The study considered a base population of 12,493 singleton live births delivered after at least 20 weeks gestation to women residing in base housing during the period 1968-1985 who were identified through birth records (ATSDR 1998). That population did not include births to mothers who resided on the base during pregnancy but were no longer residents of Onslow County at the time of delivery. Residential mobility may be substantial: according to ATSDR, “approximately one-third of the women who sought prenatal care at the Navy Regional Medical Center at Camp Lejeune moved or were transferred before they delivered” (ATSDR 1998, p. 16). Although exposures were presumed to have occurred before 1968, a starting date of January 1, 1968, was chosen because electronic files of North Carolina birth certificates began that year. The analyses assumed delivery of contaminated water via the water-distribution system through February 1985 (ATSDR 1998; Sonnenfeld et al. 2001).
ATSDR documented that 523 (4%) of the 12,493 live births were excluded because exposure to contaminated water supplies was for less than 1 week or exclusively before conception (44), or because data were missing, inconsistent, or insufficient (479), leaving 11,970 live births for the mean-birth-weight analyses. Of the11,970 live births, 6,117 (51%) were to women who resided at Tarawa Terrace at the time of birth, 31 (0.26%) were to women who resided at Hospital Point (which received water from Hadnot Point), 141 (1.2%) were to women who resided in housing units temporarily supplied by Hadnot Point during a fuel-pump failure, and 5,681 (47%) were to women who resided in housing supplied by the Holcomb Boulevard system, were considered to be unexposed, and served as a comparison group. Additional exclusions were made for the SGA analyses (eight births with gestational age under 22 weeks) and the preterm-birth analyses (the eight births excluded from the SGA analyses plus 101 births classified as implausibly heavy preterm births).
Exposure was defined by linking birth records to the base’s family housing records according to the mother’s address at delivery and the father’s name. The housing records, which contained dates of residence, were used to estimate the dates when the mother resided in base housing units. The study “assumed that each family resided in only one base housing unit during pregnancy” (ATSDR 1998, p. 21). A residential-history substudy indicated that about 55% of mothers in the study moved during their pregnancies, and 3.5% of them moved between base housing units (ATSDR 1998).
The 1998 ATSDR study included all identified births regardless of exposure, whereas the 2001 Sonnenfeld et al. study limited the exposed population to residents of Tarawa Terrace. The Tarawa Terrace residents were considered exposed to PCE from water contaminated by an off-base dry-cleaning establishment (ABC One-Hour Cleaners). ATSDR’s analysis also included births to two groups of residents who were exposed to TCE and other VOCs through the Hadnot Point water system on either a long-term or a transitory basis. Transitory exposure (called short-term in the ATSDR report) covered all births to residents who received drinking water from the Holcomb Boulevard water system and who were pregnant for at least 1 week of the 12-day period during January-February 1985 when Hadnot Point water served the Holcomb Boulevard system. In both studies, residents of the base trailer park were excluded because housing records were incomplete, and, as noted above, a few births to mothers residing on base for a very short time or during ambiguous exposure periods were excluded. The remaining births to mothers residing on the base were considered unexposed, including births to all residents of the Marine Corps Air Station, Rifle Range, and Courthouse Bay and the remaining residents of Berkeley Manor, Midway Park, Paradise Point, and Watkins Village.
Exposure was categorized further by length of residence as a proxy for duration of exposure. Duration of exposure was defined as length of time before the birth that the mother lived at the residence specified on the birth certificate. Because inclusion in the study was based on maternal residence at the time of birth, exposure duration was relative to the end of pregnancy. Duration-of-exposure analyses excluded births that occurred after exposure ended in 1985. In analyses, duration of exposure was categorized as never, 1-3 weeks, 4-10 weeks, 11-20 weeks, over 20 weeks and less than the entire pregnancy,