U.S. Department of Health and Human Services Centers for Disease Control and Prevention

1. U.S. Department of Health and Human Services Centers for Disease Control and Prevention National Center for Health Statistics National Vital Statistics System WorkshoponMeasurement of Gestational Age:Challenges to Conducting Research and SurveillanceMarch 2005, Hyattsville MD

2. Authors / Acknowledgements Joyce Martin, MPH Patty Dietz, DrPh, MPH Lucy England, MD, MSPH Cheng Qin, MD, DrPH Sharon Kirmeyer, PhD The “Birth Team”! Cincinnati, Ohio, 6/7/05

3. 2005 gestational age workshop Co-sponsored by: • Division of Reproductive Health/ NCCDPHP • Division of Vital Statistics/NCHS by Div of Reproductive Health • Nationally recognized subject-matter experts from the U.S., Canada and Sweden

4. Impetus for 2005 gestational age workshop • Preterm birth rates based on vital stats have become widely used indicators of trends in maternal/infant health • Also often used to the research causes of poor pregnancy outcome • March of DimesPrematurity Campaign (launched in 2003) • Multi-million $$ research, awareness and education • Goal is to reduce rate of preterm births • Birth certificate data is primary data source

5. Preterm birth rates: U.S., 1981-2002 0

6. Impetus for 2005 gestational age workshop • Serious issues of data quality long recognized • 2000 NCHS workshop on gestational age (GA) did not reach consensus on best approach to “cleaning” data • 2004 North Carolina study found improvements in reporting of gestational age on birth certificates during the 1990’s may have differentially affected trends in preterm birth rates by race, i.e., small improvement seen in preterm birth rates for non-Hispanic black infants may be an artifact of improved reporting

7. Preterm rates by race of mother:U.S., 1990-2003 Non-Hispanic black Hispanic Non-Hispanic white 0

8. Certificate of Live Birth& Report of Fetal Death Gestational age items: • Date Last Normal Menses Began (LMP) (national data since 1981) • Clinical estimate of gestation (national data since 1989, excludes CA) • 2003 revision = Obstetric estimate of gestation (1989 Revision)

9. NCHS published measures of gestational age • Primarily LMP-based i.e., difference between date of the last normal menses (LMP) and date of birth • Two primary manipulations of LMP-based data performed: 1. Where day of LMP date is unknown, weeks of gestation are imputed 2. Where the month/year of LMP date is unknown/invalid or inconsistent with birthweight, the clinical estimate of gestation is used

10. Imputation for missing “day” of LMP Where the “day” is missing and month and year are valid (2002=5.3%) number of weeks of gestation is imputed based on a previous record with the same: • Computed months of gestation • Race (white, black, other) • Birthweight within 500 grams

11. Distribution of records with missing “day” of month of LMP by gestational age: U.S., 1990 & 2002

12. Percent of records with weeks of gestation imputed by race/Hispanic origin of mother:U.S., 1990 and 2002 Total White (non-Hispanic) Black (non-Hispanic) Hispanic

13. Clinical estimate of gestation Clinical estimate used where: 1) LMP date (month/yr) unknown (2002=5.4%) 2) LMP- based estimate is inconsistent with birthweight (2002=0.05%) • short gestation/ high birthweight • longer gestation/very low birthweight

14. Distribution of births delivered at 28-31 weeks by birthweight: U.S., 1990-92 & 2000-02 28-31 weeks of gestation percent of births Expected birthweights for gestational age Higher-than expected birthweights for gestational age --1990-92 --2000-02 Birthweight SOURCE: National Vital Statistics System, NCHS, CDC.

15. Clinical Estimate of Gestation (weeks) Instruction: Enter the length of gestation as estimated by the attendant. Do not compute this information from the date last normal menses began and date of birth. If the attendant has not done a clinical estimate of gestation, enter “None.” Do not leave this item blank.

16. The clinical estimate of gestation • On what is it based ? • Ultrasound • Formal physical exam after delivery • Exam at 1st prenatal visit • Appearance of infant at delivery • Calculated from the LMP and DOB

17. Clinical estimate of gestation age:U.S., 1990 and 2002

18. Percent of women reported to have had ultrasound by race/Hispanic origin:U.S., 1990 and 2002 White (non-Hispanic) Black (non-Hispanic) Hispanic

19. LMP-based and the clinical estimate of gestation age: U.S., 2002 NOTE: Excludes data for CA. LMP-based distribution excludes all clinical estimates.

20. 2005 Workshop conclusions • LMP-based measure has improved • High levels of misclassification persist • Clinical estimate improved • Derivation of Clinical est remains unclear • Improvements have had differential impact on preterm birth rates by race, esp. rates for non-Hispanic blacks

21. 2005 Workshop conclusions • Unable to recommend “best” approach to cleaning these data -- don’t know what birthweight /gestational age combinations are possible, i.e., what the cut-points should be! = More research needed ! • Avoid over-reliance on Clinical Estimate • NCHS edits probably should be conservative for basic data release

22. NCHS/DVS conclusions • NCHS gestational age edits are difficult to replicate, may be too conservative, need to be revisited! • Organize group to recommend changes • to include representatives from NCHS, NAPHSIS, NCCDPHP, and outside researchers • JAMartin@cdc.gov