title: The impact of the interaction between increasing gestational age and obstetrical risk on birth outcomes: evidence of a varying optimal time of delivery.
abstract: OBJECTIVE
  To estimate the gestational age ranges that result in optimal birth outcomes for each of four risk-defined groups.
  
  
  METHODS
  Retrospective cohort study of singleton late third-trimester deliveries at a large urban teaching hospital between July 1995 and September 2003. Low-risk, advanced maternal age, hypertensive and diabetic patients were identified and grouped. Rates, by day of gestation at delivery, of cesarean delivery, major maternal perineal trauma, low 5-min APGAR score and NICU admission were determined for each study group.
  
  
  RESULTS
  Each study group had meaningful changes in rates of obstetric outcomes as a function of gestational age at delivery and these patterns differed from group to group. A unique optimal time of delivery (OTD) was estimated for each group. The low-risk group OTD was calculated to be 37 weeks 1 day to 41 weeks 0 day; the advanced maternal age group OTD was 38 weeks 5 days to 39 weeks 6 days; the hypertension group OTD was 39 weeks 2 days to 40 weeks 1 day; and the diabetes mellitus group OTD was 40 weeks 3 days to 41 weeks 1 day.
  
  
  CONCLUSIONS
  The OTD varied based on obstetrical risk. Strategies to increase the proportion of deliveries that occur within the OTD for specific risk-defined groups could theoretically improve birth outcomes.
authors.affiliation: Department of Family Medicine and Community Health, University of Pennsylvania Health System, Philadelphia, 19104, USA. james.nicholson@uphs.upenn.edu
authors.forename: J M
authors.initials: JM
authors.lastname: Nicholson
authors.forename: L C
authors.initials: LC
authors.lastname: Kellar
authors.forename: G M
authors.initials: GM
authors.lastname: Kellar
mesh_terms: D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D036861:Delivery, Obstetric / Q000379:methods / Q000592:standards*; D005260:Female; D005865:Gestational Age; D006801:Humans; D046110:Hypertension, Pregnancy-Induced; D008423:Maternal Age; D015233:Models, Statistical; D017063:Outcome Assessment, Health Care; D011247:Pregnancy; D011256:Pregnancy Outcome; D011254:Pregnancy in Diabetics; D012044:Regression Analysis; D012189:Retrospective Studies; D012307:Risk Factors