, the rates of respiratory failure (odds ratio [OR], 2.8; 95% CI, 2.0–3.9 at 37 weeks; and OR, 1.4; 95% CI, 1.0–1.9 at 38 weeks) and ventilator use (OR, 2.8; 95% CI, 2.3–3.4 at 37 weeks; and OR, 1.2; 95% CI, 1.0–1.5 at 38 weeks) were higher at 37 and 38 weeks. Additionally, respiratory distress syndrome (RDS), transient tachypnea of the newborn (TTN), pneumonia, surfactant use, and oscillator use were all higher at 37 weeks compared with 39 weeks.
We designed a retrospective cohort study to evaluate the risks of both respiratory and nonrespiratory morbidity in neonates delivered in the early term gestation window (37-39 weeks).
Materials and Methods
We carried out a retrospective cohort study of 37- to 39-week deliveries that occurred from January through December 2010 at New York Presbyterian Children’s Hospital, New York City. Electronic medical records were reviewed to determine baseline characteristics including maternal demographics, admission diagnoses, indications for delivery, mode of delivery, and prior antenatal corticosteroid exposure. Neonatal outcomes were abstracted from neonatal charts as determined by trained neonatologists. RDS was defined as the presence of respiratory symptoms (eg, tachypnea or grunting), requirement for oxygen supplementation, and confirmatory radiographic findings. TTN was determined by the presence of any tachypnea or symptoms of respiratory distress with no associated focal radiographic findings and respiratory support requirements for <24 hours. Oxygen supplementation was defined as being clinically significant when administered via continuous positive airway pressure (CPAP). A database of pertinent maternal and neonatal information was created by 3 physicians (K.G., J.C., and L.L.) and 1 medical student (E.M.). Data abstraction forms were standardized in a spreadsheet format used by each author to collect information. All discrepancies pertaining to abstracted data were resolved in conjunction with an additional author (C.G-B.). A pilot study was not performed to determine interobserver and intraobserver variability.
Gestational age was determined by using best obstetrical estimates, last menstrual period, and earliest antenatal ultrasound scans. Early term deliveries were defined as those occurring between 37 0/7 and 38 6/7 weeks’ gestation. These deliveries were compared with those occurring at 39 0/7 to 39 6/7 weeks’ gestation.
The primary neonatal outcome was composite respiratory morbidity comprised of TTN, RDS, oxygen usage, CPAP, and intubation. This composite outcome was assessed in neonates delivered in the early term period compared with those delivered at 39 weeks. Secondary outcomes included nonrespiratory morbidity such as hypoglycemia, culture-proven sepsis, necrotizing enterocolitis, hypoxic ischemic encephalopathy, and seizures. Additional information that was collected included neonatal intensive care unit (NICU) admission and Apgar scores. We excluded pregnancies with multiple gestations and suspected fetal anomalies diagnosed by antenatal ultrasound.
Associations between categorical variables were evaluated using either the χ 2 or Fisher exact tests where appropriate; continuous variables were compared based on the Student t test. Associations between the risks of respiratory morbidity and gestational age at delivery were expressed as RR and 95% CI based on the Mantel-Haenszel estimation method. Statistical analysis was performed using SAS 9.2 (SAS Institute, Inc., Cary, NC). This study was approved by the ethics committee of the institutional review board at Columbia University Medical Center, New York City.
Results
Of a total of 4474 singleton live births in 2010 in our institution, 2273 (51%) occurred between 37-39 weeks. Of these, 51% (n = 1169) were delivered in the early term (37-38 weeks) period. In the early term group, 34.6% (n = 405) were delivered at 37 weeks and 65.3% (n = 764) at 38 weeks.
Maternal and neonatal demographic characteristics stratified by gestational age groups are described in Table 1 . There was no difference between the groups with regard to maternal age, parity, or the presence of diabetes mellitus. Antenatal betamethasone for acceleration of the fetal lung was administered more frequently in the early term group as compared with the 39-week group. There were no pregnancies, however, that received betamethasone within 7 days of their eventual delivery. The rate of cesarean delivery after labor was higher in the early term group in comparison to the rate seen at 39 weeks.
| Characteristic | 37 wk (n = 405) | 38 wk (n = 764) | 39 wk (n = 1104) | P value |
|---|---|---|---|---|
| Maternal age, y | 30.8 ± 6.4 | 31.3 ± 6.4 | 30.9 ± 6.2 | .301 |
| Nulliparous | 179 (44.3) | 307 (40.1) | 439 (39.8) | .269 |
| Diabetes mellitus a | 39 (9.7) | 61 (7.9) | 76 (6.9) | .196 |
| Cesarean delivery | 161 (39.9) | 285 (37.3) | 362 (32.8) | .020 |
| Cesarean delivery (after labor) | 76 (18.8) | 140 (18.3) | 137 (12.4) | < .001 |
| Cesarean delivery (scheduled) | 85 (21) | 148 (19.4) | 226 (20.5) | .751 |
| Antenatal betamethasone b | 25 (6.2) | 20 (2.6) | 9 (0.8) | < .001 |
| Male sex | 249 (61.6) | 429 (56.1) | 558 (50.5) | < .001 |
| Birthweight, g | 2984 ± 434 | 3253 ± 436 | 3381 ± 415 | < .001 |
| NICU admission | 33 (8.2) | 34 (4.4) | 41 (3.7) | < .001 |
a Only patients with gestational diabetes mellitus class A2 or pregestational diabetes mellitus were included;
b Betamethasone was administered for promotion of fetal lung maturity.
The frequency of neonatal outcomes in the early term group as compared with the reference 39-week groups is shown in Table 2 . There was a 2-fold increased risk of composite respiratory morbidity among neonates delivered in the early term period as compared with those delivered at 39 weeks. This risk remained increased above the reference group when the data were stratified at 37 and 38 weeks (vs 39 weeks).
| Variable | 37 wk (n = 405) | 38 wk (n = 764) | 39 wk (n = 1104) | P value |
|---|---|---|---|---|
| Composite respiratory morbidity | 38 (9.4%) | 65 (8.5%) | 52 (4.7%) | .005 |
| Transient tachypnea of newborn | 14 (3.5%) | 17 (2.2%) | 22 (2.0%) | .273 |
| Respiratory distress syndrome | 4 (1.0%) | 11 (1.4%) | 5 (0.5%) | .078 |
| Oxygen usage | 35 (8.7%) | 61 (8.0%) | 47 (4.3%) | .005 |
| CPAP | 16 (4.0%) | 23 (3.0%) | 20 (1.8%) | .046 |
| Ventilation support | 0 | 2 (0.3%) | 2 (0.2%) | .600 |
| Composite nonrespiratory morbidity | 11 (2.7%) | 25 (3.3%) | 16 (1.5%) | .029 |
| Hypoglycemia | 10 (2.5%) | 25 (3.3%) | 16 (1.5%) | .031 |
| Hypoxic ischemic encephalopathy | 0 | 0 | 0 | — |
| Necrotizing enterocolitis | 0 | 0 | 0 | — |
| Culture-proven sepsis | 1 (0.3%) | 0 | 0 | .178 |
| Seizures | 1 (0.3%) | 0 | 0 | .178 |
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